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How Was the Solar System Formed? – The Nebular Hypothesis

Since time immemorial, humans have been searching for the answer of how the Universe came to be. However, it has only been within the past few centuries, with the Scientific Revolution, that the predominant theories have been empirical in nature. It was during this time, from the 16th to 18th centuries, that astronomers and physicists began to formulate evidence-based explanations of how our Sun, the planets, and the Universe began.

When it comes to the formation of our Solar System, the most widely accepted view is known as the Nebular Hypothesis . In essence, this theory states that the Sun, the planets, and all other objects in the Solar System formed from nebulous material billions of years ago. Originally proposed to explain the origin of the Solar System, this theory has gone on to become a widely accepted view of how all star systems came to be.

Nebular Hypothesis:

According to this theory, the Sun and all the planets of our Solar System began as a giant cloud of molecular gas and dust. Then, about 4.57 billion years ago, something happened that caused the cloud to collapse. This could have been the result of a passing star, or shock waves from a supernova, but the end result was a gravitational collapse at the center of the cloud.

From this collapse, pockets of dust and gas began to collect into denser regions. As the denser regions pulled in more and more matter, conservation of momentum caused it to begin rotating, while increasing pressure caused it to heat up. Most of the material ended up in a ball at the center while the rest of the matter flattened out into disk that circled around it. While the ball at the center formed the Sun, the rest of the material would form into the protoplanetary disc .

The planets formed by accretion from this disc, in which dust and gas gravitated together and coalesced to form ever larger bodies. Due to their higher boiling points, only metals and silicates could exist in solid form closer to the Sun, and these would eventually form the terrestrial planets of Mercury , Venus , Earth , and Mars . Because metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large.

In contrast, the giant planets ( Jupiter , Saturn , Uranus , and Neptune ) formed beyond the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid (i.e. the Frost Line ). The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium. Leftover debris that never became planets congregated in regions such as the Asteroid Belt , Kuiper Belt , and Oort Cloud .

Within 50 million years, the pressure and density of hydrogen in the center of the protostar became great enough for it to begin thermonuclear fusion. The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved. At this point, the Sun became a main-sequence star. Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space, ending the planetary formation process.

History of the Nebular Hypothesis:

The idea that the Solar System originated from a nebula was first proposed in 1734 by Swedish scientist and theologian Emanual Swedenborg. Immanuel Kant, who was familiar with Swedenborg’s work, developed the theory further and published it in his Universal Natural History and Theory of the Heavens  (1755). In this treatise, he argued that gaseous clouds (nebulae) slowly rotate, gradually collapsing and flattening due to gravity and forming stars and planets.

A similar but smaller and more detailed model was proposed by Pierre-Simon Laplace in his treatise Exposition du system du monde (Exposition of the system of the world), which he released in 1796. Laplace theorized that the Sun originally had an extended hot atmosphere throughout the Solar System, and that this “protostar cloud” cooled and contracted. As the cloud spun more rapidly, it threw off material that eventually condensed to form the planets.

The Laplacian nebular model was widely accepted during the 19th century, but it had some rather pronounced difficulties. The main issue was angular momentum distribution between the Sun and planets, which the nebular model could not explain. In addition, Scottish scientist James Clerk Maxwell (1831 – 1879) asserted that different rotational velocities between the inner and outer parts of a ring could not allow for condensation of material.

It was also rejected by astronomer Sir David Brewster (1781 – 1868), who stated that:

“those who believe in the Nebular Theory consider it as certain that our Earth derived its solid matter and its atmosphere from a ring thrown from the Solar atmosphere, which afterwards contracted into a solid terraqueous sphere, from which the Moon was thrown off by the same process… [Under such a view] the Moon must necessarily have carried off water and air from the watery and aerial parts of the Earth and must have an atmosphere.”

By the early 20th century, the Laplacian model had fallen out of favor, prompting scientists to seek out new theories. However, it was not until the 1970s that the modern and most widely accepted variant of the nebular hypothesis – the solar nebular disk model (SNDM) – emerged. Credit for this goes to Soviet astronomer Victor Safronov and his book Evolution of the protoplanetary cloud and formation of the Earth and the planets (1972) . In this book, almost all major problems of the planetary formation process were formulated and many were solved.

For example, the SNDM model has been successful in explaining the appearance of accretion discs around young stellar objects. Various simulations have also demonstrated that the accretion of material in these discs leads to the formation of a few Earth-sized bodies. Thus the origin of terrestrial planets is now considered to be an almost solved problem.

While originally applied only to the Solar System, the SNDM was subsequently thought by theorists to be at work throughout the Universe, and has been used to explain the formation of many of the exoplanets that have been discovered throughout our galaxy.

Although the nebular theory is widely accepted, there are still problems with it that astronomers have not been able to resolve. For example, there is the problem of tilted axes. According to the nebular theory, all planets around a star should be tilted the same way relative to the ecliptic. But as we have learned, the inner planets and outer planets have radically different axial tilts.

Whereas the inner planets range from almost 0 degree tilt, others (like Earth and Mars) are tilted significantly (23.4° and 25°, respectively), outer planets have tilts that range from Jupiter’s minor tilt of 3.13°, to Saturn and Neptune’s more pronounced tilts (26.73° and 28.32°), to Uranus’ extreme tilt of 97.77°, in which its poles are consistently facing towards the Sun.

Also, the study of extrasolar planets have allowed scientists to notice irregularities that cast doubt on the nebular hypothesis. Some of these irregularities have to do with the existence of “hot Jupiters” that orbit closely to their stars with periods of just a few days. Astronomers have adjusted the nebular hypothesis to account for some of these problems, but have yet to address all outlying questions.

Alas, it seems that it questions that have to do with origins that are the toughest to answer. Just when we think we have a satisfactory explanation, there remain those troublesome issues it just can’t account for. However, between our current models of star and planet formation, and the birth of our Universe, we have come a long way. As we learn more about neighboring star systems and explore more of the cosmos, our models are likely to mature further.

We have written many articles about the Solar System here at Universe Today. Here’s The Solar System , Did our Solar System Start with a Little Bang? , and What was Here Before the Solar System?

For more information, be sure to check out the origin of the Solar System and how the Sun and planets formed .

Astronomy Cast also has an episode on the subject – Episode 12: Where do Baby Stars Come From?

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5 Replies to “How Was the Solar System Formed? – The Nebular Hypothesis”

So… the transition from the geocentric view and eternal state the way things are evolved with appreciation of dinosaurs and plate tectonics too… and then refining the nebular idea… the Nice model… the Grand Tack model… alittle more? Now maybe the Grand Tack with the assumption of mantle breaking impacts in the early days – those first 10 millions years were heady times!

And the whole idea of “solar siblings” has been busy the last few years…

Nice overview, and I learned a lot. However, there are some salient points that I think I have picked up earlier:

“something happened that caused the cloud to collapse. This could have been the result of a passing star, or shock waves from a supernova, but the end result was a gravitational collapse at the center of the cloud.”

The study of star forming molecular clouds shows that same early, large stars form that way. In the most elaborate model which makes Earth isotope measurements easiest to predict, by free coupling the processes, the 1st generation of super massive stars would go supernova in 1-10 million years.

That blows a 1st geeration of large bubbles with massive, compressed shells that are seeded with supernova elements, as we see Earth started out with. The shells would lead to a more frequent 2nd generation of massive stars with a lifetime of 10-100 million years or so. These stars have powerful solar winds.

That blows a 2nd generation of large bubbles with massive, compressed shells, The shells would lead to a 3d generation of ~ 500 – 1000 stars of Sun size or less. In the case of the Sun the resulting mass was not enough to lead to a closed star cluster as we can see circling the Milky Way, but an open star cluster where the stars would mix with other stars over the ~ 20 orbits we have done around the MW.

“The ices that formed these planets were more plentiful”.

The astronomy course I attended looked at the core collapse model of large planets. (ASs well as the direct collapse scenario.) The core grew large rapidly and triggered gas collapse onto the planet from the disk, a large factor being the stickiness of ices at the grain stage. The terrestrial planets grow by slower accretion, and the material may have started to be cleared from the disk. by star infall or radiation pressure flow outwards, before they are finished.

An interesting problem for terrestrial planets is the “meter size problem” (IIRC the name). It was considered hard to grow grains above a cm, and when they grow they rapidly brake and fall onto the star.

Now scientists have come up with grain collapse scenarios, where grains start to follow each other for reasons of gravity and viscous properties of the disk, I think. All sorts of bodies up to protoplanets can be grown quickly and, when over the problematic size, will start to clear the disk rather than being braked by it.

“But as we have learned, the inner planets and outer planets have radically different axial tilts.”

Jupiter can be considered a clue, too massive to tilt by outside forces. The general explanation tend to be the accretion process, where the tilt would be randomized. (Venus may be an exception, since some claim it is becoming tidally locked to the Sun – Mercury is instead locked in a 3:2 resonance – and it is in fact now retrograde with a putative near axis lock.) Possible Mercury bit at least Earth and Mars (and Moon) show late great impacts.

A recent paper show that terrestrial planets would suffer impacts on the great impact scale, between 1 to 8 as norm with an average of 3. These would not be able to clear out an Earth mass atmosphere or ocean, so if Earth suffered one such impact after having volatiles delivered by late accretion/early bombardment, the Moon could result.

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14.3 Formation of the Solar System

Learning objectives.

By the end of this section, you will be able to:

• Describe the motion, chemical, and age constraints that must be met by any theory of solar system formation
• Summarize the physical and chemical changes during the solar nebula stage of solar system formation
• Explain the formation process of the terrestrial and giant planets
• Describe the main events of the further evolution of the solar system

As we have seen, the comets , asteroids , and meteorites are surviving remnants from the processes that formed the solar system. The planets, moons, and the Sun, of course, also are the products of the formation process, although the material in them has undergone a wide range of changes. We are now ready to put together the information from all these objects to discuss what is known about the origin of the solar system.

Observational Constraints

There are certain basic properties of the planetary system that any theory of its formation must explain. These may be summarized under three categories: motion constraints, chemical constraints, and age constraints. We call them constraints because they place restrictions on our theories; unless a theory can explain the observed facts, it will not survive in the competitive marketplace of ideas that characterizes the endeavor of science. Let’s take a look at these constraints one by one.

There are many regularities to the motions in the solar system. We saw that the planets all revolve around the Sun in the same direction and approximately in the plane of the Sun’s own rotation. In addition, most of the planets rotate in the same direction as they revolve, and most of the moons also move in counterclockwise orbits (when seen from the north). With the exception of the comets and other trans-neptunian objects, the motions of the system members define a disk or Frisbee shape. Nevertheless, a full theory must also be prepared to deal with the exceptions to these trends, such as the retrograde rotation (not revolution) of Venus.

In the realm of chemistry, we saw that Jupiter and Saturn have approximately the same composition—dominated by hydrogen and helium. These are the two largest planets, with sufficient gravity to hold on to any gas present when and where they formed; thus, we might expect them to be representative of the original material out of which the solar system formed. Each of the other members of the planetary system is, to some degree, lacking in the light elements. A careful examination of the composition of solid solar-system objects shows a striking progression from the metal-rich inner planets, through those made predominantly of rocky materials, out to objects with ice-dominated compositions in the outer solar system. The comets in the Oort cloud and the trans-neptunian objects in the Kuiper belt are also icy objects, whereas the asteroids represent a transitional rocky composition with abundant dark, carbon-rich material.

As we saw in Other Worlds: An Introduction to the Solar System , this general chemical pattern can be interpreted as a temperature sequence: hot near the Sun and cooler as we move outward. The inner parts of the system are generally missing those materials that could not condense (form a solid) at the high temperatures found near the Sun. However, there are (again) important exceptions to the general pattern. For example, it is difficult to explain the presence of water on Earth and Mars if these planets formed in a region where the temperature was too hot for ice to condense, unless the ice or water was brought in later from cooler regions. The extreme example is the observation that there are polar deposits of ice on both Mercury and the Moon; these are almost certainly formed and maintained by occasional comet impacts.

As far as age is concerned, we discussed that radioactive dating demonstrates that some rocks on the surface of Earth have been present for at least 3.8 billion years, and that certain lunar samples are 4.4 billion years old. The primitive meteorites all have radioactive ages near 4.5 billion years. The age of these unaltered building blocks is considered the age of the planetary system. The similarity of the measured ages tells us that planets formed and their crusts cooled within a few tens of millions of years (at most) of the beginning of the solar system. Further, detailed examination of primitive meteorites indicates that they are made primarily from material that condensed or coagulated out of a hot gas; few identifiable fragments appear to have survived from before this hot-vapor stage 4.5 billion years ago.

The Solar Nebula

All the foregoing constraints are consistent with the general idea, introduced in Other Worlds: An Introduction to the Solar System , that the solar system formed 4.5 billion years ago out of a rotating cloud of vapor and dust—which we call the solar nebula —with an initial composition similar to that of the Sun today. As the solar nebula collapsed under its own gravity, material fell toward the center, where things became more and more concentrated and hot. Increasing temperatures in the shrinking nebula vaporized most of the solid material that was originally present.

At the same time, the collapsing nebula began to rotate faster through the conservation of angular momentum (see the Orbits and Gravity and Earth, Moon, and Sky chapters). Like a figure skater pulling her arms in to spin faster, the shrinking cloud spun more quickly as time went on. Now, think about how a round object spins. Close to the poles, the spin rate is slow, and it gets faster as you get closer to the equator. In the same way, near the poles of the nebula, where orbits were slow, the nebular material fell directly into the center. Faster moving material, on the other hand, collapsed into a flat disk revolving around the central object ( Figure 14.11 ). The existence of this disk-shaped rotating nebula explains the primary motions in the solar system that we discussed in the previous section. And since they formed from a rotating disk, the planets all orbit the same way.

Picture the solar nebula at the end of the collapse phase, when it was at its hottest. With no more gravitational energy (from material falling in) to heat it, most of the nebula began to cool. The material in the center, however, where it was hottest and most crowded, formed a star that maintained high temperatures in its immediate neighborhood by producing its own energy. Turbulent motions and magnetic fields within the disk can drain away angular momentum, robbing the disk material of some of its spin. This allowed some material to continue to fall into the growing star, while the rest of the disk gradually stabilized.

The temperature within the disk decreased with increasing distance from the Sun, much as the planets’ temperatures vary with position today. As the disk cooled, the gases interacted chemically to produce compounds; eventually these compounds condensed into liquid droplets or solid grains. This is similar to the process by which raindrops on Earth condense from moist air as it rises over a mountain.

Let’s look in more detail at how material condensed at different places in the maturing disk ( Figure 14.12 ). The first materials to form solid grains were the metals and various rock-forming silicates. As the temperature dropped, these were joined throughout much of the solar nebula by sulfur compounds and by carbon- and water-rich silicates, such as those now found abundantly among the asteroids. However, in the inner parts of the disk, the temperature never dropped low enough for such materials as ice or carbonaceous organic compounds to condense, so they were lacking on the innermost planets.

Far from the Sun, cooler temperatures allowed the oxygen to combine with hydrogen and condense in the form of water (H 2 O) ice. Beyond the orbit of Saturn, carbon and nitrogen combined with hydrogen to make ices such as methane (CH 4 ) and ammonia (NH 3 ). This sequence of events explains the basic chemical composition differences among various regions of the solar system.

Example 14.1

Rotation of the solar nebula.

With P initial equal to 1,000,000 years, P final , the new rotation period, is 64 years. This is a lot shorter than the actual time Pluto takes to go around the Sun, but it gives you a sense of the kind of speeding up the conservation of angular momentum can produce. As we noted earlier, other mechanisms helped the material in the disk lose angular momentum before the planets fully formed.

Check Your Learning

The period of the rotating nebula is inversely proportional to D 2 D 2 . As we have just seen, P final P initial = ( D final D initial ) 2 . P final P initial = ( D final D initial ) 2 . Initially, we have P initial = 10 6 yr and D initial = 10 4 AU. Then, if D final is in AU, P final (in years) is given by P final = 0.01 D final 2 . P final = 0.01 D final 2 . If Jupiter’s orbit has a radius of 5.2 AU, then the diameter is 10.4 AU. The period is then 1.08 years.

Formation of the Terrestrial Planets

The grains that condensed in the solar nebula rather quickly joined into larger and larger chunks, until most of the solid material was in the form of planetesimals, chunks a few kilometers to a few tens of kilometers in diameter. Some planetesimals still survive today as comets and asteroids. Others have left their imprint on the cratered surfaces of many of the worlds we studied in earlier chapters. A substantial step up in size is required, however, to go from planetesimal to planet.

Some planetesimals were large enough to attract their neighbors gravitationally and thus to grow by the process called accretion . While the intermediate steps are not well understood, ultimately several dozen centers of accretion seem to have grown in the inner solar system. Each of these attracted surrounding planetesimals until it had acquired a mass similar to that of Mercury or Mars. At this stage, we may think of these objects as protoplanets —“not quite ready for prime time” planets.

Each of these protoplanet s continued to grow by the accretion of planetesimals. Every incoming planetesimal was accelerated by the gravity of the protoplanet, striking with enough energy to melt both the projectile and a part of the impact area. Soon the entire protoplanet was heated to above the melting temperature of rocks. The result was planetary differentiation , with heavier metals sinking toward the core and lighter silicates rising toward the surface. As they were heated, the inner protoplanets lost some of their more volatile constituents (the lighter gases), leaving more of the heavier elements and compounds behind.

Formation of the Giant Planets

In the outer solar system, where the available raw materials included ices as well as rocks, the protoplanets grew to be much larger, with masses ten times greater than Earth. These protoplanets of the outer solar system were so large that they were able to attract and hold the surrounding gas. As the hydrogen and helium rapidly collapsed onto their cores, the giant planets were heated by the energy of contraction. But although these giant planets got hotter than their terrestrial siblings, they were far too small to raise their central temperatures and pressures to the point where nuclear reactions could begin (and it is such reactions that give us our definition of a star). After glowing dull red for a few thousand years, the giant planets gradually cooled to their present state ( Figure 14.13 ).

The collapse of gas from the nebula onto the cores of the giant planets explains how these objects acquired nearly the same hydrogen-rich composition as the Sun. The process was most efficient for Jupiter and Saturn; hence, their compositions are most nearly “cosmic.” Much less gas was captured by Uranus and Neptune, which is why these two planets have compositions dominated by the icy and rocky building blocks that made up their large cores rather than by hydrogen and helium. The initial formation period ended when much of the available raw material was used up and the solar wind (the flow of atomic particles) from the young Sun blew away the remaining supply of lighter gases.

Further Evolution of the System

All the processes we have just described, from the collapse of the solar nebula to the formation of protoplanets, took place within a few million years. However, the story of the formation of the solar system was not complete at this stage; there were many planetesimals and other debris that did not initially accumulate to form the planets. What was their fate?

The comets visible to us today are merely the tip of the cosmic iceberg (if you’ll pardon the pun). Most comets are believed to be in the Oort cloud, far from the region of the planets. Additional comets and icy dwarf planets are in the Kuiper belt, which stretches beyond the orbit of Neptune. These icy pieces probably formed near the present orbits of Uranus and Neptune but were ejected from their initial orbits by the gravitational influence of the giant planets.

In the inner parts of the system, remnant planetesimals and perhaps several dozen protoplanets continued to whiz about. Over the vast span of time we are discussing, collisions among these objects were inevitable. Giant impacts at this stage may have stripped Mercury of part of its mantle and crust, reversed the rotation of Venus, and broke off part of Earth to create the Moon (all events we discussed in other chapters).

Smaller-scale impacts also added mass to the inner protoplanets. Because the gravity of the giant planets could “stir up” the orbits of the planetesimals, the material impacting on the inner protoplanets could have come from almost anywhere within the solar system. In contrast to the previous stage of accretion, therefore, this new material did not represent just a narrow range of compositions.

As a result, much of the debris striking the inner planets was ice-rich material that had condensed in the outer part of the solar nebula. As this comet-like bombardment progressed, Earth accumulated the water and various organic compounds that would later be critical to the formation of life. Mars and Venus probably also acquired abundant water and organic materials from the same source, as Mercury and the Moon are still doing to form their icy polar caps.

Gradually, as the planets swept up or ejected the remaining debris, most of the planetesimals disappeared. In two regions, however, stable orbits are possible where leftover planetesimals could avoid impacting the planets or being ejected from the system. These regions are the asteroid belt between Mars and Jupiter and the Kuiper belt beyond Neptune. The planetesimals (and their fragments) that survive in these special locations are what we now call asteroids, comets, and trans-neptunian objects.

Astronomers used to think that the solar system that emerged from this early evolution was similar to what we see today. Detailed recent studies of the orbits of the planets and asteroids, however, suggest that there were more violent events soon afterward, perhaps involving substantial changes in the orbits of Jupiter and Saturn. These two giant planets control, through their gravity, the distribution of asteroids. Working backward from our present solar system, it appears that orbital changes took place during the first few hundred million years. One consequence may have been scattering of asteroids into the inner solar system, causing the period of “heavy bombardment” recorded in the oldest lunar craters.

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September 29, 1917

17 min read

The Origin of the Solar System

An Outline of the Three Principal Hypotheses

By Harold Jeffreys

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THE question of the origin of the solar system is one that has been a source of speculation for over a hundred years; but, in spite of the attention that has been devoted to it, no really satisfactory answer has yet been obtained. There are at present three principal hypotheses that appear to contain a large element of truth, as measured by the closeness of the approximation of their consequences to the facts of the present state of the system, but none of them is wholly satisfactory. These are the Nebular Hypothesis of Laplace, the Planetesimal Hypothesis of Chamberlin and Moulton, and the Capture Theory of See. Darwings theory of Tidal Friction is scarcely a distinct hypothesis, but is mentioned separately on account of its application to all of the others. The main features of these hypotheses will be outlined in the present paper. The Hypothesis of Laplace.According to Laplace, the solar system formerly consisted of a very much flattened mass of gas, extending beyond the orbit of Neptune, and rotating like a rigid body. In consequence of radiation of energy this slowly contracted, and in so doing gained so much in angular velocity that the centrifugal force at the equator became greater than gravity, and a ring of matter was left behind along the equator. Further contraction would detach a series of rings. These were then expected to break up in such a way that each produced a gaseous planet. This might later evolve in the same way as the original nebula, thus producing satellites. The criticisms of this hypothesis in its original form are very well known, and will only be summarized here. Forest ranger beating out a fire in one of the National Forests in Oregon FIGHTING FOREST FIRES [See page 200] The angular momentum of the system when the gaseous central body extended to the orbit of any planet can be calculated, and is not nearly sufficient to cause detachment of matter. Poincare showed that this objection could be met if the nebula were initially highly heterogeneous, with all but gAtj of its mass in the central body. The matter left behind would not form definite rings; for a gas has no cohesion, and consequently the separation of matter along the equator would be continuous and lead to another gaseous nebula, not rotating like a rigid body. A ring could not condense into a planet. According to the latest work of Jeans, viscosity is inadequate to make a mass of gas as large as a Lapla- cian nebula rotate like a rigid body. No satellite could revolve in a shorter time than it takes its primary to rotate: this condition is violated by Phobos, the inner satellite of Mars, and by the particles constituting the inner edge of Saturn's ring. All satellites should revolve in the same direction as their primaries rotate: this condition is violated by one satellite of Saturn and two of Jupiter. The second, third, and fourth objections seem quite unanswerable at present. The theory of Gravitational Instability, due to Jeans, is an attempt to pass directly from the symmetrical nebula to an unsymmetrical one with a secondary nucleus, without the ring as an intermediate stage. It will be noticed that Laplace's hypothesis implies that all the planets were formerly gaseous, and hence must have been liquid before they became solid. The question of the course of evolution of a gaseous mass initially heterogeneous with several strong secondary condensations has not hitherto been considered; such a mass would be free from at least the first four of the objections offered to the standard forms of Laplace's hypothesis, and its history would serve as a hypothesis intermediate between this and the Planetesimal Hypothesis. The Planetesimal Hypothesis.This hypothesis has been formulated by Chamberlin and Moulton1 to avoid the serious defects of the Nebular Hypothesis. It really consists of two separate assumptions, either of which could be discarded without necessarily invalidating the other. The first of these involves the close approach of some wandering star to the sun. This would raise two tidal projections at opposite sides of the sun, and if the disturbance was sufficiently violent, streams of matter would be expelled from them. On account of the perturbations of their paths by the second body, these would not fall back into the sun, but would go on revolving round it as a system of secondary nuclei, with a large number of very fine particles also revolving round the sun; each particle, however small, would revolve independently, so that the system would in this respect resemble the heterogeneous nebula mentioned at the close of the last paragraph. The mathematical investigation of this hypothesis would be extremely difficult, but there seems to be no obvious objection to it. It will be seen that the nuclei would be initially liquid or gaseous, having been expelled from the sun. Thus this hypothesis implies a formerly molten earth. The smaller particles would soon become solid, but the gaseous part initially expelled and not under the influence of a secondary nucleus would remain gaseous, although its density would be very small. The orbits would be highly eccentric. The second part of the hypothesis deals with the latef- evolution of the secondary nuclei. Its authors believe that these would steadily grow by picking up the smaller particles, which are called planetesimals, and in the process they would have the eccentricities of their orbits reduced. That this is qualitatively correct can easily be proved mathematically. There is, however, a serious objection to its quantitative adequacy. Consider any arbitrary planetesimal. Its chance of colliding with another planetesimal in a definite time is proportional to the sum of the surfaces of the planetesimals, while its chance of colliding with a nucleus is proportional to the sum of the surfaces of the nuclei. Further, if the eccentricities of the planetary orbits are to be considerably affected by accretion, the mass picked up by each planet must be at least as great as the original mass of the planet. Now the more finely divided the matter is, the more surface it exposes, and hence before accretion the mass picked up must have presented a much larger surface than the planet did. Hence collisions between planetesimals must have been far commoner than collisions between planets and planetesimals. Further, as the velocity of impact must have been comparable with an orbital velocity on account of the high eccentricity of the orbits, the colliding planetesimals must in nearly all cases have turned to gas; for it is known that meteors entering the earth's atmosphere at such velocities are volatized. Hence nearly all of the planetesimals must have turned to gas before the nuclei could be much affected by accretion. We are thus back to the heterogeneous gaseous nebula. If the planetesimals moved initially in nearly circular orbits this objection does not arise, but it can then be shown that the product of the mass and the orbital eccentricity of each nucleus would diminish with the time. It can thus be seen that Jupiter could never have been smaller than Uranus is now. There is no obvious objection to this form of the hypothesis, but there is no reason to suppose that solid planetesimals did originally move in nearly circular orbits.2 A further hypothesis that has come to be associated with the present one, although not an essential part of it, is the belief that the earth has always been solid. There are many serious difficulties in the way of this. The mode of formation of the nuclei described in the first part of the Planestesimal Hypothesis implies that they were initially liquid or gaseous. This is not, however, a direct objection; one part of the hypothesis might be true and the other false, as they are not interdependent. Only one satisfactory explanation of the elevation of mountains by the folding of the earth's crust has been offered; this attributes it to a horizontal compression at the surface. Now, if a solid earth grew by the addition of small particles from outside, these would be deposited in a layer on the surface, in a perfectly unstrained condition. Thus, during the whole process of growth the same surface condition would always hold, namely, that there is no horizontal compression at the surface, however much deformation may take place within. Hence any stresses available for mountain- building must have been accumulated after accretion ceased; if the theory that the earth was formerly molten should be proved to give insufficient surface compression to account for known mountains, then a fortiori the theory of a permanently solid earth gives insufficient compression, as the available fall of temperature is less. 3. It is by no means clear that a solid earth growing by accretion would remain solid. A particle falling from an infinite distance to the earth under the earth's attraction alone would develop a velocity almost enough to volatilize it on impact, and the actual velocities must have been considerably greater than this, as the planetesimals would have a velocity relative to the earth before entering its sphere of influence. If, then, the particles required to form the earth were all brought together at once, the resulting body would be gaseous. On the other hand, if the accretion were spread over a long enough time, heat would be radiated away as fast as it was produced, and the body would remain solid. In the absence of a criterion of the rate of growth it is impossible to state whether an earth growing by accretion could remain solid or not. Holmes3 has found that the hypothesis of a cooling earth, initially in a liquid state, leads to temperatures within the crust capable of accounting for igneous activity, whereas the view that the earth is now in a steady state, its temperature gradient being maintained wholly by radio-activity, is by no means certain to lead to adequate internal temperatures. Assuming the former fluidity of the earth, he has developed a wonderfully consistent theory of the earth's thermal state. The present writer, using Holmes's data, finds4 that the available compression of the crust is of the same order of magnitude as that required to produce the existing mountain-ranges. 2Monthly Notices of R.A.S. vol. lxxvn. 1916. It seems, then, that whatever we may assume about the origin of the earth, the hypothesis that it has at some stage of its existence been liquid or gaseous agrees best with its present state. The hypothesis of Laplace, however modified, implies the former fluidity of the earth, and so does the standard form of the Planetesimal Hypothesis. The Capture Theory of See.hLike the Planetesimal Hypothesis, this has been developed during the present century to avoid the objections that have been offered to that of Laplace. The main features of the two theories are very similar. Both involve the idea of a system of secondary nuclei revolving in independent orbits about the primitive sun, with sparsely distributed small particles between them, and the impacts of the small particles on the nuclei are supposed in course of time to act on the orbits of the latter in the same way as a resisting medium; namely, the eccentricities of the orbits tend to diminish, and satellites tend to approach their primaries. The Capture Theory is not, however, stated in so precise a form as the Planetesimal Theory. It is not definitely stated whether all the small particles would revolve in the same direction or not. If they did, then there would be little or no secular effect on the mean distance of a planet. If, however, they moved indifferently in the direct and retrograde senses, then their collective effect would be the same as that of a medium at rest, and the friction encountered by the planets in their motion would cause them to approach the sun. The fact that such a secular effect is stated by See to occur implies that the particles at any point are not on an average supposed to move with the velocity appropriate to a circular orbit at that point, so that the conditions would be such as to ensure that collisions between them would be violent. The small particles are described by the somewhat vague term of “cosmical dust”; if this means that they were solid, the Capture Theory, like the Planetesimal Theory, fails on the ground that the collisions between the small particles would cause the system to degenerate to a gaseous nebula long before any important effect had been produced on the nuclei. If, on the other hand, they were discrete molecules, then the system would be a heterogeneous gaseous nebula at the commencement, and this objection does not apply. It is clear, however, that the planets cannot have entered the system from outer space, for then their orbital planes would be inclined to one another at large angles, which the subsequent action of the medium could scarcely affect, whereas actually all the major planets keep very close to the ecliptic. All must, then, be regarded as having always been members of the solar system, however much their orbits may have changed. They are supposed to be derived from the secondary nuclei of a soiral nebula. The most important difference between the Planetesimal and Capture theories lies in the history attributed to the satellites. In the former, each satellite is supposed to have always been associated with its present primary, having been near it when originally expelled from the sun. In the Capture Theory, primaries and satellites are both supposed to have initially moved independently round the sun in highly eccentric orbits. If, in the course of its movement”, a small body came sufficiently near a large one, and had a sufficiently small relative velocity, then a permanent change would take place in the character of its orbit, and it is possible that, under the influence of the resisting medium, this would ultimately lead to its becoming a satellite. The mechanism of the process has not been worked out in detail, and, in view of the extremely complicated nature of the problem, it would be very dangerous to predict whether it is feasible. All the satellites in the system are supposed to have been captured in this way by their primaries. In both hypotheses the satellites are considered to have approached their primaries after becoming associated with them owing to the secular effect of the resisting medium. 3”Padio-activity and the Earth's Thermal History,” Geol. Mag. FebruaryMarch 1915, June 1916. *Phil. Mag. vol. xxxii. Dec m':er 1916. *>The Capture Theory of Cosmical Evolution, by T. J. J. See The Theory of Tidal Friction.All the theories so far mentioned agree in the fact that each commences with a particular distribution of matter, and tries to predict the course of the changes that would follow if this were left to itself. The success or failure of such hypotheses to lead to a system resembling the present solar system is the measure of their truth or falsehood. The method is thus essentially one of trial and error, and when a theory is found unsatisfactory, the next step is to modify it in such a way as to avoid the defects that have been detected. In this way a succession of different hypotheses may be Obtained, each giving a better representation of the facts than the previous one. Destructive criticism may thus be of positive value. Such a method must necessarily yield the truth very slowly, and must further involve a large number of assumptions concerning the initial conditions; in addition, the set of initial conditions that leads to the correct final state may not be unique. The Theory of Tidal Friction, due to Sir G. H. Darwin,6 is of a totally different character. It? starts with the present conditions, and by means of a single highly plausible hypothesis obtains relations that the properties of the system must have satisfied at any epoch, provided only that this is not too remote for the calculation to be possible, and that no unknown causes have operated that could invalidate the work. The initial conditions thus obtained are then unique, and the only way of disproving the hypothesis would be to discover some new agency of sufficient magnitude to upset the course of the involution. Whatever hypothesis may ultimately be found to account for the present solar system, the Theory of Tidal Friction must therefore form a part of it. The physical basis of the theory is very simple. The attractive force due to the moon is always greatest on the side of the earth nearest to it, and least on that farthest away, while its value at the center of the earth is intermediate. The center of the earth being regarded as fixed, then, the moon tends to cause the parts of the earth nearest to and farthest from it to protrude, thus forming a bodily tide. If the earth were perfectly elastic, the high tide would always occur with the moon in the zenith or nadir; no energy would be dissipated, and there would be no secular effect. If, however, it is viscous the tides would lag somewhat, and their attractions on the moon would, in general, produce a calculable secular effect on the moon's motion and the rotation of the earth. The only case where viscosity would produce no secular effect is when the deformed body rotates in the same time as the deforming one revolves. The tide then does not move round relatively to the body, but becomes a constant fixed deformation, directly under the deforming body, and ceases to produce a secular effect. In the ultimate steady state of a viscous system, then, the viscous body will always keep the same face turned towards the perturbing one. In the solar system system there are certainly two examples of this condition, and no other explanation of it has been advanced. Mercury always keeps the same face towards the sun, and the moon towards the earth; with less certainty it is believed that the same is true of Venus and the satellites of Jupiter. Now if the viscosity of a substance be zero, that substance is a perfect fluid, and there can be no dissipation of energy inside it. If, on the other hand, it be infinite, then we have the case of perfect elasticity, and again there can be dissipation. If the viscosity steadily increase from 0 to infinity, then the rate of dissipation of energy when the same periodic stress is applied increases to a maximum and then diminishes again to zero. The balance of probability seems to imply that the earth was formerly fluid, and, if this can be granted, the fact that most of it is now almost perfectly elastic at once indicates that dissipation of energy by tidal friction must have been important in the past. On this hypothesis Sir G. H. Darwin traced the system of the earth and moon back to a state where the moon was close to the earth, the two always keeping the same face towards each other, and revolving in some time between three and five hours. The lunar orbit was practically in the plane of the equator; the initial eccentricity is uncertain, as it depends altogether on the actual variation of the viscosity with the time. Scientific Papers, vol. ii. The question that next arises is, what was the condition just before this? The natural suggestion is that the two bodies formed one mass. The cause of the separation is, however, open to some doubt. It has been thought that the rapidity of the rotation would be enough to cause instability, in which case the original body might break up into two parts. Moulton, on the other hand, has shown that the actual rotation could not be so rapid as to make the system unstable. It is more likely that Darwin's original suggestion is correct, namely, that at the epoch considered the period of rotation was nearly double the period of one of the free vibrations of the mass; consequently the amplitude of the semidiurnal tide would be enormous, and might easily lead to fission in a system not possessing much strength. The Prevalence of Direct Motion in the Solar System. On all of the theories of the origin of the solar system that have here been described it is necessary that the planets should revolve in the same direction. On the Planetesimal Theory this would be the direction of the motion of the perturbing body relative to the sun at the time of the initial disruption. In addition to this, however, all the planets except probably Uranus and Neptune have a direct rotation, and all the satellites except those of these two planets and the outer ones of Jupiter and Saturn have a direct revolution. The fact that three satellites revolve in the opposite direction to the rotation of their primaries is in flagrant contradiction to the original form of the Nebular Hypothesis. It was, however, suggested by Darwin that all the planets might have originally had a retrograde rotation, and that the friction of the solar tides has since reversed the rotation of all except the two outermost. Jupiter and Saturn would then be supposed to have produced their outer satellites before the reversal took place, and the others afterwards. An objection to this theory has been raised by Moulton, who points out that the secular retardation of the rotation of Saturn due to solar tides is only about tsooo of that of the earth, so that there probably was not time for this to occur. On the other hand, this retardation is proportional to the seventh power of the diameter of the planets: if we can grant then that these planets were formerly much more distended than at present, the viscosity remaining the same, the available time may be adequate. At the same time, solar tidal friction may be adequate to explain the facts that one of the satellites of Mars and the particles at the inner edge of Saturn's ring revolve more rapidly than their primaries rotate, which would not be the case on the unmodified Nebular Hypothesis. Direct rotation and revolution of satellites on the Planetesimal Theory are shown by Moulton to be probable as a result of a very ingenious argument involving the mode of accretion. Whether it is quantitatively adequate is not proved, and the present writer would prefer to regard these motions as having been direct since the initial disruption. Let us suppose, for instance, that disruption would occur when the disruptive force had reached a definite fraction of surface gravity. It can easily be seen that both are proportional to the diameter of the disturbed body, and hence their ratio is independent of it. Other things being equal, then, a nucleus of any size would be equally likely to be broken up and give a set of dependent nuclei, which would then revolve round it in the direct sense. Secondary nuclei expelled at the same time and close together would remain together, and their relative motion might be in either sense. Thus we should expect both direct and retrograde revolution, but the former would predominate. The fact that the retrograde satellites are on the outside of their systems is to be attributed partly to the greater stability of retrograde orbits of larger size and partly to the fact that they would experience less resistance from the medium. Capture may be possible; in the present state of our knowledge we can neither affirm nor deny it. Direct rotation is presumably to be attributed to the attraction of the disturbing body on the tidal protuberance before and during expulsion, and to secondary nuclei with direct motions falling back into the parent body. Subsequent evolution would take place in a similar way to that indicated by Darwin. The Hypothesis of a Heterogeneous Nebula.A system of nuclei revolving in a tenuous gaseous nebula would experience a viscous resistance from it, and hence would probably evolve in much the same way as See has indicated in the Capture Theory; accretion must probably be almost negligible, so that the original nuclei must have had nearly their present masses. The original eccentricities of the orbits of both planets and satellites would be considerably reduced; the inclination to the plane of the ecliptic would be small at the commencement, and would remain so; if the medium revolved the effect on the major axes of the orbit, and hence on the periods, would probably be small. Direct satellites would approach their primaries, and retrograde ones would ultimately be left on the outskirts of their subsystems. Given suitable initial conditions, then, a system might be developed that would bear a strong resemblance to the existing solar system. The resisting medium itself would gradually degenerate and approach the sun on account of its internal friction; the zodiacal light may be the last remnant of it. It may, however, be regarded as certain that there has been no large amount of resisting matter near the earth's orbit for a very long time; there has probably been ample time for the evolution of the earth and moon to take place from the state that Darwin traced them back to. The moon was then probably formed from the earth by the disruptive action of the solar tides; but, as this would be a resonance effect, increasing in amplitude over thousands of vibrations, whereas the formation of a system of nuclei in the way suggested by Moulton would take place at once, there need be no surprise that the former event led to a single satellite of of the mass of the primary, while the latter formed several, the largest having a mass of tTjjfu of its primary. The unsymmetrical nebula here considered might have been produced in the manner described in the last section. A symmetrical nebula becoming gravitationally unstable would lead to an unsymmetrical one, as was proved by Jeans, but it is difficult to see how the phenomenon of retrograde and direct motions occuring to the same subsystem could occur on this hypothesis. On the whole, then, the most plausible hypothesis seems to be that a gaseous neubla with a system of secondary and tertiary nuclei was formed round the sun by tidal disruption owing to the close passage of another star, and that this has been subsequently modified by gaseous viscosity, and at a later stage by tidal friction. The moon was probably formed from the earth by solar tidal disruption, this method being abnormal in the system, and the later evolution of the earth and moon has been dominated by bodily tidal friction.

How Did the Solar System Form?

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The solar system is a pretty busy place. It’s got all kinds of planets, moons, asteroids, and comets zipping around our Sun.

But how did this busy stellar neighborhood come to be?

Our story starts about 4.6 billion years ago, with a wispy cloud of stellar dust.

This cloud was part of a bigger cloud called a nebula.

At some point, the cloud collapsed—possibly because the shockwave of a nearby exploding star caused it to compress.

When it collapsed, it fell in on itself, creating a disk of material surrounding it.

Finally the pressure caused by the material was so great that hydrogen atoms began to fuse into helium, releasing a tremendous amount of energy. Our Sun was born!

Even though the Sun gobbled up more than 99% of all the stuff in this disk, there was still some material left over.

Bits of this material clumped together because of gravity. Big objects collided with bigger objects, forming still bigger objects. Finally some of these objects became big enough to be spheres—these spheres became planets and dwarf planets.

Rocky planets, like Earth, formed near the Sun, because icy and gaseous material couldn’t survive close to all that heat.

Gas and icy stuff collected further away, creating the gas and ice giants.

And like that, the solar system as we know it today was formed.

There are still leftover remains of the early days though.

Asteroids in the asteroid belt are the bits and pieces of the early solar system that could never quite form a planet.

Way off in the outer reaches of the solar system are comets. These icy bits haven’t changed much at all since the solar systems formation.

In fact, it is the study of asteroids and comets that allows scientists to piece together this whole long story.

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Historical Geology

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Nebular theory and the formation of the solar system

In the beginning….

How and when does the story of Earth begin? A logical place to start is with the formation of the planet, but as you’ll soon see, the formation of the planet is part of a larger story, and that story implies some backstory before the story, too. The purpose of this case study is to present our best scientific understanding of the formation of our solar system from a presolar nebula, and to put that nebula in context too.

Nebular theory

The prevailing scientific explanation for the origin of the Earth does a good job of not only explaining the Earth’s formation, but the Sun and all the other planets too. Really, it’s not “the Earth’s origin story” alone so much as it is the origin story of the whole solar system . Not only that, but our Sun is but one star among a hundred million in our galaxy, and our galaxy is one of perhaps a hundred million in the universe. So the lessons we learn by studying our own solar system can likely be applied more generally to the formation of other solar systems elsewhere, including those long ago, in galaxies far, far away. The vice versa is also true: Our understanding of our own solar system’s origin story is being refined as we learn more about exoplanets, some of which defy what we see in our own system; “ hot Jupiters ” and “ super-Earths ,” for instance, are features we see in other star systems but not our own.

When we use powerful telescopes to stare out into the galaxy, we observe plenty of other stars, but we observe other things too, including fuzzy looking features called nebulae. A nebula is a big cloud of gas and dust in space. It’s not as bright as a star because it’s not undergoing thermonuclear fusion, with the tremendous release of energy that accompanies that process. An example of a nebula that you are likely to be able to see is in the constellation Orion. Orion’s “belt,” three stars in a row, is a readily identifiable feature in the northern hemisphere’s night sky in winter. A smaller trio of light spots “dangle” from the belt; this is Orion’s sword scabbard. A cheap pair of binoculars will let you examine these objects for yourself; you will discover that the middle point of light in this smaller trio is not a star. It is a nebula called Messier 42.

Nebulae like Messier 42 are common features of the galaxy, but not as common as stars. Nebulae appear to be short-lived features, as matter is often attracted to other matter. All that stuff distributed in that tremendous volume of space is not as stable as it would be if it were all to be drawn together into a few big clumps. Particles pull together with their neighboring particles under the influence of various forces, including “static cling” or electrostatic attraction. This is the same force that makes tiny dust motes clump up into dust bunnies under your couch!

Now, electrostatic force is quite strong for pulling together small particles over small distances, but if you want to make big things like planets and stars out from a nebula, you’re going to need gravity to take over at some point. Gravity is a rather weak force. After all: every time you take a step, you’ve overcoming the gravitational pull of the entire Earth. But gravity can work very efficiently over distance, if the masses involved are large enough. So static cling was the initial organizer, until the “space dust bunnies” got large enough, then gravity was able to take over, attracting mass to mass. The net result is that the gajillions of tiny pieces of the nebula were drawn together, swirling into a denser and denser amalgamation. The nebula began to spin, flattening out from top to bottom, and flattening out into a spinning disk, something between a Frisbee and a fried egg in shape:

Once a star forms in the center, astronomers call the ring of debris around it a protoplanetary disk. Two important processes that helped organize the protoplanetary disk further were condensation and accretion.

Condensation is the process where gaseous matter sticks together to make liquid or solid matter. We have evidence of condensation in the form of small spherical objects with internal layering, kind of like “space hailstones.” These are chondrules, and they represent the earliest objects formed in our solar system. (Occasionally, we are lucky enough to find chondrules that have survived until the present day, entombed inside certain meteorites of the variety called chondrites.)

Chondrules glommed onto other chondrules, and stuck themselves together into primordial “rocks,” building up larger and larger objects. Eventually, these objects got to be big enough to pull their mass into an round shape, and we would be justified to dub them “planetesimals.” Planetesimals gobbled up nearby asteroids, and smashed into other planetesimals, merging and growing through time through the process of accretion. The kinetic force of these collisions heated the rocky and metallic material of the planetesimals, and their temperature also went up as radioactive decay heated them from within. Once warm, denser material could sink to their middles, and lighter-weight elements and compounds rose up to their surface. So not only were they maturing into spheroidal shapes, but they were also differentiating internally, separating into layers organized by density.

Meteorites that show metallic compositions represent “core” material from these planetesimals; core material that we would never get to glimpse had not their surrounding rocky material been blasted off. Iron meteorites such as the Canyon Diablo meteorite below (responsible for Arizona’s celebrated Meteor Crater) therefore are evidence of differentiation of planetesimals into layered bodies, followed by disaggregation: a polite way of saying they were later violently ripped apart by energetic collisions.

If you were to somehow weigh the nebula before condensation and accretion, and again 4.6 billion years later, we’d find the mass to be the same. Rather than being dispersed in a diffuse cloud of uncountable atoms, the condensation and accretion of the nebula resulted in exactly the same amount of stuff, but organized into a smaller and smaller number of bigger and bigger objects. The biggest of these was the Sun, comprising about 99.86% of all the mass in the solar system. Four-fifths of the remaining 0.14% makes up the planet Jupiter.  Saturn, Neptune, and Uranus are huge gas giants as well. The inner rocky planets (including Earth) make up a tiny, tiny fraction of the total mass of the whole solar system – but of course, just because they are relatively small, that doesn’t mean they are unimportant!

The process of accretion continues into the present day, though at a slower pace than the earliest days of the solar system. One place you can observe this is in the asteroid belt, where there are certain asteroids that are basically nothing more than a big 3D pile of space rocks, held together under their own gravity. Consider the asteroid called Itokawa 25143, for instance:

Only about half a kilometer long, and only a few hundred meters wide, Itokawa doesn’t even have enough gravity to pull itself into a sphere. If you were to land on the surface of Itokawa and kick a soccer-ball-sized boulder, it would readily fly off into space, as the force of your kick would be much higher than the force of gravity causing it to stay put.

Another example of accretion continuing to this day is meteorite impacts. Every time a chunk of rock in space intersects the Earth, its mass is added to that of the planet. In that instant, the solar system gets a little bit cleaner (fewer leftover bits rattling around) and the planet gets a little more massive. A spectacular example of this occurred in 1994 with Comet Shoemaker-Levy 9, a  comet which had only been discovered the previous year. Jupiter’s immense gravity broke the comet into chunks, and then swallowed them up one after another. Astronomers on Earth watched with fascination as the comet chunks, some more than a kilometer across, slammed into Jupiter’s atmosphere at 60 km/second (~134,000 mph), creating a 23,700°C fireball and enormous impact scars that were as large as the entire Earth. These scars lasted for months.

This incredibly dramatic event perhaps raises the hair on our necks, seeing the violence and power of cosmic collisions. It’s a reminder that Earthlings are not safe from accretionary impacts even today – as the dinosaurs found out. For the purposes of our current discussion, though, bear in mind that the collision was really a merger between the masses of Comet Shoemaker-Levy 9 and the planet Jupiter, and after the dust settled, the solar system had one fewer object left off by itself, and Jupiter gained a bit more mass. This is the overall trend of the accretion of our solar system from the presolar nebula: under gravity’s influence, the available mass becomes more and more concentrated through time.

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A star is born

Because the Sun is so massive, it is able to achieve tremendous pressures in its interior. These pressures are so high, they can actually force two atoms into the same space , overcoming their immense repulsion for one another, and causing their two nuclei to merge. As two atoms combine to make one more massive atom, energy is released. This process is thermonuclear fusion. Once it begins, stars begin to give off light.

The ability of stars to make big atoms from small ones is key to understanding the history of our solar system and our planet. Planet Earth is made of a wide variety of chemical elements, both lightweight and heavy. All of these elements must have been present in the nebula, in order for them to be included in Earth’s “starting mixture.” Elements formed in the Sun today stay in the Sun, fusing low-weight atoms into heavier atoms. So all the elements on Earth today came from a pre-Sun star. We can go outside on a spring day and enjoy the Sun’s warmth, but the carbon that makes up the skin that basks in that warmth was forged in the heart of another star, a star that’s gone now, a star that blew up.

This exploding star was the source of the nebula where we began this case study: it’s the backstory that occurred before the opening scene. Our solar system is like a “haunted house,” where billions of years ago, there was a vibrant, healthy main-sequence star right here, in this part of the galaxy. Perhaps it had planets orbiting it. Perhaps some of those planets harbored life. We’ll never know: the explosion wiped the slate clean, and “reset” the solar system for the iteration in which we live. The ghostly remnants of this time before our own still linger, in the very stuff we’re made from. This long-dead star fused hydrogen to build the carbon in our bodies, the iron in our blood, the oxygen we breathe, and the silicon in the rocks of our planet.

This is an incredible realization to embrace: everything you know, everything you trust, everything you are , is stardust.

Age of the solar system

So just when did all this happen? An estimate for the age of the solar system can be made using isotopes of the element lead (Pb). There are several isotopes of lead, but for the purposes of figuring out the age of the solar system, consider these four: 208 Pb, 207 Pb, 206 Pb, and 204 Pb.

208 Pb, 207 Pb, 206 Pb are all radiogenic: that is to say, they stable “daughter” isotopes that are produced from the radioactive “parent” isotopes. Each is produced from a different parent, at a different rate:

204 Pb is, as far as we know, non-radiogenic. It’s relevant to this discussion because it can serve as a ‘standard’ that can allow us to compare the other lead isotopes to one another. Just as if we wanted to compare the currencies of Namibia, Indonesia, and Chile, we might reference all three to the U.S. dollar. The dollar would serve as a standard of comparison, allowing us to better see the value of the Namibian currency relative to the Indonesian currency and the Chilean currency. That’s what 204 Pb is doing for us here.

This is a plot showing the modeled evolution of our three radiogenic lead isotopes relative to 204 Pb. It is constrained by terrestrial lead samples at the young end, and projected back in time in accordance with our measurements of how quickly these three isotopes of lead are produced by their radioactive parents. Of course, if we go back far enough in time, we run out of samples to evaluate. The Earth’s rock cycle has destroyed all its earliest rocks. They’ve been metamorphosed, or weathered, or melted – perhaps many times over! What would be really nice is to find some rocks from the early end of these curves – some samples that could verify these projections back in time are accurate.

Such samples do exist! But they are not from the Earth so much as “from the Earth’s starting materials.” If the nebular theory is correct, then a few leftover scraps of the planet’s starting materials are found in the solar system’s asteroids. Every now and again, bits of these space rocks fall to earth, and if they survive their passage through the atmosphere, we may be lucky enough to collect them, and analyze them. We call these space rocks “meteors” as they streak through the atmosphere, heating through friction and oxidizing as they fall. Those that make it all the way to Earth’s surface are known as “meteorites.” They can be often be distinguished by their scalloped fusion crust, as with this sample:

Meteorites come in several varieties, including rocky and metallic versions. It is very satisfying that when measurements of these meteorites’ lead isotopes are added to the plot above, they all fall exactly where our understanding of lead isotope production would have them: at the start of each of these model evolution curves. Each lead isotope system tells the same answer for the age of the Earth, acting like three independent witnesses corroborating one another’s testimony. And the answer they all give is 4.6 billion years ago (4.6 Ga). That’s what 208 Pb says. That’s what 207 Pb says. And that’s what 206 Pb says. They all agree, and they agree with the predicted curves based on terrestrial (Earth rock) measurements. This agreement gives us great confidence in this number. The Earth, and meteorites (former asteroids), and the solar system of which they are all a part, began about 4.6 billion years ago…

…But what came before that?

The implications of meteorites

In 1969, a meteorite fell through Earth’s atmosphere and broke up over Mexico. A great many pieces of this meteorite were recovered and made available for scientific analysis. It turned out to be a carbonaceous chondrite, the largest of its kind ever documented. It was named the Allende ( “eye-YEN-day” ) meteorite, for the tiny Chihuahuan village closest to the center of the area over which its fragments were scattered.

One of the materials making up Allende’s chondrules was the calcium feldspar called anorthite. Anorthite is an extraordinarily common mineral in Earth’s crust, but the Allende anorthite was different. For some reason, it has a large amount of magnesium in it. When geochemists determined what kind of magnesium this was, they were surprised to find that it was mostly 26 Mg, an uncommon isotope. The abundances of 25 Mg and 24 Mg were found to be about the same level as Earth rocks, but 26 Mg was elevated by about 1.3%.  And after all, magnesium doesn’t even “belong” in a feldspar. The chemical formula of anorthite is CaAl 2 Si 2 O 8 – there’s no “Mg” spot in there. Why was this odd 26 Mg in this chondritic anorthite?

One way to make 26 Mg is the break-down of radioactive 26 Al. The problem with this idea is that there is no 26 Al around today . It’s an example of an extinct isotope: an atom of aluminum so unstable that it falls apart extremely rapidly. The half-life is only 717,000 years. But because these chondrules condensed in the earliest days of the solar system, there may well have been plenty of 26 Al around at that point for them to incorporate. And Al, of course, is a key part of anorthite’s Ca Al 2 Si 2 O 8 crystal structure.

So the idea is that weird extra 26 Mg in the chondrule’s anorthite could be explained by suggesting it wasn’t always 26 Mg: Instead, it started off as 26 Al ,and it belonged in that crystal’s structure. However, over a short amount of time, it all fell apart, and that left the 26 Mg behind to mark where it had once been. If this interpretation is true, it has shocking implications for the story of our solar system.

To understand why, we first need to ask, what came before the nebula? What was the ‘pre-nebula’ situation? Where did the nebula come from, anyhow?

It turns out that nebulae are generated when old stars of a certain size explode.

These explosions are called supernovae (the plural of supernova). The “nova” part of the name comes from the fact that they are very bright in the night sky – an indication of how energetic the explosion is. They look like “new” stars to the casual observer. Supernovae occur when a star has exhausted its supply of lightweight fuel, and it runs out of small atoms that can be fused together under normal conditions. The outward-directed force ceases, and gravitationally-driven inward-directed forces suddenly dominate, collapsing the star in upon itself. This jacks up the pressures to unbelievably high levels, and is responsible for the nuclear fusion of big atoms – every atom heavier than iron is made instantaneously in the fires of the supernova.

That suite of freshly-minted atoms included a bunch of unstable isotopes, including 26 Al.

And here’s the kicker: If the 26 Al was made in a supernova, started decaying immediately, and yet enough was around that a significant portion of it could be woven into the Allende chondrules’ anorthite, that implies a very short amount of time between the obliteration of our Sun’s predecessor, and the first moments of our own. Specifically, the 717,000 year half-life of 26 Al suggests that this “transition between solar systems” played out in less than 5 million years, conceivably in only 2 million years.

That is very, very quickly.

In summary, the planet Earth is part of a solar system centered on the Sun. This solar system, with its star, its classical planets, its dwarf planets, and its “leftover” comets and asteroids, formed from a nebula full of elements in the form of gas and dust. Over time, these many very small pieces stuck together to make bigger concentrations of mass, eventually culminating in a star and a bunch of planets that orbit it. Asteroids (and asteroids that fall to Earth, called meteorites), are leftovers from this process. The starting nebula itself formed from the destruction of a previous star that had exploded in a supernova. The transition from the pre-Sun star to our solar system took place shockingly rapidly.

Further reading

Marcia Bjornerud’s book Reading the Rocks . Basic Books, 2005: 226 pages.

Jennifer A. Johnson (2019), “ Populating the periodic table: Nucleosynthesis of the elements ,” Science. 01 Feb 2019 : 474-478.

Lee, T., D. A. Papanastassiou, and G. J. Wasserburg (1976), Demonstration of 26 Mg excess in Allende and evidence for 26 Al , Geophysical Research Letters , 3(1), 41-44.

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• 1 In the beginning…
• 2 Nebular theory
• 3 A star is born
• 4 Age of the solar system
• 5 The implications of meteorites
• 7 Further reading

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Article contents

The formation and evolution of the solar system.

• Mikhail Marov Mikhail Marov International Astronomical Union
• https://doi.org/10.1093/acrefore/9780190647926.013.2
• Published online: 24 May 2018

The formation and evolution of our solar system (and planetary systems around other stars) are among the most challenging and intriguing fields of modern science. As the product of a long history of cosmic matter evolution, this important branch of astrophysics is referred to as stellar-planetary cosmogony. Interdisciplinary by way of its content, it is based on fundamental theoretical concepts and available observational data on the processes of star formation. Modern observational data on stellar evolution, disc formation, and the discovery of extrasolar planets, as well as mechanical and cosmochemical properties of the solar system, place important constraints on the different scenarios developed, each supporting the basic cosmogony concept (as rooted in the Kant-Laplace hypothesis). Basically, the sequence of events includes fragmentation of an original interstellar molecular cloud, emergence of a primordial nebula, and accretion of a protoplanetary gas-dust disk around a parent star, followed by disk instability and break-up into primary solid bodies (planetesimals) and their collisional interactions, eventually forming a planet.

Recent decades have seen major advances in the field, due to in-depth theoretical and experimental studies. Such advances have clarified a new scenario, which largely supports simultaneous stellar-planetary formation. Here, the collapse of a protosolar nebula’s inner core gives rise to fusion ignition and star birth with an accretion disc left behind: its continuing evolution resulting ultimately in protoplanets and planetary formation. Astronomical observations have allowed us to resolve in great detail the turbulent structure of gas-dust disks and their dynamics in regard to solar system origin. Indeed radio isotope dating of chondrite meteorite samples has charted the age and the chronology of key processes in the formation of the solar system. Significant progress also has been made in the theoretical study and computer modeling of protoplanetary accretion disk thermal regimes; evaporation/condensation of primordial particles depending on their radial distance, mechanisms of clustering, collisions, and dynamics. However, these breakthroughs are yet insufficient to resolve many problems intrinsically related to planetary cosmogony. Significant new questions also have been posed, which require answers. Of great importance are questions on how contemporary natural conditions appeared on solar system planets: specifically, why the three neighbor inner planets—Earth, Venus, and Mars—reveal different evolutionary paths.

• solar system
• thermal regime
• dust particles
• planetesimals

Introduction

In the recent decades great progress has been achieved in the study of our closest space environment—the solar system. Space exploration jointly with the advanced ground-based astronomical observations dramatically expanded knowledge about our star—the Sun and all eight major planets with their numerous satellites and rings, as well as about countless minor bodies—asteroids, meteoroids, and comets and interplanetary space surrounding the Sun—the heliosphere. We knew a lot about the nature of these bodies with implication to the basic ideas of fundamental scientific value concerning the solar system formation and evolution. The discovery of circumstellar discs and especially planetary systems around other stars put this challenging problem of modern astronomy on new ground and allowed us to integrate different theoretical views alongside the data of observations and computer modeling to more coherent concepts. This is one of the most intriguing branches of astrophysics that used to be referred to as planetary cosmogony (Marov, 2015 ). Being multidisciplinary by its essence, it stands at the frontiers of natural science involving mathematics, physics, and chemistry with close relevance to biology when addressing the problem of life origin and proliferation.

Planets formation is a widespread although very complex process, believed to be the succession of several stages affected by different mechanisms of physical interactions, chemical transformations, and numerous perturbations in the gas-dust disk. Scenarios and model approach to the origin of protoplanetary nebulae and evolution are generally backed by observational data. The mechanical, astrophysical, and cosmochemical characteristics of the solar system serve as the starting concept for the formation of planets around stars. The solar system planets and satellites architecture, as well as existing patterns in the systems of extrasolar planets definitely point to a unified process of every system formation though with different constraints. Data available on surface properties and matter composition for the solar system planets when confronting the samples of material from their embryos (small bodies) and “debris” (meteorites) provide an insight into the probable sources, paths, and chronology of this process.

It is generally accepted that like other planetary systems, our solar system formed from an original molecular cloud (protosolar cloud) consisting mostly of hydrogen and helium with a rather small admixture of heavier elements. The process started with the collapse of some fragment of a huge molecular cloud. A major part of its mass concentrated in the center, forming protosun while the rest flattened out into a compressed gas-dust disk, the whole system keeping rotation owing to conservation of angular momentum. In the follow-up process of the disc continuing evolution, the planets with their satellites and swarm of asteroids and comets emerged, which ultimately constituted the solar system family. The lab data on the meteoritic minerals formed during the condensation of chemical elements as well as remelting of the condensed phases allow us to judge the thermodynamic conditions in the circumsolar disk and, in turn, serve as the most important cosmochemical constraints imposed on the numerous analytical and computer models being developed.

Basic Topics: Understanding and Context

Historical highlights.

The first attempts to understand how the planets have born and solar system structured were undertaken in the Middle Ages. In the 16th century , Italian monk, doctor of theology, and author Giordano Bruno voiced against the church dogma that Earth is center of the World, arguing instead for a configuration of the solar system with Earth orbiting the Sun. But the truth is never free, and it is often necessary to pay a high price for personal conviction, sometimes with one’s life. This is what happened to Giordano Bruno: For this proclaiming of the truth, he was sentenced by inquisition to be burned on a fire. Nicolas Copernicus, who revolutionized the World system concept, had a more fortunate fate, and we refer to his concept as the real breakthrough in astronomy and philosophy in general. Immanuel Kant, father of the German classic philosophy, in 1755 published the book General Natural History and Theory of the Sky based on a hypothesis put forward in 1749 by Sweden mystic author Emmanuel Swedenborg who suggested that stars are formed in the eddy motions of space nebula matter. Kant hypothesized that planets set up of a dusty cloud that he associated with original Chaos. Famous French mathematician Pier Simon Laplace independently put forward a nearly analogous idea and gave mathematical support to it. Basically, these ideas were preserved until now and underlie the principal concepts of the solar system origin.

Indeed, the hypotheses of Kant and Laplace put forward in the 18th century about the simultaneous formation of the Sun and the protoplanetary cloud, along with the idea of rotational instability responsible for the successive separation of plane concentric rings from the cloud periphery, underlie the current views. The solar system is currently believed to have formed 4.567 billion years ago through the gravitational collapse of a dense fragment (core) of an interstellar molecular cloud with a density > 10 −20 gcm −3 , a temperature T~5–30K, a mass larger than the solar one by 10–30%, and a dust mass fraction of ~1% (see, e.g., Cassen, 1994 ; Cassen & Summers, 1984 ). It is also believed that after the central compressed core of the cloud collapses giving birth to the central star, material from the outer cloud regions continues to accrete onto the disk, causing strong turbulization of the gas-dust medium due to the difference between specific angular momentum of the falling matter and the disk particulate matter involved in the azimuthal (Keplerian) rotation. Observations backed the starting concept that a certain part of the material from the parent cloud (nebula), with an appreciable angular momentum, remains in orbit around the central clump and is incorporated into the protoplanetary disk in the process of stellar collapse. Concurrently, disk matter continues to accrete on the protostar during 1–5 Ma (Myr) and during this time the mass flow decreases by two–three orders of magnitude, while the overall process of first solid bodies formation and eventually their growing to planets take another 10–100 Ma (see Dorofeeva & Makalkin, 2004 ; Lissauer & de Pater, 2013 ; Safronov, 1969 ).

Schematic view of the solar system formation from a collapsed fragment of molecular cloud following by the formation of the proto-Sun and protoplanetary disk, its breakup into individual ring clumps of solid particles giving birth to planetesimals, and ultimately planets through collisional interactions are shown in Figure 1a . A more detailed diagram of the protoplanetary nebula evolution according to Otto Schmidt (Schmidt, 1957 ) who referred to the pioneering ideas about fragmentation of a primordial dust layer including critical wavelength and mass (Gurevich & Lebedinsky, 1950 ) is shown in Figure 1b .

Figure 1a. A basic concept of the origin of the solar system. Scheme for the formation of the solar system, from the collapse of a molecular cloud fragment through the formation of the proto-Sun and protoplanetary disk (1,2), followed by its breakup into individual ring clumps of solid particles, eventually giving birth to planetesimals (3,4). Continuing collisional interactions of planetesimals ultimately leads to the formation of planets (5). Adapted from Wikipedia.

Figure 1b. A basic concept of the origin of the solar system. Evolution of the protoplanetary nebula according to O. Schmidt. Left side: Sequence of transformations of the original gas-dust disk in blobs growing into rocks and coalescing in clumps of planetesimals. The time span is approximately 10 4 –10 5 years. Right side: These embryos of planets continue to grow through mutual collisions, eventually to become protoplanets and ultimately a planetary system, here attributed to the solar system. The time span is about 10 8 years.

It involves the sequence of transformations of the original gas-dust disk in clumps due to growing instability and formation of planetesimals in mutual collisions. These basic ideas were later developed by several authors, forming the key publication (Goldreich & Ward, 1973 ; Safronov, 1969 ).

Important Constraints

When discussing the problem of the solar system origin, we first of all address some of its obvious mechanical and cosmochemical features serving as prerequisites and placing important constraints on the developed scenarios:

All planets orbit the Sun in the same prograde (anticlockwise when looking from the North World Pole) direction, in coincidence with the Sun’s intrinsic rotation around its axis. The orbits are nearly circular and have a very small inclination to the ecliptic—the imaginary plane containing the Earth’s circumsolar orbit. Similarly, all planets (except Venus and Uranus) rotate in prograde direction and the same is true for the majority of their satellites, which argues that planetary systems formed in a unified process from the same original disk matter. Satellites are locked in resonance with the planet’s intrinsic rotation and therefore they face the planet on the same side, similar to our Moon. The outermost satellites orbiting giant planets behave more randomly, exhibiting both prograde and retrograde orbits and rotations, and they are regarded as small bodies captured later on by the planet’s gravity field.

There is a peculiar mass and angular momentum distribution in the solar system: While the Sun comprises 99.8% of the whole solar system mass, the planets comprise nearly 98% of its angular momentum. Basically, this resulted from the process of disk evolution and planets formation, though it is not yet clear how the angular momentum redistribution in early solar system history has occurred.

There is similar cosmic abundance of non-volatile chemical elements in the Sun and most primitive meteorites (carbonaceous chondrites), which are viewed as original pristine substance partially inherited from the protosolar nebula and mostly lost. There is some evidence that inner planets were formed of the matter resembling that of chondrites meteorite composition and experienced dramatic transformations in the course of evolution, while gaseous-icy giant planets preserved their chemical composition essentially unmodified since the origin while the phase compositions have definitely changed as planets grew.

There exists an obvious correlation of the planetary bulk composition with their distance from the Sun (with a small exemption for Uranus and Neptune), in support of the condensation theory that favors the emergence of different substances from the hot gas disk depending on radial temperature distribution and thus, on the distance from the Sun. Indeed, the theory of condensation, postulating the successive emergence of high temperature and low temperature condensates from the protoplanetary disk matter depending on radial distance from the Sun, may be recognized invoking some geochemical and dynamical constraints. This fractionation is believed to be responsible for the rocky inner planets close to the Sun and gaseous-icy outer planets farther away, that is, rocky composition of the terrestrial planets containing many refractory elements/compounds and the mostly gaseous and icy composition of giant planets.

The composition of asteroids in the main asteroids belt between Mars and Jupiter orbits is intermediate between the silicate/metal rich inner planets and the volatile rich outer planets, which also brings support to the condensation theory and dynamical exchange. In turn, comets are mainly composed of water ice and other frozen volatiles, and these bodies retain the most pristine matter from which the solar system formed. Migration and collisional processes throughout the solar system history and matter transport appear to play the crucial role in the subsequent planet’s evolution. Surfaces of the terrestrial planets have been scarred by asteroidal and cometary impacts and painted with a veneer of volatiles and organic compounds made of potentially life-forming elements that under certain conditions transformed into a biological infestation, at least on Earth.

Discovery of circumstellar protoplanetary gas-dust discs and extrasolar planets became the great milestone in the advancement of planetary cosmogony. Structure and composition of disks and different configurations of the exoplanetary systems placed important constraints on the origin of protosolar nebula and various scenarios of the planetary system evolution and, based on this onslaught, fueled refining theories and computer modeling of the solar system origin on the comparative approach.

Cosmochemistry and Chronology of Evolution

Of primary importance is an opportunity of getting insight into the chronology of the key physical and chemical mechanisms responsible for the early solar system evolution. Study of meteorites is the main tool of cosmochemistry aimed to reconstruct the processes of matter origin and transformations in the protoplanetary disc and forming bodies.

The time sequence was established based on the measurements of ratios of long- and short-lived isotopes and products of their decay in meteoritic materials. The main isotopic systems used in the study were U, Th-Pb, Sm-Nd, Al-Mg, Mn-Cr, Rb-Sr, I-Xe, Hf-W. Many undifferentiated meteorites (chondrites) contain the refractory inclusions of microns to cm in size enriched in refractory elements such as Al and Ca ( C alcium A luminum I nclusions or CAIs). They were assumed to belong to the ancient solid material that condensed out near the Sun ( r < 0.5 AU) at Т ~2000–1700 K (Grossman, Ebel, & Simon, 2002 ; MacPherson, 2005 ; Meibom et al., 2007 ). These objects, including some ultra-refractory mineral nodules (Ivanova, Krot, Nagashima, & MacPherson, 2012 ; Ivanova, Lorenz, Krot, & MacPherson, 2015 ), enabled a determination of the absolute age of the solar system. The measured values vary from 4567.1 ± 0.1 Ma to 4568.67 ± 0.17 Ma (Amelin et al., 2010 ; Bouvier & Wadhwa, 2009 ; Shukolyukov & Lugmair, 2003 ), with the most reliable being 4567.30 ± 0.16 Ma (Connelly et al., 2012 ). Thus, the time of the solar system origin is determined with accuracy of better than ~1 Ma, or 0.02%. Concurrently, the absolute age of iron and stony meteorites of different petrological classes was defined from 1 to a few Ma younger CAI (McSween & Huss, 2010 ). Let us note that the submillimeter chondrules ( spherules ) embedded in stony meteorites and composed of ferromagnesian silicates are dated in the range from 4567.32 ± 0.42 to 4564.71 ± 0.30 Ma indicating an age gap between CAIs and chondrules with implication that chondrules formation lasted ~3 Ma. This time scale is in accord with protoplanetary disk lifetimes inferred from astronomical observations.

One may assume that during a few million years, interval accumulation and thermal evolution (differentiation) of the parent bodies of these ancient meteorites occurred. Provisionally the first primordial parent bodies of ~100 km in size formed in the very first few million years since the solar system origin. Such a size was sufficient for the body to experience differentiation due to intense heating by the short-lived 26 Al and 60 Fe nuclides with an iron core emergence. The subsequent core and silicate shell fragmentation caused by numerous collisions have been probably responsible for the existing iron and stony meteorites. Otherwise their existence is difficult to explain, in contrast to non-differentiated chondrites that experienced no melting by the exhausted short-lived isotopes heat source.

The above time scale is in accord with the results of computer modeling that argue that accretion of matter from the disk on the protosun terminated in 1–2.5 Ma after the system formation. The dust subdisk composed presumably of 1–10 cm particles is believed to form much earlier, in 0.01–0.1 Ma at radial distance r ~1 AU. Here critical density was achieved and gravitational instability developed. Evidently, this time was sufficient for accumulation and thermal evolution of the first solid bodies. Assuming that mass of the protoplanetary disc M d was ~0.1 M S and that with account for the disc partial dissipation ~0.1 M d ultimately entered the planets, we may estimate ~10 10 of ~100 km original bodies were born in the first ~2 Ma. This idea is in accord with the models favoring distribution of asteroids from the initial generation of planetesimals of nearly similar size on which chondrules have been presumably accreted (Bottke, Nesvorny, Grimm, Morbidelli, & O’Brien, 2006 ; Morbidelli, Bottke, Nesvorny, & Levison, 2009 ; Matsumoto, Oschino, Hasegawa, & Wakita, 2017 ).

Further Advancement and Current State

We shall now discuss the state of the art in our views on principal mechanisms of the solar system origin. In modern astronomy, the key consists of high resolution images and spectral features of objects relevant to planets formation at the different stages of evolution. In computer modeling, the focus is given to the theoretical treatment and development of robust models and effective algorithms allowing us to get insight into genesis of planetary system origin from primordial matter of the outer space involving disk formation, its radial/vertical compression, and dust distribution/size grow affecting the disc structure. Unfortunately, unlike protoplanetary accretion disks whose structure and evolution are accessible to astronomical observations, the mechanism of primary solid bodies set up in the gas-dust disk and their growing to planetary embryos remains rather speculative because cannot be yet tested experimentally. Hence, computer modeling is essentially the only tool to reconstruct the multiple processes involved with the use of observational constraints for models verification.

Computer and Lab Modeling

The matter of the protoplanetary gas-dust disk is a complex system of the different phase composition, densities, temperatures, and degrees of ionization, which vary with radial distance. Basically, it is an inhomogeneous medium composed of gas and dust particles of various sizes and origin. This matter, which is generally magnetized dusty plasma, is in a state of turbulence depending on the radial and azimuthal position of a parcel of matter (Marov & Kolesnichenko, 2013 ).When the main dynamical forces controlling the rotating disc flattening (gravitational and centrifugal) are in balance, weaker factors, such as the thermal/viscous processes, turbulence, and electromagnetic phenomena dominate the disk’s evolution. They certainly affect the condensation of volatiles, including first of all water, and bear significant effect on the relative content and abundance of gaseous species and solid particles, as well as disk energetic and angular momentum transport.

When the plasma effects are disregarded, the motion of a disk medium containing dust suspended in gas can be modeled most adequately within the framework of mechanics of heterogeneous turbulized media with allowance made for the physical-chemical properties of the phases, heat and mass transport, incident radiation/opacity changes, viscosity variations, chemical reactions, phase transitions, coagulation, fragmentation, etc. The rigorous mathematical treatment of the problem is presented in Marov and Kolesnichenko ( 2013 ). Specifically, it is focused on the dynamical interaction of turbulized gas and dust including modification of the turbulence energy of the carrier phase by solid particles (i.e., the reverse effect of the dust component on the turbulent and thermal regimes of the disk gas component); the influence of turbulence on the rates of phase transitions (evaporation, condensation); on the jump-like disperse particle accumulation processes such as coagulation and fragmentation during mutual collisions between particles in the mass flow; and, finally, on the settling of solid particles through the gas to the disk midplane, where they form a flattened dust layer—a geometrically thin subdisk.

Obviously, the presence of polydisperse (different particles’ size) admixture in a turbulized medium complicates significantly the disk hydrodynamics, contributing to the realization of additional regimes of cosmic matter flow. Note that the synergetic collective self-organization processes in the thermodynamically open system of the protoplanetary disk against the background of a large-scale shear flow of cosmic matter associated with its differential rotation are regarded as very important mechanisms shaping the properties of a viscous accretion disk at various stages of its evolution (Kolesnichenko & Marov, 2006b ; Nakagawa, Sekiya, & Hayashi, 1986 ).

Whatever is the character of events under consideration, it is clear that complex physical and chemical processes accompanying evolution of the heterogeneous medium where dust particles collisions domain, are responsible for the first solids origin and planetesimal’s formation. The developed models include the sequence of changes in the aggregate state of the main protoplanetary matter components; the location of the condensation-evaporation fronts depending on the thermodynamic parameters of the disk; the role of particle sublimation and coagulation in the two-phase medium with the account for particle size distribution; the relative contribution of radiation and turbulence to the heat and mass transport; and the mechanisms for the development of streaming and gravitational instabilities with allowance made for the shear stresses in boundary layers and polydispersed, suspended dust particles (see, e.g., Armitage, 2007 ; Marov & Kolesnichenko, 2013 ).

In the most comprehensive approach, a continuum model of heterogeneous disk medium should take into account the joint influence of MHD effects and turbulence on the dynamics and heat and mass transport processes in differentially rotating matter with allowance made for the inertial properties of the polydispersed admixture of solid particles, coagulation, radiation, and changing partitioning of elements between gaseous and condensed phases. Turbulence generated at the boundaries of the protoplanetary disk layers and caused by shear flows corresponds in character to the parameters of a boundary (Ekman) layer and significantly affects the disc dynamics including the Kelvin-Helmholtz instability. It is important to emphasize that generation and maintenance of shear turbulence at various evolutionary stages of the disk involves a two-phase (gas-dust) medium with a differential angular velocity of rotation, different relative contents of dust particles, their size distribution, and coagulation processes. In general, a heterogenic mechanics approach should be applied to account for the emergence of coherent order against the background of random motions in large-scale turbulent structures. Also, the evolution of turbulence in the rotating accretion disk is supposed to be influenced by hydrodynamic helicity responsible for the cascade process of the inverse energy transfer from small to large eddies and negative viscosity appearance in the medium (Kolesnichenko & Marov, 2007 ).

Currently, a numerical solution of the bulk of problems with allowance made for the heterogeneity of a turbulent medium, radiation, diffusion, chemistry, and MHD effects is hardly possible and only limited approaches are feasible. Note that because terrestrial planets form close to the Sun, the focus in modeling is specially narrowed to the poorly resolvable inner disk regions within several astronomical units, where matter actively accretes onto the young star. This results in the dust/gas ratio, optical opacity, and the thermal regime changes, as well as the significant contribution of photochemical processes in transformation of the matter composition and transfer.

Particles Growth and Solid Bodies Formation

The key problem of the solar system origin is how the solar system bodies (original dust and condensates, as well as those produced in coagulation) progressively grew on scales ranging from nano- and micron-size particles to planetesimals and planets thus ranging over dozens orders of magnitude in mass. As mentioned above, time-dependent modeling with the different particle size distribution functions and limited lab experiments is the only tool to gain insight into the problem. Numerous attempts to reconstruct the process taking into account nebula thermal structure; evaporation fronts (EFs’) position for different components depending on radial temperature lapse rates in the disc’s midplane; particle growth by sticking limited by bouncing; fragmentation and radial drift due to nebula headwind drag of growing bodies influencing mass redistribution; etc., have been undertaken (see, e.g., Birnstiel, Dullemon, & Brauer, 2010 ; Dominik, Blum, Cuzzi, & Wurm, 2007 ; Estrada, Cuzzi, & Morgan, 2016 ; Nakagawa, Sekiya, & Hayashi, 1986 ; Ormel, Spaans, & Tielens, 2007 ; Wada, Tanaka, Suyama, Kimura, & Yamamoto, 2008 , 2009 ; Weidenschilling, 1980 ). Also, lab experiments with silicate and ice particles collisions in microgravity conditions revealed some important patterns of particles and particle aggregates formation. Some important constraints were deduced for particles bouncing and sticking, including translational energy and coefficients of restitution estimates depending on impact parameters (see, e.g., Beitz et al., 2011 ; Blum, 2004 ; Blum, Schrapler, Davidson, & Trigo-Rodriguez, 2006 ; Brisset et al., 2013 ; Chiang & Youdin, 2010 ; Güttler, Blum, Zsom, Ormel, & Dullemond, 2010 ; Hill, Heißelmann, Blum, & Fraser, 2015 ; Ida, Guilot, & Morbidelli, 2016 ; Lankowski, Teiser, & Blum, 2008 ; Schrapler, Blum, Seizinger, & Kley, 2012 ; Weidling, Güttler Blum, & Brauer, 2009 ; Weidling, Güttler, & Hium, 2011 ).

According to the modern views, the process started owing to the above-mentioned streaming/gravitational instability development in the dense subdisk formed out of the dust component settled down to the midplane of the gas-dust disk (Dorofeeva & Makalkin, 2004 ; Kolesnichenko & Marov, 2014 ; Makalkin & Dorofeeva, 1996 ; Marov et al., 2013 ; Youdin & Shu, 2002 ). This was followed by the primary porous dust clusters formation from which the first solid bodies of pebble-boulder size and eventually planetesimals of asteroid size have emerged. Some of these cm-size and larger pebbles were possibly assembled into porous clumps giving birth to comets; this approach accommodates the primordial rubble pile theory though other theory postulates that comets were made out of debris left over from the main planet-building phase, thus preserving remnants of the protosolar nebula matter. Nonetheless, collisional rubble pile theory better explains a rather small size of comet’s nucleus (~5–10 km) and even the bi-lobed shape of some of them, because of their gradual formation at low speed of leftover pebbles/grains after larger bodies have accumulated and gas in disc has disappeared, followed by gentle collisions owing to stirring the cometary orbits, specifically at the skirt of planetary system.

The sequence of solids growth and timescale of such a scenario is shown schematically in Figure 11 ;

Figure 11. Sequence of the of protosolar disk evolution. (a) Disk formation due to accretion gas and dust from the protosolar nebula and protosun emergence in the center. (b) Disk flatteniand dust particle sedimentation toward the midplane and dense dust subdisc formation; particle growth. (c) Subdisc fragmentation into dust clusters due to streaming and gravity instability development, cluster collisional interaction and solid growth, including first proto-planetesimal accumulation with gravity domain. (d) Planetesimals and planetary embryo formation, gas dissipation, and original solar system architecture setup evolving ultimately to the contemporary configuration. Time span: (a)–(b) 10 5 –10 6 yr; (c)–(d) 10 6 –10 7 yr.

the initial process is thought to last less than 10 5 –10 6 years of the overall ~10 8 years of planets formation. Note that the same short timescale is assumed for comets’ formation as well as for initial growth phase of the TNOs probably influencing the cometary-like bodies origin and evolution. The shear turbulence mechanism would promote ring-like contraction of dust in protoplanetary cloud into a thin disc ( h << r ) of non-regular shape widening toward the periphery. Solid bodies becoming planet embryos are formed from initially “loose” gas-dust (porous) clumps filling the main part of their sphere of attraction (Hill’s sphere) and slowly contracting due to internal gravitational forces. Let us recall that whereas the growth of particles during their collisions is hampered in chaotic turbulence, their coalescence and enlargement can occur inside turbulent eddies’ coherent structures promoting dust clusters set up in vortexes of baroclinic nature in a broad range of Stokes (St) number defined by the ratio of particle drag in an ambient gas to characteristic time of the system (defined as inverse Kepler frequency Ω ‎ −1 ) and dependent on size and density of a particle. Solid particles may be also concentrated in the disks with quite weak turbulence where denser than average matter structures (with large dust-to-gas ratio) would create a large population of aggregates and trigger streaming instability. This could be the case in outer zone of massive discs where rapid growth of aggregates to planetesimals was suggested (Krijt, Ormel, Dominik, & Tielens, 2016 ).

Generally, both instability mechanisms support the basic Safronov and Goldreich-Ward ideas about disc viscous accretion and subdisk matter gravitational collapse. As we have seen, in the modern models, the focus is also given to the competing processes of gas and dust photo-evaporation by the solar EUV-X-ray radiation and gas condensation/condesate growth, as well as to dust trapping, clustering, and coagulation including particles size distribution and stratified turbulence. These processes are accompanied by dust aggregates formation/restructuring through particles sticking, gas-dust coupling decrease, solids growth, and radial drift resulting ultimately in gravitationally bound planetesimals and planet embryos origin (Armitage, 2007 ; Birnstiel, Fang, & Johansen, 2016 ; Carrera, Gorti, Johansen, & Davies, 2017 ; Johansen et al., 2014 ; Marov & Kolesnichenko, 2013 ; Raettig, Klahr, & Lyra, 2015 ; Schaefer, Yang, & Johansen, 2017 ).

Although the basic scenario of ongoing particle growth is generally understood, many details of the processes involved at the different stages remain unclear. First, still uncertain are the details of physical mechanisms responsible for primary small dust particles growth before gravitational interactions of hundred meters-kilometer size bodies become effective. The process of particles’ mutual collisions is usually invoked as the factor that presumably gave rise to the aggregation of small particles to yield either dense particle clumps (Carrera, Johansen, & Davies, 2015 ) or pebble/boulder-size compact aggregates migrating in the protoplanetary disc and controlling planetesimals’ growth (Güttler, Blum, Zsom, Ormel, & Dullemond, 2010 ; Ida, Guilot, & Morbidelli, 2016 ; Krijt, Ormel, Dominik, & Tielens, 2016 ; Nakagawa, Nakazawa, & Hayashi, 1981 ; Zsom, Ormel, Güttler, Blum, & Dullemond, 2010 ; Ormel, Spaans, & Tielens, 2007 ). However, a probability of destruction rather than growth of forming bodies may be higher if collisional relative velocities and bouncing barriers for particle aggregates are significant, until gravitational attraction of planetesimals becomes dominant. Hence, an efficiency of sticking of dust particles into aggregates through collisions is a rather delicate mechanism, specifically if equal size particles of ~mm size are considered (Blum & Wurm, 2000 ). The formation of larger objects limited by bouncing is thought to be augmented by the streaming instability, gravitational collapse, collective particle behavior, and/or by static compression of fluffy dust aggregates (Kataoka, Tanaka, Okuzumi, & Wada, 2013 ; Ward, 2000 ; Yang, Johansen, & Carrera, 2016 ).

As a compromise ensuring higher efficiency of agglomerates creation we advocate for collisional integration of fluffy clusters made up by micron-sized particles rather than individual particles themselves, in support of the lab experiments mentioned above. It is further assumed in the developed models that clusters and their individual particles have fractal structure. Such an approach was thoroughly explored and validated theoretically (Kolesnichenko & Marov, 2013 ) involving concurrence of gravitational and brownian coagulation of dust monomers, aggregates growth, and porous bodies interaction/growth.

The bottom line of our numerical model is the agglomeration of small particles in the primary fluffy dust aggregates of low-bulk density and fractal nature of the latter, giving rise to their compression and formation of first dense bodies of progressively larger size. Unlike an approach used in the previous simulations (e.g., Dominik, Blum, Cuzzi, & Wurm, 2007 ; Wada, Tanaka, Suyama, Kimura, & Yamamoto, 2008 , 2009 ), the mode of particle on-head and offset collisions inside dust clusters was more strictly assessed in numerical N-body models (Marov & Rusol, 2011 , 2015a , 2015b ). Method of permeable particles and modified Newton model of collisions were utilized in terms of restitution coefficient Kr dependent on the distance between centers of particles and their relative velocity, taking into account internal structure of particles in fluffy clusters and complicated patterns of their interactions, specifically in the contact zone. This enabled us to examine collisional evolution of fluffy clusters beginning from nano-particles sticking together through electrostatic forces and growing up to compressed aggregates of larger size and different patterns. Example of 3D modeling of evolution of fractal dusty clusters with different numbers of populated submicron particles is shown in Figures 12 and 13 .

Figure 12. 3D-numerical modeling of submicron particles, collisional interaction, and growing within a grid in the primordial dusty cloud (density d = 1.5 × 10 ‒21 g/nm 3 . (a) An example of the original 3D structure of a selected grid. (b) Examples of agglomerate formation in the process of collisional evolution of particles in some grids.

Figure 13. 3D structure of fractal dusty clusters with different numbers of populated particles evolution (a–d) Red and blue colors denote positively and negatively charged particles of about 20-nm characteristic size in a quasi-neutral medium.

Figure 14 is a snapshot of a cross-section of randomly selected grids for fractal dusty clusters with different numbers of populated particles in Figure 13 .

Figure 14. Snapshot of a cross section of a randomly selected grid when getting through fractal dusty clusters with different numbers of populated particles (clusters a and b, respectively).

Of special interest is the evolution of fluffy dust clusters in mutual collisions that is critical for dusty agglomerates formation, as shown in Figure 15 .

Figure 15. Evolution of the structures of fluffy dust clusters; computer modeling of collisional interactions. (a) Fluffy clusters of characteristic size 50 nm, fractal dimension 2.55, and number of particles = 8,192. (b) Fluffy clusters of characteristic size 75 nm, fractal dimension 2.15, and number of particles = 3,072.

Examples of the respective numerical experiments carried out with clusters composed of various number of submicron particles of different fractal dimensions D β ‎ ranging from 2.025 (very fluffy structure) to 2.975 (well-packed structure) are shown in Figure 16 (Marov & Rusol, 2016 , in press ). The results of modeling of cluster interactions reveals how K r (restitution coefficient or its equivalent—collision recovery ratio) changes for clusters of different D β ‎ depending on the relative distance—between colliding i-j particles (0–1), and collision energy E ij /E int ( E ij is kinetic energy of collision; E int —the internal energy of molecular coupling preserving structure) ranging from 0.01 to 0.99. The strong dependence of K r on the parameters involved and are bound threshold were found, for example, for dense colliding clusters with relative velocity less than the critical value(corresponding to the energy of plastic deformation)bouncing occurs at K r > 0.87.

Figure 16. Restitution coefficient K r (collision recovery ratio) for clusters with various fractal dimensions D β as shown here. Note: r i , r j — radii of i and j model particles; r ij — distance between i and j colliding model particles; r col = r i +r j — collision distance; r i j r c o l — relative distance between i and j colliding particles; E i n t — cluster’s internal energy; E i j — kinetic energy of collision; E i j E i n t — relative collision energy.

The results are regarded as an important milestone toward in-depth study of “cluster-cluster coagulation” at the initial stage of the protoplanetary gas-dust disc evolution and results are addressed as precursors of primary objects growing. They are in accord with the conclusion (Wada et al., 2008 ) that a cluster with a larger number of particles is harder to destroy (easier to survive) in energetic collisions. Nonetheless, the problem is still far from being solved, especially if one keeps in mind that the global qualitative effect of disk gravity further increases collision/impact velocities and adds additional jitter to the orbital evolution of the primary bodies.

The next stage of growth of primordial seeds of bodies to planetesimals, with the allowance for mechanism of self-gravitation, seems more conceivable. At the first approximation, the mass distribution of particle agglomerates and proto-planetesimals obeys the known Smoluhovsky coagulation equation (a sort of the Boltzmann kinetic equation when dealing with coagulation process) with the account for gravity mutual attraction and fragmentation in non-gentle collisions. This interaction along with the dust sedimentation onto the formed bodies residing on intersected orbits (with chaotic velocities superimposed on quasi-circular orbital velocities) results in further growth of these protoplanetary embryos followed by the gradual scooping up of smaller bodies in due course of the swarm evolution. It is supposed, however, that there is a sort of barrier at around one meter size-range (see, e.g., Nakagawa, Hayashi, & Nakazawa, 1983 ; Weidenschilling, 1977 ; Youdin & Kenyon, 2012 ) because of the effects of body destruction and their inward drift toward the central star. The latter is caused by aerodynamic braking of these bodies in the remaining gas whose rotation velocity becomes lower than the Keplerian one. Primordial bodies beyond the meter-size barrier to proto-planetesimals growth through “coagulation” after more complete gas evacuation seems more feasible.

Obviously, competitive processes of destruction and particle trapping during radial matter exchange, along with particle concentration via streaming and/or gravitational instability, eventually results in bodies’ growth. Based on the results of modeling supported by lab simulation (Jansson, Johansen, Syed, & Blum, 2017 ) millimeter- to centimeter-size pebbles in a gravitationally bound collapsed cloud (due to, for example, disk streaming instability) would experience numerous collisions with different outcomes depending on their relative speeds and the cloud density, resulting eventually in a quite intense formation of planetesimals. As far as a planet’s accumulation is concerned, gravitationally focused mutual collisions of asteroids and planetary embryos are thought to be responsible for the emergence of progressively growing proto-planets possessing larger gravitational potentials (oligarchs), such an oligarchic growth being accompanied by catastrophic fragmentation of other bodies in a swarm of planetesimals (Chambers, 2008 ; Greenberg, Hartman, Chapman, & Wacker, 1978 ; Weidenschilling, 2010 ;Youdin & Kenyon, 2012 ). A system of satellites around a proto-planet could be explained invoking the same mechanism (Canup & Esposito, 1996 ).

A possibility of the formation of bodies from dust clusters in mass range from 10 20 to 10 22 g (corresponding to asteroids of tens to hundred kilometers in size) was shown in the numerical model (Marov et al., 2013 ). A mechanism of gravitational collapse of massive swarms of small particles and their pile-up without passing through the intermediate phases of growing was proposed to explain early formation of the ~100 km bodies, a source of very old iron meteorites, thereby circumventing the meter-size barrier problem (Cuzzi, Hogan, & Shariff, 2008 ; Johansen et al., 2007 ; Morbidelli, Bottke, Nesvorny, & Levison, 2009 ). Similarly, a prompt planetesimal’s formation in regions beyond the snow line due to streaming instability-induced gravity collapse of radially drifting millimeter-size dust particles to ensure material deposition for the giant planet cores was suggested (Armitage, Eisner, & Simon, 2016 ). Anyway, whatever particular solution of the problem, further growth occurs through interaction of planetesimals with solid and gaseous components under gravity domain and sporadic mutual collisions (Chambers, 2010 ).

Note that in the numerical experiments, some constraints were placed on the value of angular momentum of the porous (low density) gas-dust clusters (pre-planetesimals) of about Hill sphere in size and their moving before collision along heliocentric orbits. The model aimed to assist dynamical formation of planetary/satellite bodies with application to trans-Neptunian system. Arguments were drawn for consistency of the properties of wide binaries in the Kuiper Belt with a primordial origin during gravitational collapse (Nesvorny, Youdin, & Richardson, 2010 ). As the outcome, origin of the Earth-Moon system was attempted to explain in terms of the compression of a low-density cluster of 0.1 mass of the contemporary Earth formed from the collision of two original clusters on the close heliocentric orbits, which allowed the system to acquire the necessary angular momentum (Ipatov, 2014 ).

As it was said above, significant role at the final stage of planet’s growth could play migration and resonances in the forming planetary system. The commonly adopted scenario of the solar system origin involving planetesimals growing to planet embryos together with asteroid-size and comet-like bodies as remnants of planets is illustrated in Figure 17 .

Figure 17. Artist’s concept of a planetary system formation, as seen from the outer edge of the gas-dust disc around an emerged central protostar. Planetesimals growing to planet embryos, together with asteroid-size and cometlike bodies as remnants of planets’ formation, are seen. The picture resembles the commonly adopted scenario of the solar system origins.

It reflects our current general understanding of how original bodies and then planets formed though we are still far from resolving this mysterious subject. Some fundamental bottlenecks in planets formation challenging future studies were most recently summarized in Morbidelli and Raymond ( 2016 ).

Related Articles

• Accretion Processes
• Isotopic Dating
• The Orbital Architecture of Exoplanetary Systems
• Detection and Characterization Methods of Exoplanets
• Composition of Earth

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The origin of the Solar System

How did it all come together? Five major theories about the formation of the Solar System.

How did the Sun, planets and moons in the Solar System form? There is a surprising amount of debate and several strong and competing theories, but do scientists have an answer?

What are the theories for the origin of the Solar System?

Any theory about how the Solar System came to be has to account for certain, rather tricky facts. We know that the Sun sits at the centre of the Solar System with the planets in orbit around it, but these throws up five major problems:

• The Sun spins slowly, and only has 1 percent of the angular momentum of the Solar System - but 99.9 percent of its mass. Why is this?
• Terrestrial planets have solid cores - how did they form?
• What about the gas giant planets like Jupiter - were they formed differently? 
• How did planetary satellites like the Moon come into being?
• Bode's law states that the distances of the planets from the Sun follow a simple arithmetic progression. Why should this be?

Taking all these issues into account, science has suggested five key theories considered to be 'reasonable' in that they explain many (but not all) of the phenomena exhibited by the Solar System. Find out more below.

The Accretion theory

The Sun passes through a dense interstellar cloud and emerges surrounded by a dusty, gaseous envelope.

The problem is that of getting the cloud to form the planets. The terrestrial planets can form in a reasonable time, but the gaseous planets take far too long to form. The theory does not explain satellites or Bode's law and is therefore considered the weakest of those described here.

When is the next lunar eclipse?

The Protoplanet theory

A dense interstellar cloud produces a cluster of stars. Dense regions in the cloud form and coalesce; as the small blobs have random spins the resulting stars will have low rotation rates. The planets are smaller blobs captured by the star.

The small blobs would have higher rotation than is seen in the planets of the Solar System, but the theory accounts for this by having the 'planetary blobs' split into planets and satellites. However, it is not clear how the planets came to be confined to a plane or why their rotations are in the same sense.

The Capture theory

The Sun interacts with a nearby protostar, dragging a filament of material from the protostar. The low rotation speed of the Sun is explained as being due to its formation before the planets, the terrestrial planets are explained by collisions between the protoplanets close to the Sun, and the giant planets and their satellites are explained as condensations in the drawn out filament.

What was the bright object I saw in the sky last night?

The Modern Laplacian theory

French astronomer and mathematician Pierre-Simon Laplace first suggested in 1796 that the Sun and the planets formed in a rotating nebula which cooled and collapsed. The theory argued that this nebula condensed into rings, which eventually formed the planets and a central mass - the Sun. The slow spin of the Sun could not be explained.

The modern version assumes that the central condensation contains solid dust grains which create drag in the gas as the centre condenses. Eventually, after the core has been slowed, its temperature rises and the dust evaporates. The slowly rotating core becomes the Sun. The planets form from the faster rotating cloud.

The Modern Nebular theory

The planets originate in a dense disk formed from material in the gas and dust cloud that collapses to give us the Sun. The density of this disk had to be sufficient to allow the formation of the planets and yet be thin enough for the residual matter to be blown away by the Sun as its energy output increased.

In 1992 the Hubble Space Telescope obtained the first images of proto-planetary disks in the Orion nebula. They are roughly on the same scale as the Solar System and lend strong support to this theory.

There have been many attempts to develop theories for the origin of the Solar System. None of them can be described as totally satisfactory. We do believe, however, that we understand the overall mechanism.

The Sun and the planets formed from the contraction of part of a gas/dust cloud under its own gravitational pull and that the small net rotation of the cloud created a disk around the central condensation. The central condensation eventually formed the Sun, while small condensations in the disk formed the planets and their satellites. The energy from the young Sun blew away the remaining gas and dust, leaving the Solar System as we see it today.

Unique Space-inspired gifts

Explore space from the comfort of home. Introducing our accessible astronomy guides.

• Ask An Astrobiologist
• Resources Graphic Histories Coloring Pages Heroes Posters Life in the extremes Digital Backgrounds SciComm Guild

1. How did matter come together to make planets and life in the first place?

1.2. how did our solar system form.

Grades K-2 or Adult Naive Learner

• NGSS Connections for Teachers
• Concept Boundaries for Scientists

Do you know what a planet is? A planet is a big, round world, floating in space. It can be made mostly of rock or even mostly of gas, just like the air all around us.

You, me, and everyone we know lives on a planet called Earth. Our planet is in space and goes around the Sun. Now, did you know that the Sun is a star? Well, there are also seven other planets going around our star, the Sun. The Sun and the planets are part of what we call the Solar System.

The Solar System is really old. The Sun and all of the planets came from a big cloud of stuff in space. Do you know that raindrops come from clouds in the sky? Well, it turns out that stars and even planets can come from clouds in space. Our Sun came from the middle of a big cloud in space, and the planets of our solar system also formed from that same cloud, moving around the Sun in the same kind of pattern that they follow today.

Disciplinary Core Ideas

ESS1.C: The History of Planet Earth: Some events happen very quickly; others occur very slowly, over a time period much longer than one can observe. (2-ESS1-1)

PS3.B: Conservation of Energy and Energy Transfer: Sunlight warms Earth’s surface. (K-PS3-1, K-PS3-2)

Crosscutting Concepts

Patterns in the natural world can be observed, used to describe phenomena, and used as evidence. (1-ESS1-1, 1-ESS1-2)

Big Ideas: The solar system consists of Earth and seven other planets all spinning around the Sun. Planets are big, round worlds floating in space. The Earth is a planet that goes around a much larger star called the Sun. The Sun and planets formed from a big cloud of gas and dust. The Earth, moon, Sun and planets all move in a pattern called an orbit.

Boundaries: By the end of 2nd grade, seasonal patterns of Sunrise and Sunset can be observed, described and predicted. Temperature (i.e. the Sun warms Earth) is limited to relative measurements such as warmer/cooler. (K-PS3-1)

K-5 The Science of the Sun. In this unit, students focus on the Sun as the center of our solar system and as the source for all energy on Earth. By beginning with what the Sun is and how Earth relates to it in size and distance, students gain a perspective of how powerful the Sun is compared to things we have here on Earth, and the small fraction of its energy we receive. Students also gain an understanding of how Earth relates to the other planets in the solar system. The Sun as a Star (page 17) Students identify the sun as a star. The Scale of Things (page 27). Students explore the scale of the solar system. The Size of Things (page 33) Students describe the relative sizes of the planets in the solar system by making a play-doh model. What is a year (page 37) Students act out the motion of Earth as it travels (revolves) around the Sun. Goddard Space Flight Center/NASA. https://sdo.gsfc.nasa.gov/assets/docs/UnitPlanElementary.pdf

2-12 Toilet Paper Solar System. Even in our own “cosmic neighborhood,” distances in space are so vast they are difficult to imagine. In this activity, participants build a scale model of the distances in the solar system using a roll of toilet paper. https://astrosociety.org/file_download/inline/cfdf9b2c-5947-4c19-9a23-a790ac3c7ae0

Grades 3-5 or Adult Emerging Learner

For us to learn about where we came from, we need to understand how our solar system formed.

The Sun and the planets and all of the asteroids and comets and other stuff in our solar system all formed from a really big cloud of gas and dust in space. There are clouds of gas and dust all around our galaxy. Sometimes these clouds can slowly turn into stars and planets when enough material is available and clumps together forming massive collections of ice and rock.

Do you know what kind of pattern the planets make when they go around the Sun? It kind of looks like a big circle, right? Well, when the planets were first forming from that cloud in space, the cloud itself was spinning in the same way, with the Sun forming in the middle. That’s why we see the planets moving around the Sun the way that they do today! We call that pattern of how a planet moves around the Sun an “orbit.” Have you heard of anything else that has an “orbit”? Our Moon orbits around our Earth, just like our Earth orbits around our Sun, and our entire solar system is also orbiting around the galaxy. Orbits are really important for us to learn about if we want to know where we came from.

ESS1.C: The History of Planet Earth: Local, regional, and global patterns of rock formations reveal changes over time due to earth forces, such as earthquakes. The presence and location of certain fossil types indicate the order in which rock layers were formed. (4-ESS1-1)

PS1.A: Structure and Properties of Matter: Matter of any type can be subdivided into particles that are too small to see, but even then the matter still exists and can be detected by other means. (5-PS1-1)

PS2.B: Types of Interactions: Objects in contact exert forces on each other. (3-PS2-1) The gravitational force of Earth acting on an object near Earth’s surface pulls that object toward the planet’s center. (5-PS2-1)

Patterns can be used as evidence to support an explanation. (4-ESS1-1, 4-ESS2-2) *Science assumes consistent patterns in natural systems. (4-ESS1-1)

Big Ideas: The Solar system formed through condensation from a big cloud of gas and dust. The solar system consists of Earth and seven other planets all orbiting around the Sun. The Sun, moon, and planets all move in predictable patterns called orbits. Many of these orbits are observable from Earth. The entire solar system orbits around the Milky Way galaxy.

Boundaries: In this grade band, students are learning about the different positions of the Sun, moon, and stars as observable from Earth at different times of the day, month, and year. Students are not yet defining the unseen particles or explaining the atomic-scale mechanism of condensation.

3-5 SpaceMath Problem 543: Timeline for Planet Formation. Students calculate time intervals in millions and billions of years from a timeline of events [Topics: time calculations; integers] https://spacemath.gsfc.nasa.gov/Grade35/10Page6.pdf

3-5 SpaceMath Problem 541: How to Build a Planet. Students study planet growth by using a clay model of planetessimals combining to form a planet by investigating volume addition with spheres. [Topics: graphing; counting] https://spacemath.gsfc.nasa.gov/Grade35/10Page4.pdf

3-5, 6-8, 9-12 Marsbound! In this NGSS aligned activity (three 45-minute sessions), students in grades become NASA project managers and design their own NASA mission to Mars. Mars is significant in astrobiology and more needs to be learned about this planet and its potential for life. Students create a mission that must balance the return of science data with mission limitations such as power, mass and budget. Risk factors play a role and add to the excitement in this interactive mission planning activity. Arizona State University/NASA. http://marsed.asu.edu/lesson_plans/marsbound

3-5 or 6-8 Strange New Planet. This 5E hands-on lesson (2-3 hours) engages students in how scientists gain information from looking at things from different perspectives. Students gain knowledge about simulated planetary surfaces through a variety of missions such as Earth-based telescopes to landed missions. They learn the importance of remote sensing techniques for exploration and observation. NASA /Arizona State University. http://marsed.asu.edu/strange-new-planet

4-8 SpaceMath Problem 300: Does Anybody Really Know What Time It Is? Students use tabulated data for the number of days in a year from 900 million years ago to the present, to estimate the rate at which an Earth day has changed using a linear model. [Topics: graphing; finding slopes; forecasting] https://spacemath.gsfc.nasa.gov/earth/6Page58.pdf

4-12 Meet the Planets. In this activity, kids identify the planets in the solar system, observe and describe their characteristics and features, and build a scale model out of everyday materials. They are also introduced to moons, comets, and asteroids. (Finding life Beyond Earth, page 13) NOVA . https://d43fweuh3sg51.cloudfront.net/media/assets/wgbh/nvfl/nvfl_doc_collection/nvfl_doc_collection.pdf

5-12 Exploring Meteorite Mysteries: The Meteorite Asteroid Connection (4.1). In this lesson, students build an exact-scale model of the inner solar system; the scale allows the model to fit within a normal classroom and also allows the representation of Earth to be visible without magnification. Students chart where most asteroids are, compared to the Earth, and see that a few asteroids come close to the Earth. Students see that the solar system is mostly empty space unlike the way it appears on most charts and maps. NASA . https://er.jsc.nasa.gov/seh/Exploring_Meteorite_Mysteries.pdf

5-12 Exploring Meteorite Mysteries: Building Blocks of Planets (10.1). Chondrites are the most primitive type of rock available for study. The chondrules that make up chondrites are considered the building blocks of planets. In this lesson, students experiment with balloons and static electricity to illustrate the theories about how dust particles collected into larger clusters. Students also manipulate magnetic marbles and steel balls to illustrate the accretion of chondritic material into larger bodies like planets and asteroids. NASA . https://er.jsc.nasa.gov/seh/Exploring_Meteorite_Mysteries.pdf

5-12 Exploring Meteorite Mysteries: Exploration Proposal (17.1). Exploration of the outer Solar System provides clues to the beginnings of the solar system. This is a group-participation simulation based on the premise that water and other resources from the asteroid belt are required for deep space exploration. Students brainstorm or investigate to identify useful resources, including water, that might be found on an asteroid. NASA . https://er.jsc.nasa.gov/seh/Exploring_Meteorite_Mysteries.pdf

5-12 Big Explosions and Strong Gravity. In this one-two day activity, students work in groups to examine the crushing ability of gravity, equilibrium, and a model for the creation of heavy elements through a supernova. This active lesson helps students visualize the variation and life cycle of stars. NASA http://imagine.gsfc.nasa.gov/educators/programs/bigexplosions/activities/supernova_demos.html

Grades 6-8 or Adult Building Learner

Earth is the only world that we know of that has life. All of the plants and animals and microbes and other living things on Earth have evolved here. So, for us to understand where life as we know it came from, we need to understand where our planet came from.

The Sun and the planets and all of the other stuff in our solar system all formed from a really big cloud of gas and dust in space. We call such a cloud a “nebula” and more than one of them we refer to as “nebulae.” There are nebulae all around our galaxy, and it’s from these nebulae that stars and planets form. Nebulae are massive clouds of dust and debris in space and have all the ingredients to form stars and planets. When enough material is available, it begins to stick together forming a large mass. In time, the mass can grow large enough to form a planet or even a new star.

We currently think that our solar system formed from a large nebula, perhaps after the explosion of a nearby star. Some big stars can explode, something called a supernova, and that explosion has enough energy to make the gas and dust in nearby nebulae start swirling and spinning about. As this happened, it caused a lot of the material in the nebula to fall into its center, and that’s where the Sun started forming. Meanwhile, the rest of the gas and dust in the nebula began colliding and sticking together, making little pieces of metal and rock. Those small pieces then collided with each other, forming larger pieces, which then collided with each other to form even larger ones. These were young planets, and eventually, over a long time and through many, many collisions, our eight planets were formed – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

We call the pattern that the planets make when they go around the Sun an “orbit.” Well, when the planets were first forming from that cloud in space, the cloud itself was spinning in the same direction as the orbits of the planets today, with the Sun forming in the middle and also spinning in the same direction. That’s why we see the planets moving around the Sun the way that they do today!

You might also know that the Moon orbits around Earth. For something to be a moon, it needs to be in orbit around a planet. One thing that makes a planet is that a planet has to be orbiting a star. But star systems also have orbits. They orbit around their entire galaxy. So, orbits are really important for us to learn about if we want to know where we came from.

ESS1.A: The Universe and Its Stars: - Patterns of the apparent motion of the Sun, the Moon, and stars in the sky can be observed, described, predicted, and explained with models. (MS-ESS1-1) - Earth and its solar system are part of the Milky Way galaxy, which is one of many galaxies in the universe. (MS-ESS1-2)

ESS1.B: Earth and the Solar System: - The solar system consists of the Sun and a collection of objects, including planets, their moons, and asteroids that are held in orbit around the Sun by its gravitational pull on them. (MS-ESS1-2, MS-ESS1-3) - This model of the solar system can explain eclipses of the Sun and the Moon. Earth’s spin axis is fixed in direction over the short-term but tilted relative to its orbit around the Sun. The seasons are a result of that tilt and are caused by the differential intensity of sunlight on different areas of Earth across the year. (MS-ESS1-1) - The solar system appears to have formed from a disk of dust and gas, drawn together by gravity. (MS-ESS1-2)

PS1.A: Structure and Properties of Matter: All substances are made from some 100 different types of atoms, which combine with one another in various ways. Atoms form molecules that range in size from two to thousands of atoms. Pure substances are made from a single type of atom or molecule; each pure substance has characteristic physical and chemical properties that can be used to identify it. (MS-PS1-1)

Cause and effect relationships may be used to predict phenomena in natural or designed systems. (MS-PS1-4)

Big Ideas: Condensation causes rain drops to form inside of clouds, and sometimes can cause entire star systems to form inside of clouds. The Solar system formed through condensation from big clouds of gas and dust called nebulae after a supernova, or the explosion of a large star. Planets move around the Sun in an orbit, and the Solar system orbits around the entire galaxy.

Boundaries: Emphasis is on gravity as the force that holds together the solar system and Milky Way galaxy and controls orbital motions within them. (MS-ESS1-2) Does not include Kepler’s Laws of orbital motion or the apparent retrograde motion of the planets as viewed from Earth. (MS-ESS1-2)

6-8 SpaceMath Problem 542: The Late Heavy Bombardment Era. Students estimate the average arrival time of large asteroids that impacted the moon. They work with the formula for the volume of a sphere to estimate how much additional mass was added to the moon and Earth during this era. [Topics: volume of spheres; proportions] https://spacemath.gsfc.nasa.gov/earth/10Page5.pdf

6-8 SpaceMath Problem 60: When is a planet not a planet? In 2003, Dr. Michael Brown and his colleagues at CalTech discovered an object nearly 30% larger than Pluto, which is designated as 2003UB313. Is 2003UB313 really a planet? In this activity, students examine this topic by surveying various internet resources that attempt to define the astronomical term ‘planet’. [Topics: non-mathematical essay; reading to be informed] https://spacemath.gsfc.nasa.gov/astrob/2page17.pdf

6-8 SpaceMath Problem 59: Getting A Round in the Solar System! How big does a body have to be before it becomes round? In this activity, students examine images of asteroids and planetary moons to determine the critical size for an object to become round under the action of its own gravitational field. [Topics: data analysis; decimals; ratios; graphing] https://spacemath.gsfc.nasa.gov/astrob/2page20.pdf

6-8 Explore! Jupiter’s Family Secrets. This one-hour lesson for formal or informal education settings has students connecting their own life story to a cultural creation story and then to the “life” story of Jupiter, including the Big Bang as the beginning of the universe, the creation of elements through stars and the creation of the solar system. JPL /NASA. http://www.lpi.usra.edu/education/explore/solar_system/activities/birthday/

6-9 Rising Stargirls Teaching and Activity Handbook. 1.2. Art & the Cosmic Connection: (page 19). This activity engages students in space and science education by becoming explorers. Using the elements of art: line, color, texture, shape, and value: students learn to analyze the mysterious surfaces of our rocky celestial neighbors; planets, moons, comets and asteroids, as well as the Earth. Name That Planet (page 25) Students communicate their knowledge about the solar system using different modes of communication—visual, verbal, and kinesthetic. Distance Calculation (page 27) Students calculate the distances between planets using a unit of measurement that is personal to them—themselves! Rising Stargirls activities fuse science and the arts to create enlightened future scientists and imaginative thinkers. Rising Stargirls. https://static1.squarespace.com/static/54d01d6be4b07f8719d7f29e/t/5748c58ec2ea517f705c7cc6/1464386959806/Rising_Stargirls_Teaching_Handbook.compressed.pdf

6-12 Science Fiction Stories with Good Astronomy & Physics: A Topical List: Cosmology. 1.2. The Astronomical Society of the Pacific created this list of short stories and novels that use more or less accurate science and can be used for teaching or reinforcing astronomy or physics concepts including the origin of the universe. https://astrosociety.org/file_download/inline/621a63fc-04d5-4794-8d2b-38e7195056e9

6-12 Where are the Small Worlds? Through an immersive digital experience (1-2 hours), students use a simulation/model of the solar system in order to investigate small worlds in order to learn more about the solar system and its origin. The experience can be standalone or has options to track student tasks or modify the simulation as needed by the teacher. Arizona State University. https://infiniscope.org/lesson/where-are-the-small-worlds/

6-12 Astrobiology Math. This collection of math problems provides an authentic glimpse of modern astrobiology science and engineering issues, often involving actual research data. Students explore concepts in astrobiology through calculations. Relevant topics include Habitable Zones and Stellar Luminosity (page 57) and Ice or Water? (page 49). NASA . https://www.nasa.gov/pdf/637832main_Astrobiology_Math.pdf

6-12 Pocket Solar System. This activity involves making a simple model to give students an overview of the distances between the orbits of the planets and other objects in our solar system. It is also a good tool for reviewing fractions. https://astrosociety.org/file_download/inline/5c27818a-e947-46ad-a9dc-f4af157af7d8

6-12 Origins: The Universe. In this web interactive, scientists use a giant eye in the southern sky to unravel how galaxies are born. Video, pictures, and print weave information for the learner as they more deeply understand the scientific pursuit of astrobiology. UW-Madison. https://origins.wisc.edu/

7-9 SpaceMath Problem 8: Making a Model Planet. Students use the formula for a sphere, and the concept of density, to make a mathematical model of a planet based on its mass, radius and the density of several possible materials (ice, silicate rock, iron, basalt). [Topics: volume of sphere; mass = density x volume; decimal math; scientific notation] https://spacemath.gsfc.nasa.gov/astrob/Week14.pdf

Grades 9-12 or Adult Sophisticated Learner

As the physical context for life as we know it, it is important to learn about Earth’s origins so we can understand life’s origins. Although life may exist in situations other than that of a planet orbiting a star, it makes sense to explore the phenomenon of planetary system formation as a context for the emergence and evolution of life.

The story of the formation of our solar system begins in a region of space of called a “giant molecular cloud”. You might have heard before that a cloud of gas and dust in space is also called a “nebula,” so the scientific theory for how stars and planets form from molecular clouds is also sometimes called the Nebular Theory. Nebular Theory tells us that a process known as “gravitational contraction” occurred, causing parts of the cloud to clump together, which would allow for the Sun and planets to form from it.

Before gravitational contraction, the majority of the material within the giant molecular cloud that formed our solar system consisted of hydrogen and helium produced at the time of the big bang, with small amounts of heavier elements such as carbon and oxygen which were made via nucleosynthesis in prior generations of stars (see 1.1 above). The material in this giant cloud was not uniformly distributed – there were regions of higher density (more dust and gas within a specific volume of space) and regions of lower density (less gas and dust within that same volume).

Evidence from meteorites suggests that the energy produced by a nearby exploding star (a supernova) passed through a higher density region in the cloud and caused it to begin to swirl and twist about. This area of the cloud is sometimes called the pre-solar nebula (“pre” = before; “solar” = star or Sun). As molecules in the pre-solar nebula were swirling about, some of them started bumping into each other and sometimes would even stick together. As more and more of these clumps formed, gravity caused them to start sticking together and to fall into the center of the pre-solar nebula, which only caused gravity to pull even more of the material into the center of the cloud, and this is the process that’s referred to as gravitational contraction.

While all of this was happening, the action of molecules bumping into each other over and over slowly caused the pre-solar nebula to flatten into a spinning disk of dust and gas. This is sometimes called a circumstellar disk (“circum” = around; “stellar” = star) or protoplanetary disk (“proto” = first or before). Almost all of the material in the disk collected in the center, giving rise to the young Sun. However, some of the particles in the spinning disk began colliding with each other and sticking together, forming larger and larger fragments. The larger a fragment became, the more mass it had and therefore the more gravitational pull it exerted. Which in turn drew more and more material to it, and the larger it became, and so on. This process is called “accretion,” and resulted in the production of many planetesimals (small objects that build up into planets), and eventually, the planets themselves.

While the young Sun was starting to heat up in the middle of the protoplanetary disk, it warmed up the disk so much that nothing could stay solid really close to the Sun (it all melted). A little further out from the Sun, stuff like metal and rock was able to cool enough to make solid materials for forming the planets. But it was still so hot there that molecules that are often liquids or gases here on Earth (like water, ammonia, carbon dioxide and methane) couldn’t easily stick to the solid planet-forming materials. Those molecules could only really be added to planets that were a lot further from the Sun, where it was cold enough for them to clump together with the other solid stuff. This is why we have gas giant planets like Jupiter and Saturn which are very different from the rocky planets like Earth and Venus.

ESS1.A: The universe and its Stars: Nearly all observable matter in the universe is hydrogen or helium, which formed in the first minutes after the big bang. Elements other than these remnants of the big bang continue to form within the cores of stars. (HS-ESS1-2) *Nuclear fusion within stars produces all atomic nuclei lighter than and including iron, and the process releases the energy seen as starlight. Heavier elements are produced when certain massive stars achieve a supernova stage and explode. (HS-ESS1-2, HS-ESS1-3) *Stars go through a sequence of developmental stages — they are formed; evolve in size, mass, and brightness; and eventually burn out. Material from earlier stars that exploded as supernovas is recycled to form younger stars and their planetary systems.

ESS1.B: Earth and the Solar System: Kepler’s laws describe common features of the motions of orbiting objects, including their elliptical paths around the Sun. (HS-ESS1-4) *The solar system consists of the Sun and a collection of objects of varying sizes and conditions — including planets and their moons — that are held in orbit around the Sun by its gravitational pull on them. This system appears to have formed from a disk of dust and gas, drawn together by gravity.

PS1.C: Nuclear Processes: Nuclear processes, including fusion, fission, and radioactive decays of unstable nuclei, involve release or absorption of energy. The total number of neutrons plus protons does not change in any nuclear process. (HS-PS1-8)

Scientific knowledge is based on the assumption that natural laws operate today as they did in the past and they will continue to doe so in the future (HS-ESS1-2). Science assumes the universe is a vast single system in which basic laws are consistent. (HS-ESS1-2)

Big Ideas: The phenomenon of planetary system formation serves as a context for the emergence and evolution of life. A cloud of gas and dust in space is called a “nebula”. The Nebular Theory is the scientific theory for how stars and planets form from molecular clouds and their own gravity. The majority of the material within the giant molecular cloud that formed our solar system consisted of hydrogen and helium produced at the time of the big bang. Nuclear fusion within stars forms heavier elements under extreme pressure and temperature. The larger the star, the heavier the elements that can be produced through fusion and Supernova. Heavier elements were also made via nucleosynthesis. The circumstellar disk gave rise to the young Sun.

Boundaries: Emphasis is on the way nucleosynthesis, and therefore the different elements created, varies as a function of the mass of a star and the stage of its lifetime.(HS-ESS1-3) Does not include details of the atomic and subatomic processes involved with the Sun’s nuclear fusion. (HS-ESS1-1)

9-10 Voyages through Time: Cosmic Evolution. This comprehensive integrated curriculum includes the universe, the totality of all things that exist, origins (beginning with an explosion of space and time and the expansion of a hot, dense mass of elementary particles and photons), and how it has evolved over billions of years into the stars and galaxies we observe today. Sample lesson on the website and the curriculum is available for purchase. SETI . http://www.voyagesthroughtime.org/cosmic/index.html

9-11 SpaceMath Problem 302: How to Build a Planet from the Inside Out. Students model a planet using a spherical core and shell with different densities. The goal is to create a planet of the right size, and with the correct mass using common planet building materials. [Topics: geometry; volume; scientific notation; mass=density x volume] https://spacemath.gsfc.nasa.gov/astrob/6Page72.pdf

9-12 Genesis Science Modules: Cosmic Chemistry: Planetary Diversity. The goal of this module is to acquaint students with the planets of the solar system and some current models for their origin and evolution. The lessons in the Genesis Science Modules challenge students to look for patterns in data, to generate observations, and critically analyze where the data does not fit with the current nebular model. This mini-unit reveals the essence of scientific research and argument within the context of the formation of solar systems. JPL /NASA http://genesismission.jpl.nasa.gov/educate/scimodule/PlanetaryDiversity/index.html

9-12 A101 Slide Set: From Supernovae to Planets. This slide set explains the discoveries of the SOFIA mission and the implications of the new data explaining how supernovae and dust push planet formation and how this is the physical context for life. SOFIA /NASA https://slideplayer.com/slide/8679314/ Teacher’s Guide:

https://www.astrosociety.org/edu/higher-ed/files/A101ss.SOFIA_SupernovaePlanets.v3.pdf

11-12 SpaceMath Problem 305: From Asteroids to Planets. Students explore how long it takes to form a small planet from a collection of asteroids in a planet-forming disk of matter orbiting a star based on a very simple physical model. [Topics: integral calculus] https://spacemath.gsfc.nasa.gov/astrob/6Page82.pdf

11-12 SpaceMath Problem 304: From Dust Balls to Asteroids. Students calculate how long it takes to form an asteroid-sized body using a simple differential equation based on a very simple physical model. [Topics: integral calculus] https://spacemath.gsfc.nasa.gov/astrob/6Page81.pdf

11-12 SpaceMath Problem 303: From Dust Grains to Dust Balls. Students create a model of how dust grains grow to centimeter-sized dust balls as part of forming a planet based on a very simple physical model. [Topics: integral calculus] https://spacemath.gsfc.nasa.gov/astrob/6Page80.pdf

Storyline Extensions

The planets are named after stories from long ago:.

Our planets are named Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Seven of the planets are named after gods from Roman mythology. These are Mercury, Venus, Mars, Jupiter, Saturn, and Neptune. However, Uranus is a name from Greek mythology (Uranus was the god of the sky). Also, the name for our planet, Earth, comes from Old English, and appears to have come from people who lived in Northern Europe long ago.

Our location in the galaxy:

Our Milky Way galaxy is really big! If we could travel outside of the galaxy and look back at it, it would look like a big disk of dust and gas and stars, with a big bulging sphere of stars near the middle. The disk of the galaxy is about 100,000 lightyears in diameter. That means that it takes light about 100,000 years to travel from one side to the other. Our little solar system (little in comparison to the galaxy, that is) lies about 30,000 lightyears from the center of the galaxy. Just as moons orbit around planets, and planets orbit around stars, star systems also orbit around the center of the galaxy. Our own solar system is traveling through the galaxy at over 500,000 miles per hour! And our very long orbit around the galaxy takes almost 250 million years! But we’re not alone out here. There are lots of other stars and other worlds in the galaxy. Our best estimates right now are that there are about 100-400 billion stars in the Milky Way. And, even though we’ve only just begun finding exoplanets, some astronomers believe there is evidence for more planets than stars in the milky way and other galaxies. That’s an awful lot of worlds!

The sun was born when a dense gas cloud collapsed, 4.6 billion years ago

Associate Professor, Department of Earth Sciences, Carleton University

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Hanika Rizo receives funding from NSERC.

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While the upcoming total solar eclipse is a special moment to reflect on our place in the universe, scientists have been studying the birth of the sun and the formation of our solar system for a long time.

Our solar system today is mainly composed of a central star — the sun — along with an inner solar system with rocky planets, and an outer solar system with gas and ice giant planets. However, it hasn’t always been that way.

How was the sun formed?

Our solar system formed from the gravitational collapse of a “dense” giant molecular cloud of gas and dust, composed mainly of hydrogen, a bit of helium, and about one per cent of heavier elements. After the cloud collapsed, the majority of the mass concentrated onto the centre, creating our sun.

The star continued to contract until it reached its final size and density. Hydrogen fusion ignited the sun’s core, causing the star to emit light and heat.

Around the sun, the leftovers — about 0.5 to one per cent of the mass of the sun — created a protoplanetary disk, where planets subsequently formed.

Protoplanetary disks in the process of making planets are not just theory — they have actually been observed, such as the disk around HL Tauri , a young star with rings and gaps that are likely signs of forming planets.

We have a pretty good idea of when that collapse took place in our solar system because we can analyze the first (or oldest) solids that condensed out from the protoplanetary disk gas. This detailed analysis is only possible in our solar system, since we cannot directly collect material from other solar systems.

These solid fragments, called calcium-aluminum rich inclusions (CAIs), have been found in some of the oldest meteorites, and age-dated to 4,567.3 million years . This is when our solar system came into being, and provides the age for the birth of our sun.

Element factories

Very dense molecular clouds can collapse due to their own gravity . However, the collapse of our protosolar nebula was likely triggered by the perturbation from the passing shock wave of an exploding massive star, called a supernova . This shock wave compressed enough of the molecular cloud to start collapsing it, and form a central star and a planetary disk around it.

The evidence for this hypothesis is found in the isotope composition of some chemical elements in pre-solar grains. Pre-solar grains are tiny silicon-carbide minerals (under a micrometre in size), and can be found in parts per million quantities in some meteorites. These pre-solar grains have isotope compositions that cannot be explained by chemical or physical processes occurring in our solar system, and are better explained by these grains forming elsewhere .

The isotope composition of pre-solar grains implies that, after the supernova, these grains travelled into space, and they got trapped into our molecular cloud, which then collapsed, keeping those grains inside the meteorites that we study today.

How much older is the sun than the Earth?

The age of 4,567 million years found for the CAIs is often used as the age of the Earth. However, after the formation of CAIs, it likely took tens to a few hundreds of millions of years for Earth to form. Although we have determined the age of our solar system very precisely, debates still persist regarding the age of our own planet Earth.

The challenge comes from the fact that the Earth is an active planet, and is very efficient at recycling and reworking its oldest rocks, resetting their geochronological information.

More than 98 per cent of the proto-Earth’s mass might have been already melded together by the time a giant impact hit the proto-Earth . That giant impact added the remaining two per cent to Earth, and also led to the formation of our moon .

The giant impact, occurring somewhere between 70 to 120 million years after the CAIs formation, could provide the best determination for the age of the Earth. Independent age estimates can also be obtained from estimating the timing of Earth’s magma ocean solidification, a consequence of the moon-forming giant impact.

Studies attempting to determine the timing of magma ocean solidification provide ages between 100 and 150 million years after the birth of the sun .

The upcoming total solar eclipse is an opportunity for everyone to appreciate the wonders of our solar system, which took about 4.6 billion years to evolve.

It is truly a cosmic coincidence that total solar eclipses can be seen on Earth : the sun happens to be about 400 times larger than the moon, which is 400 times closer than the sun.

If you were on Mars or Venus, you would not be so lucky as to witness this phenomenon!

Johanna Teske of the Carnegie Institution for Science contributed to writing this article. She is a staff scientist, and researches the compositions of exoplanets.

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14.3: Formation of the Solar System

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Learning Objectives

By the end of this section, you will be able to:

• Describe the motion, chemical, and age constraints that must be met by any theory of solar system formation
• Summarize the physical and chemical changes during the solar nebula stage of solar system formation
• Explain the formation process of the terrestrial and giant planets
• Describe the main events of the further evolution of the solar system

As we have seen, the comets, asteroids, and meteorites are surviving remnants from the processes that formed the solar system. The planets, moons, and the Sun, of course, also are the products of the formation process, although the material in them has undergone a wide range of changes. We are now ready to put together the information from all these objects to discuss what is known about the origin of the solar system.

Observational Constraints

There are certain basic properties of the planetary system that any theory of its formation must explain. These may be summarized under three categories: motion constraints, chemical constraints, and age constraints. We call them constraints because they place restrictions on our theories; unless a theory can explain the observed facts, it will not survive in the competitive marketplace of ideas that characterizes the endeavor of science. Let’s take a look at these constraints one by one.

There are many regularities to the motions in the solar system. We saw that the planets all revolve around the Sun in the same direction and approximately in the plane of the Sun’s own rotation. In addition, most of the planets rotate in the same direction as they revolve, and most of the moons also move in counterclockwise orbits (when seen from the north). With the exception of the comets and other trans-neptunian objects, the motions of the system members define a disk or Frisbee shape. Nevertheless, a full theory must also be prepared to deal with the exceptions to these trends, such as the retrograde rotation (not revolution) of Venus.

In the realm of chemistry, we saw that Jupiter and Saturn have approximately the same composition—dominated by hydrogen and helium. These are the two largest planets, with sufficient gravity to hold on to any gas present when and where they formed; thus, we might expect them to be representative of the original material out of which the solar system formed. Each of the other members of the planetary system is, to some degree, lacking in the light elements. A careful examination of the composition of solid solar-system objects shows a striking progression from the metal-rich inner planets, through those made predominantly of rocky materials, out to objects with ice-dominated compositions in the outer solar system. The comets in the Oort cloud and the trans-neptunian objects in the Kuiper belt are also icy objects, whereas the asteroids represent a transitional rocky composition with abundant dark, carbon-rich material.

As we saw in Other Worlds: An Introduction to the Solar System, this general chemical pattern can be interpreted as a temperature sequence: hot near the Sun and cooler as we move outward. The inner parts of the system are generally missing those materials that could not condense (form a solid) at the high temperatures found near the Sun. However, there are (again) important exceptions to the general pattern. For example, it is difficult to explain the presence of water on Earth and Mars if these planets formed in a region where the temperature was too hot for ice to condense, unless the ice or water was brought in later from cooler regions. The extreme example is the observation that there are polar deposits of ice on both Mercury and the Moon; these are almost certainly formed and maintained by occasional comet impacts.

As far as age is concerned, we discussed that radioactive dating demonstrates that some rocks on the surface of Earth have been present for at least 3.8 billion years, and that certain lunar samples are 4.4 billion years old. The primitive meteorites all have radioactive ages near 4.5 billion years. The age of these unaltered building blocks is considered the age of the planetary system. The similarity of the measured ages tells us that planets formed and their crusts cooled within a few tens of millions of years (at most) of the beginning of the solar system. Further, detailed examination of primitive meteorites indicates that they are made primarily from material that condensed or coagulated out of a hot gas; few identifiable fragments appear to have survived from before this hot-vapor stage 4.5 billion years ago.

The Solar Nebula

All the foregoing constraints are consistent with the general idea, introduced in Other Worlds: An Introduction to the Solar System, that the solar system formed 4.5 billion years ago out of a rotating cloud of vapor and dust—which we call the solar nebula—with an initial composition similar to that of the Sun today. As the solar nebula collapsed under its own gravity, material fell toward the center, where things became more and more concentrated and hot. Increasing temperatures in the shrinking nebula vaporized most of the solid material that was originally present.

At the same time, the collapsing nebula began to rotate faster through the conservation of angular momentum (see the Orbits and Gravity and Earth, Moon, and Sky chapters). Like a figure skater pulling her arms in to spin faster, the shrinking cloud spun more quickly as time went on. Now, think about how a round object spins. Close to the poles, the spin rate is slow, and it gets faster as you get closer to the equator. In the same way, near the poles of the nebula, where orbits were slow, the nebular material fell directly into the center. Faster moving material, on the other hand, collapsed into a flat disk revolving around the central object (Figure \(\PageIndex{1}\)). The existence of this disk-shaped rotating nebula explains the primary motions in the solar system that we discussed in the previous section. And since they formed from a rotating disk, the planets all orbit the same way.

Picture the solar nebula at the end of the collapse phase, when it was at its hottest. With no more gravitational energy (from material falling in) to heat it, most of the nebula began to cool. The material in the center, however, where it was hottest and most crowded, formed a star that maintained high temperatures in its immediate neighborhood by producing its own energy. Turbulent motions and magnetic fields within the disk can drain away angular momentum, robbing the disk material of some of its spin. This allowed some material to continue to fall into the growing star, while the rest of the disk gradually stabilized.

The temperature within the disk decreased with increasing distance from the Sun, much as the planets’ temperatures vary with position today. As the disk cooled, the gases interacted chemically to produce compounds; eventually these compounds condensed into liquid droplets or solid grains. This is similar to the process by which raindrops on Earth condense from moist air as it rises over a mountain.

Let’s look in more detail at how material condensed at different places in the maturing disk (Figure \(\PageIndex{2}\)). The first materials to form solid grains were the metals and various rock-forming silicates. As the temperature dropped, these were joined throughout much of the solar nebula by sulfur compounds and by carbon- and water-rich silicates, such as those now found abundantly among the asteroids. However, in the inner parts of the disk, the temperature never dropped low enough for such materials as ice or carbonaceous organic compounds to condense, so they were lacking on the innermost planets.

Far from the Sun, cooler temperatures allowed the oxygen to combine with hydrogen and condense in the form of water (H 2 O) ice. Beyond the orbit of Saturn, carbon and nitrogen combined with hydrogen to make ices such as methane (CH 4 ) and ammonia (NH 3 ). This sequence of events explains the basic chemical composition differences among various regions of the solar system.

Example \(\PageIndex{1}\): rotation of the solar nebula

We can use the concept of angular momentum to trace the evolution of the collapsing solar nebula. The angular momentum of an object is proportional to the square of its size (diameter) divided by its period of rotation (\(D^2/P\)). If angular momentum is conserved, then any change in the size of a nebula must be compensated for by a proportional change in period, in order to keep \(D^2/P\) constant. Suppose the solar nebula began with a diameter of 10,000 AU and a rotation period of 1 million years. What is its rotation period when it has shrunk to the size of Pluto’s orbit, which Appendix F tells us has a radius of about 40 AU?

We are given that the final diameter of the solar nebula is about 80 AU. Noting the initial state before the collapse and the final state at Pluto’s orbit, then

\[\frac{P_{\text{final}}}{P_{\text{initial}}}= \left( \frac{D_{\text{final}}}{D_{\text{initial}}} \right)^2= \left( \frac{80}{10,000} \right)^2=(0.008)^2=0.000064 \nonumber\]

With \(P_{\text{initial}}\) equal to 1,000,000 years, \(P_{\text{final}}\), the new rotation period, is 64 years. This is a lot shorter than the actual time Pluto takes to go around the Sun, but it gives you a sense of the kind of speeding up the conservation of angular momentum can produce. As we noted earlier, other mechanisms helped the material in the disk lose angular momentum before the planets fully formed.

Exercise \(\PageIndex{1}\)

What would the rotation period of the nebula in our example be when it had shrunk to the size of Jupiter’s orbit?

The period of the rotating nebula is inversely proportional to \(D^2\). As we have just seen, \(\frac{P_{\text{final}}}{P_{\text{initial}}} = \left( \frac{D_{\text{final}}}{D_{\text{initial}}} \right)^2\). Initially, we have \(P_{\text{initial}} = 106 yr and \(D_{\text{initial}}\) = 104 AU. Then, if \(D_{\text{final}}\) is in AU, \(P_{\text{final}}\) (in years) is given by \(P_{\text{final}}=0.01D^2_{\text{final}}\). If Jupiter’s orbit has a radius of 5.2 AU, then the diameter is 10.4 AU. The period is then 1.08 years.

Formation of the Terrestrial Planets

The grains that condensed in the solar nebula rather quickly joined into larger and larger chunks, until most of the solid material was in the form of planetesimals, chunks a few kilometers to a few tens of kilometers in diameter. Some planetesimals still survive today as comets and asteroids. Others have left their imprint on the cratered surfaces of many of the worlds we studied in earlier chapters. A substantial step up in size is required, however, to go from planetesimal to planet.

Some planetesimals were large enough to attract their neighbors gravitationally and thus to grow by the process called accretion . While the intermediate steps are not well understood, ultimately several dozen centers of accretion seem to have grown in the inner solar system. Each of these attracted surrounding planetesimals until it had acquired a mass similar to that of Mercury or Mars. At this stage, we may think of these objects as protoplanets —“not quite ready for prime time” planets.

Each of these protoplanets continued to grow by the accretion of planetesimals. Every incoming planetesimal was accelerated by the gravity of the protoplanet, striking with enough energy to melt both the projectile and a part of the impact area. Soon the entire protoplanet was heated to above the melting temperature of rocks. The result was planetary differentiation , with heavier metals sinking toward the core and lighter silicates rising toward the surface. As they were heated, the inner protoplanets lost some of their more volatile constituents (the lighter gases), leaving more of the heavier elements and compounds behind.

Formation of the Giant Planets

In the outer solar system, where the available raw materials included ices as well as rocks, the protoplanets grew to be much larger, with masses ten times greater than Earth. These protoplanets of the outer solar system were so large that they were able to attract and hold the surrounding gas. As the hydrogen and helium rapidly collapsed onto their cores, the giant planets were heated by the energy of contraction. But although these giant planets got hotter than their terrestrial siblings, they were far too small to raise their central temperatures and pressures to the point where nuclear reactions could begin (and it is such reactions that give us our definition of a star). After glowing dull red for a few thousand years, the giant planets gradually cooled to their present state (Figure \(\PageIndex{3}\)).

The collapse of gas from the nebula onto the cores of the giant planets explains how these objects acquired nearly the same hydrogen-rich composition as the Sun. The process was most efficient for Jupiter and Saturn; hence, their compositions are most nearly “cosmic.” Much less gas was captured by Uranus and Neptune, which is why these two planets have compositions dominated by the icy and rocky building blocks that made up their large cores rather than by hydrogen and helium. The initial formation period ended when much of the available raw material was used up and the solar wind (the flow of atomic particles) from the young Sun blew away the remaining supply of lighter gases.

Further Evolution of the System

All the processes we have just described, from the collapse of the solar nebula to the formation of protoplanets, took place within a few million years. However, the story of the formation of the solar system was not complete at this stage; there were many planetesimals and other debris that did not initially accumulate to form the planets. What was their fate?

The comets visible to us today are merely the tip of the cosmic iceberg (if you’ll pardon the pun). Most comets are believed to be in the Oort cloud, far from the region of the planets. Additional comets and icy dwarf planets are in the Kuiper belt, which stretches beyond the orbit of Neptune. These icy pieces probably formed near the present orbits of Uranus and Neptune but were ejected from their initial orbits by the gravitational influence of the giant planets.

In the inner parts of the system, remnant planetesimals and perhaps several dozen protoplanets continued to whiz about. Over the vast span of time we are discussing, collisions among these objects were inevitable. Giant impacts at this stage probably stripped Mercury of part of its mantle and crust, reversed the rotation of Venus, and broke off part of Earth to create the Moon (all events we discussed in other chapters).

Smaller-scale impacts also added mass to the inner protoplanets. Because the gravity of the giant planets could “stir up” the orbits of the planetesimals, the material impacting on the inner protoplanets could have come from almost anywhere within the solar system. In contrast to the previous stage of accretion, therefore, this new material did not represent just a narrow range of compositions.

As a result, much of the debris striking the inner planets was ice-rich material that had condensed in the outer part of the solar nebula. As this comet-like bombardment progressed, Earth accumulated the water and various organic compounds that would later be critical to the formation of life. Mars and Venus probably also acquired abundant water and organic materials from the same source, as Mercury and the Moon are still doing to form their icy polar caps.

Gradually, as the planets swept up or ejected the remaining debris, most of the planetesimals disappeared. In two regions, however, stable orbits are possible where leftover planetesimals could avoid impacting the planets or being ejected from the system. These regions are the asteroid belt between Mars and Jupiter and the Kuiper belt beyond Neptune. The planetesimals (and their fragments) that survive in these special locations are what we now call asteroids, comets, and trans-neptunian objects.

Astronomers used to think that the solar system that emerged from this early evolution was similar to what we see today. Detailed recent studies of the orbits of the planets and asteroids, however, suggest that there were more violent events soon afterward, perhaps involving substantial changes in the orbits of Jupiter and Saturn. These two giant planets control, through their gravity, the distribution of asteroids. Working backward from our present solar system, it appears that orbital changes took place during the first few hundred million years. One consequence may have been scattering of asteroids into the inner solar system, causing the period of “heavy bombardment” recorded in the oldest lunar craters.

Key Concepts and Summary

A viable theory of solar system formation must take into account motion constraints, chemical constraints, and age constraints. Meteorites, comets, and asteroids are survivors of the solar nebula out of which the solar system formed. This nebula was the result of the collapse of an interstellar cloud of gas and dust, which contracted (conserving its angular momentum) to form our star, the Sun, surrounded by a thin, spinning disk of dust and vapor. Condensation in the disk led to the formation of planetesimals, which became the building blocks of the planets. Accretion of infalling materials heated the planets, leading to their differentiation. The giant planets were also able to attract and hold gas from the solar nebula. After a few million years of violent impacts, most of the debris was swept up or ejected, leaving only the asteroids and cometary remnants surviving to the present.

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15.2: Origin of the Solar System—The Nebular Hypothesis

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• Chris Johnson, Matthew D. Affolter, Paul Inkenbrandt, & Cam Mosher
• Salt Lake Community College via OpenGeology

Our solar system formed at the same time as our Sun as described in the nebular hypothesis. The nebular hypothesis is the idea that a spinning cloud of dust made of mostly light elements, called a nebula, flattened into a protoplanetary disk, and became a solar system consisting of a star with orbiting planets [ 12 ]. The spinning nebula collected the vast majority of material in its center, which is why the sun Accounts for over 99% of the mass in our solar system.

Planet Arrangement and Segregation

As our solar system formed, the nebular cloud of dispersed particles developed distinct temperature zones. Temperatures were very high close to the center, only allowing condensation of metals and silicate minerals with high melting points. Farther from the Sun, the temperatures were lower, allowing the condensation of lighter gaseous molecules such as methane, ammonia, carbon dioxide, and water [ 13 ]. This temperature differentiation resulted in the inner four planets of the solar system becoming rocky, and the outer four planets becoming gas giants.

Both rocky and gaseous planets have a similar growth model. Particles of dust, floating in the disc were attracted to each other by static charges and eventually, gravity. As the clumps of dust became bigger, they interacted with each other—colliding, sticking, and forming proto-planets. The planets continued to grow over the course of many thousands or millions of years, as material from the protoplanetary disc was added. Both rocky and gaseous planets started with a solid core. Rocky planets built more rock on that core, while gas planets added gas and ice. Ice giants formed later and on the furthest edges of the disc, accumulating less gas and more ice. That is why the gas-giant planets Jupiter and Saturn are composed of mostly hydrogen and helium gas, more than 90%. The ice giants Uranus and Neptune are composed of mostly methane ices and only about 20% hydrogen and helium gases.

The planetary composition of the gas giants is clearly different from the rocky planets. Their size is also dramatically different for two reasons: First, the original planetary nebula contained more gases and ices than metals and rocks. There was abundant hydrogen, carbon, oxygen, nitrogen, and less silicon and iron, giving the outer planets more building material. Second, the stronger gravitational pull of these giant planets allowed them to collect large quantities of hydrogen and helium, which could not be collected by the weaker gravity of the smaller planets.

Jupiter’s massive gravity further shaped the solar system and growth of the inner rocky planets. As the nebula started to coalesce into planets, Jupiter’s gravity accelerated the movement of nearby materials, generating destructive collisions rather than constructively gluing material together [ 14 ]. These collisions created the asteroid belt, an unfinished planet, located between Mars and Jupiter. This asteroid belt is the source of most meteorites that currently impact the Earth. Study of asteroids and meteorites help geologist to determine the age of Earth and the composition of its core, mantle, and crust. Jupiter’s gravity may also explain Mars’ smaller mass, with the larger planet consuming material as it migrated from the inner to the outer edge of the solar system [ 15 ].

Pluto and Planet Definition

The outermost part of the solar system is known as the Kuiper belt, which is a scattering of rocky and icy bodies. Beyond that is the Oort cloud, a zone filled with small and dispersed ice traces. These two locations are where most comets form and continue to orbit, and objects found here have relatively irregular orbits compared to the rest of the solar system. Pluto, formerly the ninth planet, is located in this region of space. The XXVIth General Assembly of the International Astronomical Union (IAU) stripped Pluto of planetary status in 2006 because scientists discovered an object more massive than Pluto, which they named Eris. The IAU decided against including Eris as a planet, and therefore, excluded Pluto as well. The IAU narrowed the definition of a planet to three criteria:

• Enough mass to have gravitational forces that force it to be rounded
• Not massive enough to create a fusion
• Large enough to be in a cleared orbit, free of other planetesimals that should have been incorporated at the time the planet formed. Pluto passed the first two parts of the definition, but not the third. Pluto and Eris are currently classified as dwarf planets

12. Montmerle T, Augereau J-C, Chaussidon M, et al (2006) Solar System Formation and Early Evolution: the First 100 Million Years. In: From Suns to Life: A Chronological Approach to the History of Life on Earth. Springer New York, pp 39–95

13. Martin RG, Livio M (2012) On the evolution of the snow line in protoplanetary discs. Mon Not R Aston Soc Lett 425:L6–L9

14. Petit J-M, Morbidelli A, Chambers J (2001) The Primordial Excitation and Clearing of the Asteroid Belt. Icarus 153:338–347. https://doi.org/10.1006/icar.2001.6702

15. Walsh KJ, Morbidelli A, Raymond SN, et al (2011) A low mass for Mars from Jupiter’s early gas-driven migration. Nature 475:206–209

What were those red spots during the solar eclipse? An astronomer explains

A s skies darkened Monday over North America during a rare solar eclipse , many people noticed bright dots — flickering spots that were reddish, pink and orange in hue — along the periphery of the moon and Sun.

Those brilliant red spots, say experts, are called solar prominences. Here's what that means.

What are solar prominences?

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Solar prominences, explains NASA , are large, bright loops of plasma anchored to the Sun's surface in the photosphere — the visible surface of the Sun — that extend into the Sun's outer atmosphere. These prominences are sometimes visible during a solar eclipse.

"They're sort of like an appendage coming out of the chromosphere of the Sun," said Butler University  Physics & Astronomy professor Brian Murphy. "That red color people noticed is fluorescing hydrogen gas, just in the right temperature and density to give us this pinkish color."

Experts say you can think of solar prominences as being akin to the clouds in Earth's atmosphere, but instead of suspended water vapor, these giant tendrils of hot gas are trapped by magnetic fields. They can reach distances of 93,000 miles above the Sun's surface and last for several months, according to NASA , unless they erupt.

As a solar prominence flows along a tangled and twisted structure of magnetic fields, says NASA, they either collapse back into the Sun's surface or can become unstable, break and burst outward, releasing the plasma in a dazzling display of energy.

Was that a solar flare during the eclipse?

Many photos Monday managed to catch a bright spot at the bottom of the eclipse. Murphy explained people were probably witnessing a "post eruptive solar prominence," which was likely the location of a solar flare.

When a solar prominence is 'post eruptive'

"Think of a rubber band stretched in the shape of a horseshoe magnet," Murphy said. "Now imagine that rubber band breaking at the top. That's when you get material ejected quite often from the sun or a solar flare, when the magnetic field is reconnecting — that's what we mean by 'post eruptive'."

What is a solar flare?

A solar flare is an intense burst of radiation near a sunspot that releases magnetic energy out into space,  according to NASA . These giant explosions from the sun send energy, light, and particles throughout the solar system. Flares can last several minutes to several hours. Sometimes this burst of energy can cause geomagnetic storms on Earth.

What's the difference between a solar flare and a coronal mass ejection?

While coronal mass ejections (CMEs) and solar flares are both tremendous explosions of energy that occur on the Sun, they move at different speeds.

Solar flares, NASA says, are some of the most powerful explosions in the solar system. Particles from a solar flare can travel at the speed of light and reach Earth in minutes. CMEs, explains NASA , are large clouds of solar plasma and magnetic fields from the Sun that can take up to three days to reach our planet.

Credits: NASA’s Goddard Space Flight Center/Mary Pat Hrybyk-Keith

Community science groups across U.S. capture video of 2024 solar eclipse

Murphy said community science groups at 35 telescope stations across the country collaborated Monday to capture video clips of the total eclipse as it stretched from Texas to New England.

These 2-3 minute clips, as part of a National Science Foundation project, will later all be combined into a 60-minute video to help study the Sun's corona. Speaking on behalf of the Citizen Continental-America Telescope Eclipse (Citizen CATE) 2024 project, Murphy said he and others were excited to see the solar prominence — those red dots — in their recordings.

"It was pretty obvious to the eye as well," he said, "which made it even better."

Others are reading: Thousands gather to watch 'indescribable' total solar eclipse in Indianapolis

John Tufts covers trending news for the Indianapolis Star. Send him a news tip at  [email protected] . Follow him on X at  @JTuftsReports .

This article originally appeared on Indianapolis Star: What were those red spots during the solar eclipse? An astronomer explains

Cosmic Samples and the Origin of the Solar System

Formation of the solar system, learning objectives.

By the end of this section, you will be able to:

• Describe the motion, chemical, and age constraints that must be met by any theory of solar system formation
• Summarize the physical and chemical changes during the solar nebula stage of solar system formation
• Explain the formation process of the terrestrial and giant planets
• Describe the main events of the further evolution of the solar system

As we have seen, the comets , asteroids , and meteorites are surviving remnants from the processes that formed the solar system. The planets, moons, and the Sun, of course, also are the products of the formation process, although the material in them has undergone a wide range of changes. We are now ready to put together the information from all these objects to discuss what is known about the origin of the solar system.

Observational Constraints

There are certain basic properties of the planetary system that any theory of its formation must explain. These may be summarized under three categories: motion constraints, chemical constraints, and age constraints. We call them constraints because they place restrictions on our theories; unless a theory can explain the observed facts, it will not survive in the competitive marketplace of ideas that characterizes the endeavor of science. Let’s take a look at these constraints one by one.

There are many regularities to the motions in the solar system. We saw that the planets all revolve around the Sun in the same direction and approximately in the plane of the Sun’s own rotation. In addition, most of the planets rotate in the same direction as they revolve, and most of the moons also move in counterclockwise orbits (when seen from the north). With the exception of the comets and other trans-neptunian objects, the motions of the system members define a disk or Frisbee shape. Nevertheless, a full theory must also be prepared to deal with the exceptions to these trends, such as the retrograde rotation (not revolution) of Venus.

In the realm of chemistry, we saw that Jupiter and Saturn have approximately the same composition—dominated by hydrogen and helium. These are the two largest planets, with sufficient gravity to hold on to any gas present when and where they formed; thus, we might expect them to be representative of the original material out of which the solar system formed. Each of the other members of the planetary system is, to some degree, lacking in the light elements. A careful examination of the composition of solid solar-system objects shows a striking progression from the metal-rich inner planets, through those made predominantly of rocky materials, out to objects with ice-dominated compositions in the outer solar system. The comets in the Oort cloud and the trans-neptunian objects in the Kuiper belt are also icy objects, whereas the asteroids represent a transitional rocky composition with abundant dark, carbon-rich material.

As we saw in Other Worlds: An Introduction to the Solar System , this general chemical pattern can be interpreted as a temperature sequence: hot near the Sun and cooler as we move outward. The inner parts of the system are generally missing those materials that could not condense (form a solid) at the high temperatures found near the Sun. However, there are (again) important exceptions to the general pattern. For example, it is difficult to explain the presence of water on Earth and Mars if these planets formed in a region where the temperature was too hot for ice to condense, unless the ice or water was brought in later from cooler regions. The extreme example is the observation that there are polar deposits of ice on both Mercury and the Moon; these are almost certainly formed and maintained by occasional comet impacts.

As far as age is concerned, we discussed that radioactive dating demonstrates that some rocks on the surface of Earth have been present for at least 3.8 billion years, and that certain lunar samples are 4.4 billion years old. The primitive meteorites all have radioactive ages near 4.5 billion years. The age of these unaltered building blocks is considered the age of the planetary system. The similarity of the measured ages tells us that planets formed and their crusts cooled within a few tens of millions of years (at most) of the beginning of the solar system. Further, detailed examination of primitive meteorites indicates that they are made primarily from material that condensed or coagulated out of a hot gas; few identifiable fragments appear to have survived from before this hot-vapor stage 4.5 billion years ago.

The Solar Nebula

All the foregoing constraints are consistent with the general idea, introduced in Other Worlds: An Introduction to the Solar System , that the solar system formed 4.5 billion years ago out of a rotating cloud of vapor and dust—which we call the solar nebula —with an initial composition similar to that of the Sun today. As the solar nebula collapsed under its own gravity, material fell toward the center, where things became more and more concentrated and hot. Increasing temperatures in the shrinking nebula vaporized most of the solid material that was originally present.

At the same time, the collapsing nebula began to rotate faster through the conservation of angular momentum (see the Orbits and Gravity and Earth, Moon, and Sky chapters). Like a figure skater pulling her arms in to spin faster, the shrinking cloud spun more quickly as time went on. Now, think about how a round object spins. Close to the poles, the spin rate is slow, and it gets faster as you get closer to the equator. In the same way, near the poles of the nebula, where orbits were slow, the nebular material fell directly into the center. Faster moving material, on the other hand, collapsed into a flat disk revolving around the central object (Figure 1). The existence of this disk-shaped rotating nebula explains the primary motions in the solar system that we discussed in the previous section. And since they formed from a rotating disk, the planets all orbit the same way.

Figure 1: Steps in Forming the Solar System . This illustration shows the steps in the formation of the solar system from the solar nebula. As the nebula shrinks, its rotation causes it to flatten into a disk. Much of the material is concentrated in the hot center, which will ultimately become a star. Away from the center, solid particles can condense as the nebula cools, giving rise to planetesimals, the building blocks of the planets and moons.

Picture the solar nebula at the end of the collapse phase, when it was at its hottest. With no more gravitational energy (from material falling in) to heat it, most of the nebula began to cool. The material in the center, however, where it was hottest and most crowded, formed a star that maintained high temperatures in its immediate neighborhood by producing its own energy. Turbulent motions and magnetic fields within the disk can drain away angular momentum, robbing the disk material of some of its spin. This allowed some material to continue to fall into the growing star, while the rest of the disk gradually stabilized.

The temperature within the disk decreased with increasing distance from the Sun, much as the planets’ temperatures vary with position today. As the disk cooled, the gases interacted chemically to produce compounds; eventually these compounds condensed into liquid droplets or solid grains. This is similar to the process by which raindrops on Earth condense from moist air as it rises over a mountain.

Let’s look in more detail at how material condensed at different places in the maturing disk (Figure 2). The first materials to form solid grains were the metals and various rock-forming silicates. As the temperature dropped, these were joined throughout much of the solar nebula by sulfur compounds and by carbon- and water-rich silicates, such as those now found abundantly among the asteroids. However, in the inner parts of the disk, the temperature never dropped low enough for such materials as ice or carbonaceous organic compounds to condense, so they were lacking on the innermost planets.

Figure 2: Chemical Condensation Sequence in the Solar Nebula . The scale along the bottom shows temperature; above are the materials that would condense out at each temperature under the conditions expected to prevail in the nebula.

Far from the Sun, cooler temperatures allowed the oxygen to combine with hydrogen and condense in the form of water (H 2 O) ice. Beyond the orbit of Saturn, carbon and nitrogen combined with hydrogen to make ices such as methane (CH 4 ) and ammonia (NH 3 ). This sequence of events explains the basic chemical composition differences among various regions of the solar system.

Example 1: Rotation of the Solar Nebula

We can use the concept of angular momentum to trace the evolution of the collapsing solar nebula. The angular momentum of an object is proportional to the square of its size (diameter) times its period of rotation ( D 2 / P ). If angular momentum is conserved, then any change in the size of a nebula must be compensated for by a proportional change in period, in order to keep D 2 / P constant. Suppose the solar nebula began with a diameter of 10,000 AU and a rotation period of 1 million years. What is its rotation period when it has shrunk to the size of Pluto’s orbit, which Appendix F tells us has a radius of about 40 AU?

We are given that the final diameter of the solar nebula is about 80 AU. Noting the initial state before the collapse and the final state at Pluto’s orbit, then

[latex]\frac{{P}_{\text{final}}}{{P}_{\text{initial}}}={\left(\frac{{D}_{\text{final}}}{{D}_{\text{initial}}}\right)}^{2}={\left(\frac{80}{10,000}\right)}^{2}={\left(0.008\right)}^{2}=0.000064[/latex]

With P initial equal to 1,000,000 years, P final , the new rotation period, is 64 years. This is a lot shorter than the actual time Pluto takes to go around the Sun, but it gives you a sense of the kind of speeding up the conservation of angular momentum can produce. As we noted earlier, other mechanisms helped the material in the disk lose angular momentum before the planets fully formed.

Check Your Learning

What would the rotation period of the nebula in our example be when it had shrunk to the size of Jupiter’s orbit?

Formation of the Terrestrial Planets

The grains that condensed in the solar nebula rather quickly joined into larger and larger chunks, until most of the solid material was in the form of planetesimals, chunks a few kilometers to a few tens of kilometers in diameter. Some planetesimals still survive today as comets and asteroids. Others have left their imprint on the cratered surfaces of many of the worlds we studied in earlier chapters. A substantial step up in size is required, however, to go from planetesimal to planet.

Some planetesimals were large enough to attract their neighbors gravitationally and thus to grow by the process called accretion . While the intermediate steps are not well understood, ultimately several dozen centers of accretion seem to have grown in the inner solar system. Each of these attracted surrounding planetesimals until it had acquired a mass similar to that of Mercury or Mars. At this stage, we may think of these objects as protoplanets —”not quite ready for prime time” planets.

Each of these protoplanet s continued to grow by the accretion of planetesimals. Every incoming planetesimal was accelerated by the gravity of the protoplanet, striking with enough energy to melt both the projectile and a part of the impact area. Soon the entire protoplanet was heated to above the melting temperature of rocks. The result was planetary differentiation , with heavier metals sinking toward the core and lighter silicates rising toward the surface. As they were heated, the inner protoplanets lost some of their more volatile constituents (the lighter gases), leaving more of the heavier elements and compounds behind.

Formation of the Giant Planets

In the outer solar system, where the available raw materials included ices as well as rocks, the protoplanets grew to be much larger, with masses ten times greater than Earth. These protoplanets of the outer solar system were so large that they were able to attract and hold the surrounding gas. As the hydrogen and helium rapidly collapsed onto their cores, the giant planets were heated by the energy of contraction. But although these giant planets got hotter than their terrestrial siblings, they were far too small to raise their central temperatures and pressures to the point where nuclear reactions could begin (and it is such reactions that give us our definition of a star). After glowing dull red for a few thousand years, the giant planets gradually cooled to their present state (Figure 3).

Figure 3: Saturn Seen in Infrared . This image from the Cassini spacecraft is stitched together from 65 individual observations. Sunlight reflected at a wavelength of 2 micrometers is shown as blue, sunlight reflected at 3 micrometers is shown as green, and heat radiated from Saturn’s interior at 5 micrometers is red. For example, Saturn’s rings reflect sunlight at 2 micrometers, but not at 3 and 5 micrometers, so they appear blue. Saturn’s south polar regions are seen glowing with internal heat. (credit: modification of work by NASA/JPL/University of Arizona)

The collapse of gas from the nebula onto the cores of the giant planets explains how these objects acquired nearly the same hydrogen-rich composition as the Sun. The process was most efficient for Jupiter and Saturn; hence, their compositions are most nearly “cosmic.” Much less gas was captured by Uranus and Neptune, which is why these two planets have compositions dominated by the icy and rocky building blocks that made up their large cores rather than by hydrogen and helium. The initial formation period ended when much of the available raw material was used up and the solar wind (the flow of atomic particles) from the young Sun blew away the remaining supply of lighter gases.

Further Evolution of the System

All the processes we have just described, from the collapse of the solar nebula to the formation of protoplanets, took place within a few million years. However, the story of the formation of the solar system was not complete at this stage; there were many planetesimals and other debris that did not initially accumulate to form the planets. What was their fate?

The comets visible to us today are merely the tip of the cosmic iceberg (if you’ll pardon the pun). Most comets are believed to be in the Oort cloud, far from the region of the planets. Additional comets and icy dwarf planets are in the Kuiper belt, which stretches beyond the orbit of Neptune. These icy pieces probably formed near the present orbits of Uranus and Neptune but were ejected from their initial orbits by the gravitational influence of the giant planets.

In the inner parts of the system, remnant planetesimals and perhaps several dozen protoplanets continued to whiz about. Over the vast span of time we are discussing, collisions among these objects were inevitable. Giant impacts at this stage probably stripped Mercury of part of its mantle and crust, reversed the rotation of Venus, and broke off part of Earth to create the Moon (all events we discussed in other chapters).

Smaller-scale impacts also added mass to the inner protoplanets. Because the gravity of the giant planets could “stir up” the orbits of the planetesimals, the material impacting on the inner protoplanets could have come from almost anywhere within the solar system. In contrast to the previous stage of accretion, therefore, this new material did not represent just a narrow range of compositions.

As a result, much of the debris striking the inner planets was ice-rich material that had condensed in the outer part of the solar nebula. As this comet-like bombardment progressed, Earth accumulated the water and various organic compounds that would later be critical to the formation of life. Mars and Venus probably also acquired abundant water and organic materials from the same source, as Mercury and the Moon are still doing to form their icy polar caps.

Gradually, as the planets swept up or ejected the remaining debris, most of the planetesimals disappeared. In two regions, however, stable orbits are possible where leftover planetesimals could avoid impacting the planets or being ejected from the system. These regions are the asteroid belt between Mars and Jupiter and the Kuiper belt beyond Neptune. The planetesimals (and their fragments) that survive in these special locations are what we now call asteroids, comets, and trans-neptunian objects.

Astronomers used to think that the solar system that emerged from this early evolution was similar to what we see today. Detailed recent studies of the orbits of the planets and asteroids, however, suggest that there were more violent events soon afterward, perhaps involving substantial changes in the orbits of Jupiter and Saturn. These two giant planets control, through their gravity, the distribution of asteroids. Working backward from our present solar system, it appears that orbital changes took place during the first few hundred million years. One consequence may have been scattering of asteroids into the inner solar system, causing the period of “heavy bombardment” recorded in the oldest lunar craters.

Key concepts and summary

A viable theory of solar system formation must take into account motion constraints, chemical constraints, and age constraints. Meteorites, comets, and asteroids are survivors of the solar nebula out of which the solar system formed. This nebula was the result of the collapse of an interstellar cloud of gas and dust, which contracted (conserving its angular momentum) to form our star, the Sun, surrounded by a thin, spinning disk of dust and vapor. Condensation in the disk led to the formation of planetesimals, which became the building blocks of the planets. Accretion of infalling materials heated the planets, leading to their differentiation. The giant planets were also able to attract and hold gas from the solar nebula. After a few million years of violent impacts, most of the debris was swept up or ejected, leaving only the asteroids and cometary remnants surviving to the present.

accretion: the gradual accumulation of mass, as by a planet forming from colliding particles in the solar nebula

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