3 hypothesis of the origin of the solar system

September 29, 1917

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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.

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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.

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• Published: 22 December 2021

PLANET FORMATION

Archaeology of the Solar System

• Chris Ormel   ORCID: orcid.org/0000-0003-4672-8411 1  

Nature Astronomy volume  6 ,  pages 16–17 ( 2022 ) Cite this article

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A new model for the origin of the Solar System proposes planet building blocks formed fast from material that was transported outwards to cooler regions. It claims to be consistent with the properties of ancient meteorites.

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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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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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Chapter Contents

• 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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3.1 Origin of Earth and the Solar System

Modified from Karla Panchuk in "Physical Geology" by Steven Earle*

According to the Big Bang theory , the universe blinked violently into existence 13.77 billion years ago (Figure 3.1.1). The Big Bang is often described as an explosion, but imagining it as an enormous fireball isn’t accurate. The Big Bang involved a sudden expansion of matter, energy, and space from a single point. The kind of Hollywood explosion that might come to mind involves expansion of matter and energy within space, but during the big bang, space itself was created.

At the start of the Big Bang, the universe was too hot and dense to be anything but a sizzle of particles smaller than atoms, but as it expanded, it also cooled. Eventually some of the particles collided and stuck together. Those collisions produced hydrogen and helium, the most common elements in the universe, along with a small amount of lithium. Gravity caused clouds of these early elements to coalesce into stars, and it was inside these stars that heavier elements were formed

Our solar system began to form around 5 billion years ago, roughly 8.7 billion years after the Big Bang. A solar system consists of a collection of objects orbiting one or more central stars. All solar systems start out the same way. They begin in a cloud of gas and dust called a nebula . Nebulae are some of the most beautiful objects that have been photographed in space, with vibrant colors from the gases and dust they contain, and brilliant twinkling from the many stars that have formed within them (Figure 3.1.2). The gas consists largely of hydrogen and helium, and the dust consists of tiny mineral grains, ice crystals, and organic particles.

A solar system begins to form when a small patch within a nebula (small by the standards of the universe, that is) begins to collapse upon itself. Exactly how this starts isn’t clear, although it might be triggered by the violent behavior of nearby stars as they progress through their life cycles. Energy and matter released by these stars might compress the gas and dust in nearby neighborhoods within the nebula. Once it is triggered, the collapse of gas and dust within that patch continues for two reasons. One of those reasons is that gravitational force pulls gas molecules and dust particles together. But early in the process, those particles are very small, so the gravitational force between them isn’t strong. So how do they come together? The answer is that dust first accumulates in loose clumps for the same reason dust bunnies form under your bed: static electricity. As the small patch within a nebula condenses, a star begins to form from material drawn into the center of the patch, and the remaining dust and gas settle into a disk that rotates around the star. The disk is where planets eventually form, so it’s called a protoplanetary disk . In Figure 3.1.3 the image in the upper left shows an artist’s impression of a protoplanetary disk, and the image in the upper right shows an actual protoplanetary disk surrounding the star HL Tauri. Notice the dark rings in the protoplanetary disk. These are gaps where planets are beginning to form. The rings are there because incipient planets are beginning to collect the dust and gas in their orbits. There is an analogy for this in our own solar system, because the dark rings are akin to the gaps in the rings of Saturn (Fig. 3.1.3, lower left), where moons can be found (Fig. 3.1.3, lower right).

These three types of planets are not mixed together randomly within our solar system. Instead they occur in a systematic way, with terrestrial planets closest to the sun, followed by the Jovian planets and then the ice giants. Part of the reason for this arrangement is the frost line (also referred to as the snow line ). The frost line separated the inner part of the protoplanetary disk closer to the sun, where it was too hot to permit anything but silicate minerals and metal to crystallize, from the outer part of the disk farther from the Sun, where it was cool enough to allow ice to form. As a result, the objects that formed in the inner part of the protoplanetary disk consist largely of rock and metal, while the objects that formed in the outer part consist largely of gas and ice. The young sun also blasted the solar system with raging solar winds (winds made up of energetic particles), which helped to drive lighter molecules toward the outer part of the protoplanetary disk.

The objects in our solar system formed by accretion . Early in this process, mineral and rock particles collected in fluffy clumps because of static electricity. As the mass of the clumps increased, gravity became more important, pulling material from farther away and growing these solid masses into larger and larger bodies. Eventually the mass of the objects became large enough that their gravity was strong enough to hang onto gas molecules, because gas molecules are very light.

Our Earth formed though this process of accretion about 4.6 billion years ago. The early Earth was very hot and had a molten, fluid composition, with lost of geological and volcanic activity on the surface. The Earth’s heat came from a variety of processes:

• Heat came from the decay of radioactive elements within the Earth, specifically the decay of 235U, 238U, 40K, and 232Th, which are primarily present in the mantle. The total heat produced that way has been decreasing over time (because these isotopes are getting used up), and is now roughly 25% of what it was when Earth formed. This means that Earth’s interior is slowly becoming cooler.
• Heat came from the thermal energy already contained within the objects that accreted to form the Earth.
• Heat came from collisions. When objects hit Earth, some of the energy from their motion went into deforming Earth, and some of it was transformed into heat. (The very worst collision that Earth experienced was with a planet named Theia, which was approximately the size of Mars. Not long after Earth formed, Theia struck Earth. When Theia slammed into Earth, Theia’s metal core merged with Earth’s core , and debris from the outer silicate layers was cast into space, forming a ring of rubble around Earth. The material within the ring coalesced into a new body in orbit around Earth, giving us our moon. Remarkably, the debris may have coalesced in 10 years or fewer! This scenario for the formation of the moon is called the giant impact hypothesis .)
• As Earth became larger, its gravitational force became stronger. This increased Earth’s ability to draw objects to it, but it also caused the material making Earth to be compressed, rather like Earth giving itself a giant gravitational hug. Compression causes materials to heat up.

Heating had a very important consequence for Earth’s structure. As Earth grew, it collected a mixture of silicate mineral grains as well as iron and nickel. These materials were scattered throughout Earth. That changed when Earth began to heat up: it got so hot that both the silicate minerals and the metals melted. The metal melt was much denser than the silicate mineral melt, so the metal melt sank to Earth’s center to become its core , and the silicate melt rose upward to become Earth’s crust and mantle . In other words, Earth unmixed itself. The separation of silicate minerals and metals into a rocky outer layer and a metallic core, respectively, is called differentiation . Gravity has since pulled Earth into an almost spherical shape with a radius of 6371 km, and a circumference of about 40,000 km. However, it is not a perfect sphere, as the Earth’s rotation causes an equatorial bulge, so that the Earth’s circumference is 21 km (0.3%) wider at the equator than it is pole to pole. Thus it is technically an “oblate spheroid.”

If we were to take an inventory of the elements that make up Earth, we would find that 95% of Earth’s mass comes from only four elements: oxygen, magnesium, silicon, and iron. Most of the remaining 5% comes from aluminum, calcium, nickel, hydrogen, and sulphur. We know that the Big Bang made hydrogen, helium, and lithium, but where did the rest of the elements come from? The answer is that the other elements were made by stars. The heat and pressure within stars cause smaller atoms to smash together and fuse into new, larger atoms. For example, when hydrogen atoms smash together and fuse, helium is formed. Large amounts of energy are released when some atoms fuse and that energy is what causes stars to shine.

It takes larger stars to make elements as heavy as iron and nickel. Our Sun is an average star; after it uses up its hydrogen fuel to make helium, and then some of that helium is fused to make small amounts of beryllium, carbon, nitrogen, oxygen, and fluorine, it will be at the end of its life. It will stop making atoms and will cool down and bloat until its middle reaches the orbit of Mars. In contrast, large stars end their lives in spectacular fashion, exploding as supernovae and casting off newly formed atoms —including the elements heavier than iron — into space. It took many generations of stars creating heavier elements and casting them into space before heavier elements were abundant enough to form planets like Earth.

*”Physical Geology” by Steven Earle used under a CC-BY 4.0 international license. Download this book for free at http://open.bccampus.ca

the theory that the universe started with a giant expansion approximately 13.77 billion years ago (3.1)

a star and the planets surrounding it (3.1)

a cloud of interstellar dust and gases (3.1)

a rotating cloud of gas and dust surrounding a young star (3.1)

a planet with a rocky mantle and crust and metallic core (e.g., Earth) (3.1)

a gas giant (3.1)

a large planet composed mostly of hydrogen and helium (e.g. Jupiter) (3.1)

a planet that is comprised mainly of gases heavier than hydrogen and helium, including oxygen, carbon, nitrogen, and sulfur (e.g., Uranus and Neptune) (3.1)

in the context of planetary systems the boundary beyond which volatile components (e.g., water, carbon dioxide, methane, ammonia etc.) are frozen (3.1)

in astronomy the radius around a star at which represents the boundary between gases (or liquids) and solids (3.1)

a stream of ionized (charged) particles away from the Sun (3.1)

the process by which solid celestial bodies are added to existing bodies during collisions (3.1)

the metallic interior part of the Earth, extending from a depth of 2900 km to the center (3.2)

the theory that the Moon formed when a Mars-sized planet (Theia) collided with the Earth at 4.5 billion years ago (3.1)

the uppermost layer of the Earth, ranging in thickness from about 5 km (in the oceans) to over 50 km (on the continents) (3.2)

the middle layer of the Earth, dominated by iron and magnesium rich silicate minerals and extending for about 2900 km from the base of the crust to the top of the core (3.2)

the un-mixing of a magma, typically by the physical separation of minerals that crystallize early and settle towards the bottom (3.1)

Introduction to Oceanography Copyright © by Paul Webb is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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6.4: Origin of the Solar System

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Where did it all come from? That is a question that philosophers and astronomers have pondered for thousands of years. While cannot rewind time and watch the formation of the Solar System form the beginning, we can look at the Solar System as it is today for clues as to its origins. From that, we can develop a model to describe how it may have gotten that way. Any model for the origin of the Solar System must be consistent with the laws of physics as first described by Isaac Newton and later expanded by Albert Einstein with his theory of general and special relativity.

A theory on the origin of the Solar System must also be able to account for what we can observe today. Some observations that we can make about the Solar System include the following:

The Sun contains over 99% of the Solar System’s mass while the planets contribute only about 0.2%

Angular Momentum

In contracts, planets have most of the Solar System’s angular momentum.

Patterns of Motion

All the planets orbit the Sun in the same direction and in roughly the same plane. All of them are inclined less than 10 degrees from the ecliptic with Mercury having the greatest inclination. In contrast, the dwarf planets like Pluto and Eris have much more tilted orbits, at 17 and 44 degrees to the ecliptic, respectively. Also, the Sun’s rotational equator lies very close the ecliptic. The planets all orbit the Sun in the same west-to-east direction, called prograde revolution and the Sun also rotates in the same prograde direction. Also, except for Venus and Uranus, the planets also rotate in the prograde direction. The planets also have small obliquity, the tilt between equatorial and orbital planes, with Uranus again being the exception. The planetary orbits are also nearly circular.

Two Types of Planets

As we noted, planets are either rocky terrestrial bodies or gaseous Jovian worlds. Their distribution varies roughly with their distance from the Sun with the dense-metal-rich terrestrial planets in the inner system and the giant, hydrogen-right planets in the outer system. The distances between the planets roughly conform to a simple rule, called Bode’s Rule , which states the planets have a geometry spacing between their orbits. Mercury, the inner-most planet, has the highest metal composition, followed by the three more rocky planets, Venus, Earth, and Mars. Then, asteroids occupy a strange gap in the planetary distribution. After the asteroids, come Jupiter and Saturn, two planets made mostly of hydrogen and helium. Next come Uranus and Neptune, similar to the other Jovian planets, but with more hydrogen compounds like methane and ammonia, giving them a blue or greenish blue hue. Beyond Neptune, we have the Kuiper belt full of small icy bodies and even more such bodies beyond until we head into interstellar space and the Oort cloud beyond.

The Jovian planets also have numerous moons, making them appear to be miniature versions of the Solar System while the terrestrial planets have relatively few moons.

Many bodies, especially those that have little or no erosion or plate tectonics, like Mercury and the Moon, show evidence of numerous impacts. These impacts have left craters behind and our dating of the craters of the Moon indicate that the majority of these impacts occurred within the first few hundred million years of the Solar System’s history.

Asteroids and Comets

Examination of meteorites show that they differ in geological properties from all known planetary and lunar rocks. As for comets, most of them orbit in a large, almost spherical swarm around the Solar System called the Oort cloud. Kuiper belt objects resemble comets in their composition.

The Exceptions

Finally, our model of the Solar System must also account for the anomalies that we observe, such as Earth’s large Moon, Venus’ retrograde motion, and Uranus’ extreme axial tilt.

Overall, the universe is mostly hydrogen and helium, however, the terrestrial planets, including the Earth, are mostly made of heavier elements. Where did those heavier elements come from? Massive stars can fuse hydrogen and helium in heavier elements such as oxygen, carbon, nitrogen, silicon, etc. However, even the most massive stars cannot fuse atoms inside their cores into elements heavier than iron. Those elements can only be formed in the supernova explosions that mark the death of the most massive stars. These elements where then hurled into space by the force of the supernovae. Eventually, they coalesced into nebulae (Latin for clouds) made up of hydrogen, helium, and dust particles.

6.5.1 Nebular Theory

According to nebular theory , one of these clouds began to contract. The cause of this contraction is unclear, but perhaps it was force of a dying star going supernova that pushed the cloud into contracting. Kant and Laplace first proposed the nebular collapse theory over two centuries ago and since then, astronomers have found a great deal of evidence to support it. For example, we can see young stars forming in massive nebulae like such as the Eagle Nebula. In addition, we can see that the star Beta Pictoris has a disc of warm matter surrounding it, which may indicate early planetary formation.

https:/commons.wikimedia.org/wiki/File:Protoplanetary_disk_of_RX_J1615_(eso1640b).jpg; 

As the cloud collapses, it must speed up due to conservation of angular momentum (Chapter 3). Just as a skate can start herself rotating slowly with her arms fully extended and then speed herself up by bringing her arms into her body, a cloud will contract as it contracts further. Meanwhile, collisions between particles will cause cloud to flatten and form a disc around the early star. Collisions between gas particles will reduce random motions and up and down motions, resulting in flattening as the particles eventually settle into motions that orbit around the forming star. The material in the disc eventually forms into the planets. Initially, the early Sun rotated much faster than it does now, however, friction between its magnetic field and the nebula slowed it down.

Note that some astronomers had propose a rival theory to the nebular theory in which the planets formed from material torn off the surface of the Sun as it made a close encounter with another massive object. However, this encounter theory could not explain the motions of the planets and their distribution.

6.5.2 Formation of the Planets

The planets formed through condensation, which occurs when gas cools and forms tiny solid particles. These dust grains act as condensation nuclei. Temperature governs where particular materials condense out. Astronomers mark a boundary in the between where rocky particles condense and where icy particles condense the frost line or the snow line . Inside the snow line, where it is too hot for hydrogen compounds to form ices, the terrestrial planets form, with metals condense closer in, making Mercury the most metallic planet. Outside the frost line, the Jovian planets form from ice particles.

The small particles of rock and metal inside the snow line eventually start to clump together. Initially, friction between the particles creates electrostatic charges which cause the particles to stick together. Over time, some of these particles form larger bodies called planetesimals, which are large enough for gravity to take over the accretion process. Gravity also reshapes the bodies, forming them into spheres. Gravity pulls all matter toward an objects center of mass, so above a certain mass, all bodies take on a spherical shape. More smaller objects collect into a few large ones. As the planetesimal collided, their energy converted thermal energy, causing them melt. These hot molten bodies continue to gather the remaining mass in their orbital path, becoming the four terrestrial planets. As they become molten, the denser elements like iron and nickel sink into the center, forming the planetary cores.

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Meanwhile, outside the snow line, ices form the basis of the particles that accrete into planetesimals. A combination of photos and the solar wind (charged particles flowing out from the Sun) blew the lighter gases into the outer Solar system. The gravity of these larger planets pulled the hydrogen and helium together. Meanwhile, these large gas giants formed discs of their own which condensed into the moons of the Jovian planets.

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Like the terrestrial planets, the Jovian planets also underwent differentiation, forming rocky cores surrounded by gases ices. Jupiter won the race to gobble up the most matter, leaving only a small amount of debris in the asteroid belt marking the boundary between the inner and outer solar system. Uranus and Neptune, being further out and colder, could gather more ices than Jupiter and Saturn, giving them higher methane, ammonia, and water contents.

Leftover debris then formed into the asteroids and comets. Asteroids being closer in than the Jovian planets, could form out of both dust and ice. Some of them formed large enough bodies to undergo differentiation. Larger asteroids like Ceres, are spherical with metallic cores surrounded by ice and rock layers. Comets and the Kuiper Belt objects like Pluto, formed mostly from ices and frozen nitrogen, which smaller amounts of rock compared to the asteroids.

The early few million years of the Solar System had a lot more planetesimals that are present today. Perturbations from Jupiter’s gravity disrupted them from their ordinary orbits and flung them into the inner solar system. Others were flung into the outer solar system, colliding with the moons of the Jovian planets or being gobbled up by the gas giants themselves. This Late Heavy Bombardment period lasted between 4.1 and 3. 8 billion years ago and turned much of the solar system into a shooting gallery. Massive collisions left many of the Moons and planets like Mercury heavily cratered. One major collision probably led to the formation of the Moon (Chapter 8). Icy planetesimals from beyond the snow line likely brought water to the Earth. Mars’ two moons, Phobos and Deimos, may be captured planetesimals leftover from the Late Heavy Bombardment. Other large collisions may have caused the odd rotation of planets like Venus and Uranus.

https:/commons.wikimedia.org/wiki/File:Lunar_cataclysm.jpg; 

6.4.3 Measuring the age of rocks

How do we know how old the Solar System is? It turns out, the rocks we find on Earth do not help us answer this question much. Because the Earth has the most active surface of the terrestrial planets, there are few places on Earth where the crust is very old. As the various tectonic plates (Chapter 8) move, old crust disappears into the mantle while new crust forms as magma forces it way to the surface. The movement of plates raise up mountains and then wind and water wear them down. As a result, the Earth likely has the youngest surface of the terrestrial planets.

https://en.wikiversity.org/wiki/File...ingmineral.png

The main way we date rock is through radiometric dating . Some isotopes are radioactive and decay into more stable atoms over time. Radioactive decay is a random process and we can never predict when a single atom may decay. As result, the decay rate is governed by a statistical model based on an exponential decay curve. Radioactive isotopes decay according to their respective half-life , the amount of time it takes for half of the atoms present to decay. When a rock forms from the cooling of magma, the quantity of certain isotopes becomes fixed. By measuring the ratio of the radioactive isotopes to their decay product, we can calculate how many half-lives have passed since the rock formed.

The isotope most people may be familiar with is Carbon-14. However, its half-live is only about 5700 years, which is too short to be useful for dating ancient rocks. Carbon-14 dating is mostly used in archeology to date artifacts made of wood, bone or some other organic material.

Some dating methods that are useful for dating rocks include:

·         rubidium-87 decays to strontium-87 with a half-life of 49 billion years.

·         uranium-238 decays (in a series of steps) to lead-206 with a half-life of 4.5 billion years.

·         potassium-40 decays to argon-40 with a half-life of 1.25 billion years.

Potassium-40 decays into argon-40 with a half-life of 1.25 billion years. This is a much more useful half-life for rock dating. The table below summarizes the potassium/argon ratio over a period of a few half-lives, giving the approximate age of a rock as the ratio is measured.

For example, say a rock contains potassium-40 when it forms. After one half-life (1.25 billion years), half of the potassium-40 will have decayed into argon-40, giving a potassium-40/argon-40 ratio of 1:1. After two half-lives (2.5 billion years), the ratio will become 1:3. After three half-lives (3.75 billion years), the ratio will become 1: 7. After four half-lives (5 billion years), the ratio will be down to 1:15.

Using radiometric dating techniques such as potassium-argon dating, scientists have determined that the oldest rocks ever found on the surface of the Earth have an age of about 4.28 billion years, meaning the Earth must be at least than old. But since the Earth’s surface has been under a constant reformation, we cannot be certain that the oldest rocks on the surface are the same age as the Earth itself or the Solar System. Fortunately, we have other rocks besides those found on Earth that we can date. One group has been falling onto the Earth since the earliest days of the Solar System. The other group, we can go and bring back. These are meteorites and the lunar rocks from the Apollo program.

Radiometric dating of lunar rocks has found that the oldest of them are about 4.4 billion years old. This gives us a minimum age for the Moon and given that the Moon likely formed from a collision with the Earth, the Earth must be even older. However, the Moon does not have active plate tectonics, so its surface must be older than the Earth’s surface. Most lunar craters date from between 4.1 and 3.8 billion years ago, giving us a time frame for the Late Heavy Bombardment. Finally, we have found that the oldest meteorites are 4.55 billion years old. Given that these meteorites formed out of the same disc debris that the planets formed from, the planets likely formed sometime around 4.5 billion years ago. Calculations then indicate that to give enough time for the Sun and the planetary disc to form, the nebula contraction probably began around 4.6 billion years ago.

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10.02: Origin of the Solar System—The Nebular Hypothesis

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• Chris Johnson, Matthew D. Affolter, Paul Inkenbrandt, & Cam Mosher
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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

35 Facts About Our Solar System That'll Make You Stop & Think

Posted: April 14, 2024 | Last updated: April 14, 2024

Do you know these facts about our solar system?

The origin of the solar system has been debated by scientists, theologians, and other grand high muckity-mucks since time immemorial. While there’s no pinpointing an exact date and time when our solar system arrived on the premises, the good people at NASA have given a ballpark estimate of 4.5 billion years .

According to NASA, a cloud of interstellar gas and dust collapsed after getting caught in the shockwave of an exploding star. The collapsed cloud formed a swirling body of matter whose gravitational pull drew more matter into its center. The pressure there created a massive amount of energy, which eventually formed our sun.

Got all that? If not, don’t worry – we’ve created a rundown of facts about our solar system that you can use to familiarize yourself with it. On the other hand, if you got straight A’s in science class and consider yourself an authority on all things solar, here’s a chance to test your knowledge and see how much you really know.

1. Earth is not even close to the center of the galaxy

For most of human history, scientists and theologians alike believed in the geocentric theory of the universe, which basically boils down to the belief that Earth is at the center of the universe, and everything revolves around it, including the sun. As it turns out, we’re  approximately 165 quadrillion miles away  from the center of the Milky Way.

2. The closest planet to the sun has ice

Mercury is the closest planet to the sun, but despite that, it has some  ice on the surface . The ice is found in craters that are never exposed to sunlight, but don’t mistake that for a sign that human beings could live there. The average temperature on Mercury is  354 degrees Fahrenheit , it can go as high as 800 degrees and at nighttime can reach -330 degrees, which puts it firmly outside the reach of “bring a sweater” weather.

3. Pluto used to be a planet but it isn’t any more

For a long time, it was an article of faith that our solar system has nine planets. Then, in 2006,  the International Astronomical Union decided  that one of them, Pluto, did not meet the criteria for being a full-fledged planet, because it had not yet “cleared the neighborhood around its orbit.” It was designated a “dwarf planet” and now the solar system consists of eight planets.

4. Neptune has really, really high-speed wind

Neptune has winds that blow at speeds that easily outpace anything that we have on Earth. According to NASA, some of the winds can blow at speeds of  over 1,100 miles per hour , and to put that in perspective, the speed of sound back here on Earth is just 767 miles per hour.

5. There might be a 'Planet X,' and it’s really far away

Pluto may have been demoted from a planet to a dwarf planet, but we may still have a ninth planet just the same. Known to the  cognoscenti  as “Planet X,” it’s believed to orbit the sun even farther out than Pluto does, and may take  between 10,000 and 20,000  of our years to go around the sun just once. But remember, this is all still theoretical, according to the Caltech astronomers who did the research, so don’t strap on your astronaut suit just yet.

6. Why is the existence of 'Planet X' still just a theory?

According to Jim Green, director of NASA's Planetary Science Division, the possible existence of a “Planet X” is a prediction based on early modeling. Some of the dwarf planets in the Kuiper Belt  follow orbits that cluster together , and the gravity from a potential planet may be what causes them to do this. Or maybe not. It’s still just a theory or, as Green said, “It's the start of a process that could lead to an exciting result."

7. If 'Planet X' turns out to be real, can they call it the Death Star?

No. The naming rights for a newly discovered planet go to  the person who made the discovery . So as tempting as it might be to give it some way cool name out of science fiction, the name will almost certainly end up being the name of a lucky scientist or astronomer whose diligent work has led to the discovery of a new world.

8. 'Planet X' could probably be detected with existing technology

The search for Planet X may seem a little fruitless at first. After all, if it’s even further away than Pluto, would modern telescopes even be able to detect it? Perhaps surprisingly,  the answer is yes , although it would be so faint that it would be difficult to detect for all but the most experienced astronomers.

9. Your weight on Earth is six times what it would be on the moon

Let’s say you weigh 150 pounds on Earth. Then let’s say you went to the moon. You would find yourself weighing  a considerably lighter  25 pounds. At the same time, 17. If you weigh the same 150 pounds on Earth, you would weigh 28 times that on the sun, or 4,200 pounds.

10. The hottest planet isn’t the one closest to the sun

Mercury is the closest planet to the sun, but that doesn’t necessarily mean it’s the hottest. It has no atmosphere, so it can’t retain the heat of the nearby sun. Venus, meanwhile, has an atmosphere that’s 100 times thicker than the one we have on Earth, and it’s composed almost entirely of carbon dioxide, so it reaches  an average temperature of 875 degrees Fahrenheit , while the temperature on Mercury tops out at 800 degrees Fahrenheit.

11. This planet has the most volcanoes

Venus isn’t only the hottest planet in the solar system. It’s also the planet that boasts the largest number of volcanoes. The number of volcanoes on Venus  exceeds 1,600 .

12. This planet also has no moons

Venus may be the hottest planet in the solar system, and it may be the planet with the most volcanoes, but for reasons that scientists have been unable to determine,  it has no moons . Not even one! There is, however, research to suggest that it might have had one long ago.

13. Pluto isn’t the only dwarf planet

When Pluto got downgraded from a planet to a dwarf planet, it joined four other bodies that are also classified as dwarf planets. One of them is Eris, one of the  largest known dwarf planets  in the solar system.  It’s also the planet that inadvertently got Pluto demoted to dwarf planet status – at first it appeared larger than Pluto, and it caused the scientific community to reappraise its criteria for planets. When all was said and done, Pluto was a dwarf planet. Stupid Eris!

14. The sun is crazy big

You’ve probably been aware that the sun is very large for a long time and don’t need anyone from the scientific community to tell you that it’s large. But do you know how large? It’s so large that it accounts for  99.86% of the entire solar system , and if that doesn’t put it in perspective for you, think of it this way – all the planets, including Earth and the dwarf planets that we can barely see from Earth live in that remaining 0.14% of the solar system that isn’t the sun.

15. Mars used to have a thicker atmosphere than it does now

While Mercury has no atmosphere to speak of, Mars has a thin atmosphere, although it’s not enough to support the existence of water on the planet. At the same time, people who have studied the planet’s surface have observed patterns that are most likely due to the presence of water at some point. Scientists believe that Mars  gradually lost atmosphere  to the force of the sun’s energy over the course of millions of years.

16. If you like solar eclipses, it’s a good thing you live on Earth

A solar eclipse is what you get when the sun, moon and Earth are perfectly aligned, causing the moon to cast its shadow on Earth. They don’t happen all that often, but you know where they  never  happen?  Anywhere else , that’s where! If you’re not on Earth, you can’t see a solar eclipse, so solar eclipse fans, stay here on Earth. It will be worth it every few years.

17. Earth is also your go-to destination for water, ice, and vapor

Earth is also the only game in town if you like living on a planet that can have water in three different states –  liquid, solid, and vapor . So if you like drinking ice water at the sauna, Earth is the only place where this will be possible.

18. Mars has the tallest known mountain in the solar system

Mount Everest, the tallest mountain on Earth, is approximately 5.5 miles high, and people have perished attempting to conquer it. Meanwhile, the tallest mountain on Mars is more than double that at  14 miles high . Sadly, both men who first climbed Everest successfully, Sir Edmund Hillary and Tenzing Norgay, have long since passed away and cannot lead an expedition to the top of that Martian mountain.

19. Neptune’s winds are the strongest in the solar system

Earlier we said that at over 1,100 miles per hour, Neptune’s winds exceed anything we have on Earth, including the speed of sound. What we didn’t say is  how much  over 1,100 miles per hour those winds could blow – at their strongest they can reach speeds of  1,304 miles per hour , easily outpacing any other planet in the solar system.

20. You emphasize the first syllable in “Uranus”

This one goes out to all the high school science teachers out there. The seventh planet from the sun has a name that has caused much mirth for schoolchildren and nonstop headaches for the people tasked with teaching them. Well, the party’s over, kids. The planet’s  name is pronounced  by emphasizing the first syllable ­– the “YER” – and the letter “A” gets pronounced not as a long vowel sound, but as a schwa. You’re welcome.

21. OK, fine, the long 'A' sound works too

Technically,  either pronunciation  of the word “Uranus” is correct, including the one with the long “A” sound. But boy, your science teacher will thank you for making the job a little easier by pronouncing it the first way. Think of it as a passive form of grade-grubbing.

22. Uranus has the coldest planetary atmosphere in the entire solar system

Venus is the hottest planet in the solar system, and the  yin  to its hot-weather  yang  is Uranus, which is the planet with the coldest temperature in the solar system.  At its coldest , it can reach temperatures of -371.56 degrees Fahrenheit.

23. Mars has the biggest dust storms in the solar system

If you like dust storms, go to Mars immediately! The planet has  the biggest ones in the entire solar system , they can last for months, and can cover the entire planet.

24. As much as half the water on Earth comes from interstellar ice

A recent model of the early solar system’s chemistry revealed something interesting about our own planet. As much as half of the water on Earth originally came  from interstellar ice  that was in abundant supply as the sun formed.

25. Uranus is the third-largest planet in the solar system

In the battle for largest planet, Uranus takes a not-too-shabby third place with a  radius of 15,759 miles . In second place is Saturn, whose 36,184 miles radius makes it 945% the size of Earth, and the winner of the biggest planet trophy is Jupiter, whose 43,441 mile radius makes it 1,120% the size of Earth.

26. Venus is the most similar planet to Earth in the solar system

Let’s say the 1996 movie  Independence Day  were to come true and belligerent aliens from a faraway galaxy came to destroy the Earth. Where would you go? Well, if size, orbit, and composition are the only issues,  your best bet is to move to Venus , which has those attributes in very similar proportion to those found here. Sure, it’s completely inhospitable to human life, but the size is very similar.

27. The planets all follow the same path in the sky

The planets in our solar system all orbit in the same direction and are all on the same plane. The path that the planets follow is called the ecliptic, which  supports the theory  that the solar system formed from a large cloud of dust and gas that condensed and began to spin.

28. It takes 240 million years for the solar system to orbit the galaxy once

If you want to be there at the exact moment the solar system completes one orbit of the galaxy, we suggest that you make other plans instead. Unless you’re planning some diet and exercise regimen that will keep you alive forever, you’re unlikely to be there at the moment of truth, since the solar system takes a full  240 million years  to orbit the galaxy just once. And you thought  The Irishman  was long!

29. If you like traveling at 1,000 miles per hour, stay on Earth

If you’re ever stuck on a long line at the Department of Motor Vehicles that just doesn’t seem to be moving, take heart! The very planet where you live, Earth, is actually spinning  at 1,000 miles per hour and orbits the sun at 67,000 miles per hour . So even if you feel like you’re going nowhere, you’re doing it really fast.

30. If you want to see what the solar system looks like, visit Sweden

Sweden isn’t just the land of meatballs and ABBA. It also has the world’s largest scale model of the solar system. According to  Sweden Solar System , the Globe in Stockholm represents the sun and the scale is 1:20 million.

31. Please clear your calendar for the next four decades

The solar system is bigger than anyone can imagine, but how big is that exactly? Just ask the Voyager 1 space probe, which was launched by NASA in 1977 to explore the outermost reaches of the solar system. 35 years later, in 2012, it  passed the sun’s gas and magnetic environment , a trip of 11 billion miles.

32. How old are Saturn’s rings? No one really knows

From Earth, the rings of Saturn seem like sleek, unbroken circles. On closer inspection, one can observe that  the rings actually consist of rock and ice debris  that’s attracted to the planet’s gravitational pull.

33. There’s only one asteroid field in the solar system that we know of

For many of us on Earth, our first exposure to an asteroid field was when Han Solo flew the Millennium Falcon through one in 1980’s  The Empire Strikes Back . As it turns out, that depiction was not 100% accurate. Apart from the fact that the only one in the solar system that we know of is  between Mars and Jupiter , the individual asteroids themselves have a lot of space between them, making Solo’s daredevil evasion tactics wholly unnecessary.

34. Uranus is stormy and tempestuous

A lot can happen in a few decades, both to individual planets and to those of us back here on Earth who like to observe those planets. When Voyager 2 flew past Uranus it looked like a very calm planet, but due to improvements in technology it’s now possible to see that the planet gets  a lot more storm activity  than previously thought. More remarkable still is that in 2007, the planet got closer to the sun.

35. You’re inside the sun right now

A lot of people think that the sun is just that fiery orb in the sky, but there’s actually a lot more of it than people realize. In fact, you’re sitting inside the sun right now. The ball of light in the sky that we see every day is  93 million miles away , but its outer atmosphere – known as the heliosphere – stretches far beyond what we can see with our eyes. The solar atmosphere not only encompasses Earth, but also reaches to such distant planets as Jupiter, Saturn, Uranus, and Neptune.

This article was produced and syndicated by MediaFeed.org .

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31. The Encounter Theories of the Origin of the Solar System

From the book a source book in astronomy and astrophysics, 1900–1975.

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