nebular hypothesis essay

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How Was the Solar System Formed? – The Nebular Hypothesis

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

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

Nebular Hypothesis:

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

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

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

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

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

History of the Nebular Hypothesis:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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September 29, 1917

17 min read

The Origin of the Solar System

An Outline of the Three Principal Hypotheses

By Harold Jeffreys

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

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44 The Nebular Theory

A protostar is an object in which no nuclear fusion has occurred, unlike a star that is undergoing nuclear fusion. A protostar becomes a star when nuclear fusion begins. Most likely the next step was that the nebula flattened into a disk called the  Protoplanetary Disk  ; planets eventually formed from and in this disk.

Three processes occurred with the nebular collapse:

• Temperatures continued to increase
• The solar nebula spun faster and faster
• The solar nebula disk flattened

The orderly motions of the solar system today are a direct result of the solar system’s beginnings in a spinning, flattened cloud of gas and dust.

Introduction to Astronomy Copyright © by Lumen Learning is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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8 The Rise and Fall of the Nebular Hypothesis

• Published: August 2023
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The first to explain the origin of the planets and moons was Pierre-Simon Laplace in his 1796 book, Exposition for the System of the World . His theory would dominate science throughout the next century and come to be accepted as a given. He held that the solar system had begun as a hot, rotating gas cloud. As it spun, centrifugal force threw off blobs of gas that coagulated into planets. The planets then repeated the process to create their moons. By the last few decades of the eighteenth century, enough evidence had come to light to call the nebular hypothesis into question, if not to falsify it. This opened the way for three different theories for the origin of the Moon. The fission theory resembled the nebular hypothesis in holding that the gravity of the Sun had pulled off a bulge in the proto-Earth which became the Moon. The co-accretion theory held that the Moon and the Earth had formed near each other and thus were like sister planets. The capture theory imagined that the Moon had started out in some distant region of the solar system but drew near enough to be captured into orbit by the Earth’s gravity.

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

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

1.2. how did our solar system form.

Grades K-2 or Adult Naive Learner

• NGSS Connections for Teachers
• Concept Boundaries for Scientists

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

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

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

Disciplinary Core Ideas

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

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

Crosscutting Concepts

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

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

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

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

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

Grades 3-5 or Adult Emerging Learner

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Grades 6-8 or Adult Building Learner

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Grades 9-12 or Adult Sophisticated Learner

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Storyline Extensions

The planets are named after stories from long ago:.

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

Our location in the galaxy:

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

solar system: The Nebular Hypothesis

• The Nebular Hypothesis

The nebular hypothesis, developed by Immanuel Kant and given scientific form by P. S. Laplace at the end of the 18th cent., assumed that the solar system in its first state was a nebula, a hot, slowly rotating mass of rarefied matter, which gradually cooled and contracted, the rotation becoming more rapid, in turn giving the nebula a flattened, disklike shape. In time, rings of gaseous matter became separated from the outer part of the disk, until the diminished nebula at the center was surrounded by a series of rings. Out of the material of each ring a great ball was formed, which by shrinking eventually became a planet. The mass at the center of the system condensed to form the sun. The objections to this hypothesis were based on observations of angular momentum that conflicted with the theory.

Sections in this article:

• Introduction
• Contemporary Theories
• The Planetesimal and Tidal Theories
• Origin of the Solar System
• Physical Properties
• Planetary Motion
• The Planets
• Bibliography

The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2024, Columbia University Press. All rights reserved.

See more Encyclopedia articles on: Astronomy: General

e-LUMINESCIENCES: the blog of Jean-Pierre Luminet

Cosmogenesis (8) : The Nebular Hypothesis

Sequel of the preceding post Cosmogenesis (7) : The Date of the Creation

The Nebular Hypothesis

The ancient Babylonians had a different idea of how the world began. They believed that it had evolved rather than being created instantaneously. Assyrian inscriptions have been found which suggest that the cosmos evolved after the Great Flood and that the animal kingdom originated from earth and water. This idea was at least partially incorporated into a monotheist doctrine and found its way into the sacred texts of the Jews, neighbors and disciples of the Babylonians. It was also taken up by the early Ionian philosophers, including Anaximander and Anaximenes, and by the Stoics and atomists.

Democritus developed a theory that the world had originated from the void, a vast region in which atoms were swirling in a whirlpool or vortex. The heaviest matter was sucked into the center of the vortex and condensed to form the earth. The lightest matter was thrown to the outside where it revolved so rapidly that it eventually ignited to form the stars and planets. These celestial bodies, as well as the earth itself, were kept in position by centrifugal force. This concept admitted the possibility that the universe contained an infinite number of objects. It also anticipated the 19th century theory of the origin of the solar system, known as the nebular hypothesis, according to which a “primitive nebula” condensed to form the sun and planets.

The idea of universal evolution had a strong influence on classical thought and developed in various directions during Greek and Roman times. In the first century BC Lucretius extended the theories of atomism and evolution to cover every natural phenomenon [i] and argued that all living things originated from earth. Two centuries later, in his medical treatise On the Use of the Parts of the Body [ii] , the Greek physician Galen (Claudius Galenus) expressed the essentially Stoic view that matter is eternal and that even God is subject to the laws of nature: contrary to the literal interpretation of the Genesis story, he could not have “formed man from the dust of the ground”; he could only have shaped the dust according to the laws governing the behaviour of matter. The Church Fathers, who insisted that the Creation was instantaneous, rejected any sort of evolutionary theory; to them the ideas of the Stoics and atomists were heretical.

In the second half of the 16th century the idea of universal evolution began to be incorporated into the new system of scientific thought resulting from the work of Copernicus, Kepler, Galileo, Descartes and Newton. According to Descartes, for example, space consisted of “whirlpools” of matter whose motion was governed by the laws of physics. Newton, with his theory of universal attraction, was accused of having substituted gravitation for providence, for having replaced God’s spiritual influence on the cosmos by a material mechanism [iii] . A new view of the world had nevertheless been established, whereby the workings of the universe were subject not to the whim of the Almighty but to the laws of physics – it was an irreversible step.

The Descartes system of whirlpools.

In the 18th century Newtonian theory came to dominate astronomical theory. The scriptures could no longer account for the origin of the world but Newton’s “uncreated” universe was no more satisfactory from a philosophical point of view. Moreover, since the earth no longer had a privileged position in relation to other celestial bodies (as it had in a geocentric universe), why should it have been created first? Science had established a new order of creation: first the stars, then the sun and finally the earth.

In the mid-18th century it began to be assumed that the early universe had been filled with some elementary fluid, a primeval substance from which the various celestial bodies had progressively emerged – an idea deriving largely from the Swedish mystic Emanuel Swedenborg. In his Prodromus Principiorum Rerum Naturalium (On the Principles of Natural Things), published in Germany in 1734, Swedenborg made the hypothesis that the planets, including the earth, had once been part of the sun and had separated themselves from it long ago; the solar system as a whole had originally been a nebula – like those we can now see in space – and the sun and the planets had only emerged as separate entities after a long period of evolution. It was therefore Swedenborg who first postulated what we now call the “nebular hypothesis”, although it is often attributed to Buffon.

In 1745, independently of Swedenborg, the French scientist had suggested ways in which celestial objects might have been formed and attempted to explain why all the planets orbited the sun in the same direction. According to Buffon the force that had created the solar system was the impact of a comet; this had thrown lumps of matter, which had been in the process of fusing with the sun, far enough from it not to be drawn back by its gravitational pull (this idea would be taken up again in the early 20th century by the English physicist James Jeans, but unsuccessfully). It is interesting to note that Buffon’s concept of opposing forces – centrifugal and gravitational – supports a myth which dates back to Heraclitus and parts of which are to be found in the Vedas: that of a great “pulsation”, a constant alternation in the balance between attraction and repulsion. Today’s astrophysicists reckon that these two forces coexist, in permanent opposition, in the solar system as well as in every galaxy.

The English scientist Thomas Wright published his major work, An Original Theory or New Hypothesis of the Universe, in 1750 and five years later completed his Universal Architecture (not published in his lifetime). His aim was nothing less than to reveal the Creator’s plan. Astronomy shows us what the universe looks like and determines our position within it but only religion, Wright argued, can give us a true picture of the Creation itself. He wanted to unify what we see through a telescope and what we know of the divine world of the Holy Trinity. The universe must therefore comprise a central region (the kingdom of God and the angels), a sphere surrounding that central kingdom (housing the sun and all the stars with their entourages of planets and living things) and a nebulous outer zone (the realm of the damned).

Despite its intention to reconcile science and religion, Wright’s work influenced rationalists like Herschel, Laplace and the German philosopher Immanuel Kant, whose Theory of the Heavens expressed a number of original ideas on cosmology. Kant applied the principles of Newtonian physics to the nebular hypothesis, giving it a consistency it had previously lacked. As far as the formation of the solar system (and of all other star systems) was concerned Kant had a grandiose vision of a primordial age when the infinite reaches of space were filled with matter, from which the planets and stars were formed. Dark and silent this veil of matter contained the seeds of the universe as we know it. Diderot’s Lettre sur les aveugles à l’usage de ceux qui voient (Note on the Blind for Those who See) of 1749 is a literary presentiment of this primeval state: “How many disfigured, misshapen worlds must have disintegrated and were perhaps being reformed and disintegrating again every second far away in space… where matter swirls and will continue to swirl in great masses until it has achieved a form in which it may survive.”

The French mathematician and astronomer, Pierre Simon, marquis de Laplace, defended the nebular hypothesis even more strongly than Kant, supporting it with mathematical reasoning as well as with reference to celestial mechanics. He proved that our solar system and other planetary and lunar systems were the result of nebulous masses acting in accordance with natural laws, as were the movements of those planets and moons and their relative sizes and distances from each other. Laplace derived his concept of a “primitive nebula” from the observations of astronomers such as Charles Messier and William Herschel, who had used the latest telescopes to catalogue hundreds of nebulous bodies. Some of these appeared to consist not of masses of stars but of clouds of opaque matter, which Laplace concluded must condense into stars. A man who constantly proclaimed, “I do not make hypotheses”, Laplace went on to make the most sensational hypothesis of the century: that the solar system had originated from a primitive nebulosity, a flat disc of slowly rotating matter, which had coalesced into lumps as it contracted and cooled. First its core had formed into a fireball (the infant sun) from which “wisps” of gas had escaped and quickly formed into rings surrounding the core; these rings, initially revolving in ellipses, then broke up into lumps, which condensed into young planets, emerging shining from their misty cocoon.

To believers in the Creation Laplace’s hypothesis was just another form of atheism, since it displaced God from His position as Creator of the stars, and opponents of the theory were delighted when telescopes revealed that some nebulosities were in fact clusters of stars: surely the same was true of all nebulae and it was only a matter of time before more powerful telescopes would prove the fact. The nebular hypothesis therefore remained unsubstantiated until the advent of spectroscopy, which allowed the light emitted by stars to be analyzed. In 1814 the German physicist Joseph von Fraunhofer discovered that the spectrum of a hot gas was broken up by dark lines (now known as Fraunhofer’s lines), caused by chemical elements in the gas. During the 1860s astronomers like Angelo Secchi in Italy and William Huggins in England undertook a systematic study of star spectra, thereby founding the discipline of astrochemistry. Like for the spectra of terrestrial objects, those of celestial objects reveal not only the presence of chemical elements, but also whether the object is solid or gaseous.

Many nebulae were thus proven to be enormous clouds of gas: no telescope, however powerful, would ever show them to consist of stars. Some of them even had a bright central point, indicating that a star was in the process of formation. The publication in the late 19th century of observations by the Irish astronomer William Parsons and by the Dane Heinrich Louis d’Arrest, accompanied by detailed drawings of nebulae, finally confirmed Laplace’s theory and established the nebular hypothesis as part of accepted cosmogony. It also proved to be a major contribution to physics, since it explained a number of the processes of star formation in terms of thermodynamics.

Scientists in various other fields of research dealt further blows to the traditional Creation myth: Darwin, of course, with his theory of evolution, but also archaeologists and philologists, who were studying ancient monuments and hieroglyphs. Historians such as Oppert, Rawlinson and Smith [iv] had succeeded in deciphering the inscriptions found in the great library of Assurbanipal (Sardanapalus) at Nineveh and an account of a deluge in the Epic of Gilgamesh appeared to be the source not only of the Babylonian myths but also of the story of the Great Flood in the bible. Genesis could therefore no longer be considered a reliable account of the Creation, as revealed to Moses by God Himself; it had become just one of many stories about the origin of the world, all of which had been influenced by various cultures and reflected the scientific knowledge of the people who had first conceived and recounted them.

[i] De Natura Rerum, book V.

[ii] De UsuPartium Corporis Humani, 11.14.

[iii] On this controversy see for example McCosh, The Religious Aspect of Evolution, New York, 1890, pp. 103-04.

[iv] George Smith, Chaldean Account of Genesis, New York, 1876, pp. 74-75.

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I felt dizzy and wept, for my eyes had seen that secret and conjectured object whose name is common to all men but which no man has looked upon — the unimaginable universe. Jorge luis Borges, The Aleph (1949)

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Nebular Theory Might Explain How Our Solar System Formed

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Our solar system contains the sun, inner rocky planets, the gas giants , or the outer planets, and other celestial bodies, but how they all formed is something that scientists have debated over time.

The nebular theory , also known as nebular hypothesis , presents one explanation of how the solar system formed. Pierre-Simon, Marquis de Laplace proposed the theory in 1796, stating that solar systems originate from vast clouds of gas and dust, known as solar nebula, within interstellar space.

Learn more about this solar system formation theory and some of the criticism it faced.

What Is the Nebular Theory?

Criticisms of the nebular theory, solar nebular disk model.

Laplace said the material from which the solar system and Earth derived was once a slowly rotating cloud, or nebula, of extremely hot gas. The gas cooled and the nebula began to shrink. As the nebula became smaller, it rotated more rapidly, becoming somewhat flattened at the poles.

A combination of centrifugal force, produced by the nebula's rotation, and gravitational force, from the mass of the nebula, left behind rings of gas as the nebula shrank. These rings condensed into planets and their satellites, while the remaining part of the nebula formed the sun.

The planet formation hypothesis, widely accepted for about a hundred years, has several serious flaws. The most serious concern is the speed of rotation of the sun.

When calculated mathematically on the basis of the known orbital momentum, of the planets, the nebular hypothesis predicts that the sun must rotate about 50 times more rapidly than it actually does. There is also some doubt that the rings pictured by Laplace would ever condense into planets.

In the early 20th century, scientists rejected the nebular hypothesis for the planetesimal hypothesis, which proposes that planets formed from material drawn out of the sun. This theory, too, proved unsatisfactory.

Later theories have revived the concept of a nebular origin for the planets. An educational NASA website states: "You might have heard before that a cloud of gas and dust in space is also called a 'nebula,' so the scientific theory for how stars and planets form from molecular clouds is also sometimes called the Nebular Theory. Nebular Theory tells us that a process known as 'gravitational contraction' occurred, causing parts of the cloud to clump together, which would allow for the Sun and planets to form from it."

Victor Safronov , a Russian astronomer, helped lay the groundwork for the modern understanding of the Solar Nebular Disk Model. His work, particularly in the 1960s and 1970s, was instrumental in shaping our comprehension of how planets form from a protoplanetary disk.

At a time when others did not want to focus on the planetary formation process, Safronov used math to try to explain how the giant planets, inner planets and more came to be. A decade after his research, he published a book presenting his work.

George Wetherill's research also contributed to this area, specifically on the dynamics of planetesimal growth and planetary accretion.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.

Please copy/paste the following text to properly cite this HowStuffWorks.com article:

The Solar System’s Nebular Model Essay

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Introduction

Pre-solar nebular, planetesimals, formation of the moons, features explained by nebular model.

Nebular model, an explanation about the origin of the solar system was first proposed by Laplace in 1796. He suggested that the matter from which the solar system formed was at one time a nebula or a slowly rotating cloud of hot gas and dust. The dust and gas cooled and the cloud began to shrink. As the cloud became smaller, it began to spin more rapidly, and somehow became flattened. The rotation could have resulted from a combination of centrifugal forces while gravitational force caused the fragments of gaseous matter to be left behind. The rings fused into planets and moons while the larger part of the cloud formed the sun.

The nebular model is the most widely accepted hypothesis in cosmology, but has several flaws. First concerns about the speed of the rotating sun. The model predicts the speed of the rotating sun to be 50 times fast that its actual speed. Secondly, there are doubts that the rings hypothesized to form the planets would ever condense. However, this model seems to explain most of the phenomena observed in our solar system. These have been considered to be the evidence of the theory for a very long time. This has been supported by scans of the universe which indicate the process to be taking place elsewhere. In response to this, the paper is aimed at explaining the nebular model of the solar system in details and the features of the solar system that the model can explain. Despite the many theories about the formation of the solar system, the nebular model seems to be the most inclusive and which is associated with observable evidence.

The nebular model maintains that our solar system began to form when a fragment of a giant cloud of dust and gas began to collapse due to the gravitational forces exceeding the forces related to gas pressure that expanded it (Montmerle, et al, 2006. p.47). This collapse was triggered by a range of perturbations such as density waves in rotating galaxies and a supernova blast wave. Montmerle and others (2006) assert that the cloud had the size of about 20 pc while the collapsing fragment was about 1pascecs across (p.47). The fragments continued to collapse resulting to the formation of dense cores, about 0.01 to 0.1 pc in size11. The pre-solar nebular (one of the collapsing fragments) was to form our solar system. The mass of this fragment which was to form the sun was just larger that the mass of the sun. This part contained elements like hydrogen, helium as well as lithium while the other included heavier elements formed earlier.

Stable daughter nuclei of transitory isotopes like iron-60 which form only in exploding transitory stars have been revealed from studies of earliest meteorites. Therefore, a supernova must have occurred in a region near the sun. it is likely that the hypothesized formation of the sun was initiated by the supernova shock waves. The nebular became denser and caused the fragmentation. And since only enormous, transitory stars produce supernova, this formation must have taken place in the region where massive stars are formed, probably similar to Orion Nebular (Hester, Desch, Healy & Leshin, 2004. p.1117). Revelations from Kuiper belt and the strange materials it contains suggest that the formation of the sun occurred within a cluster of stars. The width of the cluster was between 6.5 and 19.5 light years and a mass of about 3,000 suns (Simon & Zwart, 2009. p.13).

In the Nebular model, the collapsing cloud begins to spin faster and faster because of the angular momentum being conserved. The condensation of the matter in the cloud was characterized by the bombardments of the molecules with escalating frequency, and their kinetic energy changing into heat energy. The core of the nebular, where the mass was concentrated, acquired much heat than the surrounding regions. For many years, the competing forces associated with gas pressure, gravity, rotation, and magnetic fields caused the contracting cloud to flatten into a spinning pancake shape (protoplanetary disc) and formed a hot, dense protostar or a star prior to hydrogen fusion at the core (Greaves, 2005. p.68).

During this stage in the formation of solar system, the sun is suggested to have been developed into a T Tauri star. In the presence of T Tauri star, it means that there are protoplanetary plates having smaller masses than the star itself. These discs may extend to several hundred light years and are somewhat cool, with the highest temperature being a thousand kelvins only (Küker, Henning & Rüdiger, 2003. p.397).

The Hubble Space Telescope observed potoplanetary discs that are 1,000 AU wide in regions where stars are formed like the Orion Nebular. With time, the temperature and pressure of the center of the sun went to an extent that the hydrogen gas contained started to fuse and created a source of energy within the disc that counteracted the gravitational contraction leading to a hydrostatic balance. At this juncture, the sun entered into the principal phase of its evolution, often referred to as the main sequence. Stars in this phase obtain energy from hydrogen fusion in their centers. Since the formation halted, the sun has existed as a main sequence.

According to nebular model, the planets in our solar system formed from the same nebula as the sun, the sola nebula. The cloud fragments left from the formation of the sun were responsible for the planets formation (Boss, & Durisen, 2005. p.137). The common and accepted method by which this formation took place is referred to as accretion. In this method, the planets started out as dust particles orbiting around the inner protostar. These particles collected into bigger objects through fusion and later collided to form planetesimals. Through further collisions, these bodies ultimately increased in size at a rate of several centimeters annually for several million years that followed.

The solar nebular formed from very hot gases and dust and the heat could not allow volatile molecules to condense. Therefore, only silicates and metals (heavier elements) were the only constituents in the formation of terrestrial planets. These rocky planets became the so called terrestrial and include: Mercury the closest to sun; Venus the second nearest, Earth the third nearest and finery Mars the farthest of the rocky planets. Compounds with high melting points are very scarce in the universe and so the rocky planets grew to relatively small sizes. The terrestrial planetesimals grew to a small fraction of the Earth masses and stopped accumulating matter approximately 100,000 years after the sun was formed. Through collision and fusions of the terrestrial embryos that followed, the rocky planets are believed to have grown into their present sizes (Lin, 2008. p.58).

During the formation of the rocky planets, these planets remained engrossed in a gas and dust cloud disc. And because the gas had its own pressure plus the gravitational pressure, it orbited slower than the forming planets. The difference in pressure resulted in a drag which changed the angular momentum causing the planets to eventually move to new positions. The temperature differences in the disc controlled the rate by which the planets moved, yet the overall trend was for the planets nearest to the core to move inward as the nebular dissipated, leaving them in their present orbits.

Jovian planets essentially formed further away from the sun. This is past the snow line which is the area between Jupiter and Mars. In this region, the temperatures were lower and volatile elements could condense. These materials constituted the larger part of the Jovian planets. These compounds are more abundant in the universe than silicates and metals that formed the rocky planets. As the planet increased in size, they were able to consume the lighter gases that were most abundant in the solar nebular. Formation of the planets past the snow line collected to several times the earth masses in a period of about three million years. At present the Jovian planets comprise almost 99 percent of the total mass rotating around the sun.

Furthermore, it is believed that the existence of Jupiter near the snow line is not an accident. As the falling ice approached the snow boundary, it encountered a change in temperature and evaporated causing the surrounding area to accumulate a lot of water. This resulted in a reduction of the pressure and dust particles could spiral faster and thus stopped moving towards the sun. Effectively, the snow line formed a barrier that made the matter to accumulate fast at a short distance from the sun. The excess matter combined into a large body of several Earth masses that grew swiftly by acquiring hydrogen from the adjacent disc to the largest planet in the solar system. The lower masses for Saturn resulted from its later formation when most of the gas to consume had been swallowed by Jupiter.

All the T Tauri stars including the sun are characterized by strong stellar wind. Other stable stars may have weaker winds. Neptune and Uranus must have formed after Saturn and Jupiter, when the stellar winds had cleared much of the matter within the disc. Therefore, the planets acquired very small amounts of hydrogen and helium, probably one Earth mass each. In effect, the two planets are usually called the “failed cores”. Though, the formation assumptions of these planets bring a problem relating to the time taken to form them.

The actual distance of the planet from the sun suggests that the planets formation or the accretion could have taken a much longer period of time. This means that the formation of the planets took place a closer distance from the sun…probably near or between Saturn and Jupiter…and they later migrated to their present positions (Levison et al., 2007. p.258). Planet migration was on both directions during their formation, either to the warmer region or the cold regions. The growth of the planetisimals could have halted after many years when the strong stellar winds forced the material out of the solar nebular into interstellar region.

Moons have been known to revolve around many planets as well as other bodies in the Solar system. Three mechanisms could have been responsible for the formation of the moons and include: from a solar disc through co-formation, from bombarding fragments and confinement of the passing objects.

Saturn and Jupiter have several large moons including Europa, Io, Titan, and Ganymede, which might have formed from discs surrounding each planet in a similar way the giant planets originated from the discs surrounding the sun (Takato, et al., 2004. p.2224). This formation is indicated by the nearness of the moons to the planets and their relatively larger sizes. The indicated attributes cannot be attained through capture neither can the moons form from bombardment fragments due to their gaseous nature. Moons further away from the giant planets tend to have smaller sizes and have peculiar orbits with random inclinations. Only captured bodies could be having such characteristics. Unfortunately, such moons have been reported to be revolving in the opposite direction. Evidence on the capturing of passing objects is based on Triton which is a moon of Neptune which has many irregularities. This moon could probably have been captured with Kuiper belt.

The formation of the moons within the terrestrial planets could probably have been a consequence of collision or capture. Phobos as well as Deimos is believed to be captured asteroids outside the snow line. Stevenson (1987) suggested that the Earth’s moon might have formed from a particular, slanting collision (p.271). The object that caused the impact had a mass similar to that of Mars, while the collision possibly took place as the end of giants bombardment period approached. The bombardment released some of the fragments into the orbit, which then united to form the moon (Canup & Asphaug, 2001. p.710). Perhaps, the collision was the last in the sequence of fusions that formed the primary. The earth-Sun Lagrangian points are also believed to be some of the areas where the moons could have been formed. Charon, a moon in Pluto is believed to have formed from a large collision.

The nebular model explains many features of the sola system including:

• Density difference between Jovial and terrestrial planets
• Terrestrial planets have fewer and smaller moons than Jovian planets
• Most of the planets carry a disc shape
• All the planets revolve in one direction

The terrestrial planets have approximately similar densities as none accreted much of the low-density material, which still existed in vapour form in the region near the core of the solar system. Moving towards Mars and Asteroids, volatile condensates such as water are more available leading to lower densities. However, the bodies could grow big enough to allow for the gravitational swallowing of gases. Saturn and Jupiter grew large enough to accrete gasses which lowered their densities. The increasing density of the outermost planets is due to the low-density methane condensates which makes the planets to have higher levels of heavy materials which increase the density.

The nebular model explains the formation of moons to have occurred as a result of co-formation, impact fragments, and capture of passing objects. Only co-formation was possible in the region beyond frost line and resulted in moon almost the size of terrestrial planets. The circum-planetary disc as a result of solar nebular fragmentations resulted in the formation of many fragments that fused to form the planets and moons. However, the moons in the terrestrial planets formed as a result of bombardments and capture of the passing objects.

The collisions were not very frequent and could have occurred outside the terrestrial region sending the fragments into the interstellar region. In addition, the passing objects could only be captured within the Kuiper belt region in order to form a moon. Unfortunately, many objects passed outside this region and thus only a few could be captured.

Due to the competing forces from gas pressure, gravitational force, and angular momentum, the constricting nebular starts to flatten resulting into a spinning flattened object with a swelling at the core. The decreasing angular momentum support close to the poles indicates that the material will easily fall close to the top, and not at the equator. Therefore, there results a swell which finally leads to the disk shape. The disc should not necessarily be flat, but is typically thicker on the outside than inside.

The entire solar system including the sun and the planets were formed from the solar nebular in which an angular motion was involved. In turn, the resulting fragments also spiraled in the same direction but with varying velocities due to their accumulating materials. As the disc materials reduced and the stellar winds brew them into the interstellar region, the revolving bodies could not change the aspects that defined their flow and rotated on a specific orbit at a specified speed. However, the nebular model does not explain satisfactorily about the moons that rotate in the opposite direction, yet it claims to have been formed together with the planets. The only explanation could be that the moons formed from either collision outside the solar system or capture of a passing object to assume their present direction. There is also a possibility that the moons collided with other bodies and thus changing their direction of rotation.

The nebular model of the solar system is a comprehensive theory that explains the origin of the solar system basing on the existence of a cloud of gases and dust. The cloud contracted to form a disc-shaped nebular. The nebular then contracted to form small planetisimals. The planetisimals fused to form the planets. The moons could have formed from bombardments or capture of passing objects, but most of the larger moons for the jovial planets formed just like the planets. The nebular theory can explain why the planets revolve in the same direction or why most planets are disc shaped. It can also explain the difference in densities between jovial planets and terrestrial planets and why the rocky planets have fewer moons that the giant planets.

Boss, A. P. & Durisen, R. H. 2005. Chondrule-forming shock fronts in the solar nebula: a possible unified scenario for planet and chondrite formation. The Astrophysical Journal , 621(2), pp.137–140. Web.

Canup, R. M. & Asphaug, E. 2001. Origin of the Moon in a giant impact near the end of the Earth’s formation. Nature , 412 (6848), pp.708–12. Web.

Greaves, J. S. 2005. Disks around stars and the growth of planetary systems. Science , 307 (5706), pp.68. Web.

Hester, J. J., Desch, S. J., Healy, K. R. & Leshin, L. A. 2004. The cradle of the Solar System. Science , 304 (5674), pp.1116-1117. Web.

Küker, M., Henning, T. & Rüdiger, G. 2003. Magnetic star-disk coupling in classical t Tauri systems. Astrophysical Journal , 589 (1), pp.397. Web.

Levison, H. F. et al 2007. Origin of the structure of the Kuiper belt during a dynamical instability in the orbits of Uranus and Neptune. Icarus , 196 (1), pp.258. Web.

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• Published: 03 February 1870

Where are the Nebulæ?

• HERBERT SPENCER 1  

Nature volume  1 ,  pages 359–360 ( 1870 ) Cite this article

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MR. PROCTOR'S interesting paper in your last number reminded me of an essay on “The Nebular Hypothesis,” originally published in 1858, and re-published, along with others, in a volume in 1863 (“Essays: Scientific, Political, and Speculative.” Second Series), in which I had occasion to discuss the question he raises. In that essay I ventured to call in question the inference drawn from the revelations of Lord Rosse's telescope, that nebulæ are remote sidereal systems—an inference at that time generally accepted in the scientific world. On referring back to this essay, I find that, besides sundry of the reasons enumerated by Mr. Proctor for rejecting this inference, I have pointed out one which he has omitted.

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• Journal of Emerging Technologies and Innovative Research - The Origin of Solar System "The Nebular Model"
• IOPscience - Mass Distribution and Planet Formation in the Solar Nebula
• Geosciences LibreTexts - Origin of the Solar System—The Nebular Hypothesis

solar nebula , gaseous cloud from which, in the so-called nebular hypothesis of the origin of the solar system , the Sun and planets formed by condensation. Swedish philosopher Emanuel Swedenborg in 1734 proposed that the planets formed out of a nebular crust that had surrounded the Sun and then broken apart. In 1755 the German philosopher Immanuel Kant suggested that a nebula in slow rotation, gradually pulled together by its own gravitational force and flattened into a spinning disk, gave birth to the Sun and planets. A similar model, but with the planets being formed before the Sun, was proposed by the French astronomer and mathematician Pierre-Simon Laplace in 1796. During the late 19th century the Kant-Laplace views were criticized by the British physicist James Clerk Maxwell , who showed that, if all the matter contained in the known planets had once been distributed around the Sun in the form of a disk, the shearing forces of differential rotation would have prevented the condensation of individual planets. Another objection was that the Sun possesses less angular momentum (dependent on the total mass, its distribution, and the speed of rotation) than the theory seemed to require. For several decades most astronomers preferred the so-called collision theory, in which the planets were considered to have been formed as a result of a close approach to the Sun by some other star . Objections to the collision theory more convincing than those against the nebular hypothesis were raised, however, especially as the latter was modified in the 1940s. The masses of the original planets ( see protoplanet ) were assumed to be larger than in the earlier version of the theory, and the apparent discrepancy in angular momentum was attributed to magnetic forces connecting the Sun and planets. The nebular hypothesis has thus become the prevailing theory of the origin of the solar system.

The Nebular Hypothesis - A False Paradigm Misleading Scientists

• Myers, L. S.

Science has reached a turning point in history after being misled for 250 years by Immanuel Kant's nebular hypothesis, the most fundamental assumption in science. The nebular hypothesis assumes all nine planets were created 4.5 billion years ago (Ga) as molten bodies that cooled with the same size and chemical composition they have today. Reevaluation of the nebular hypothesis proves it has been wrong since its inception. The proof has lain in plain sight for centuries-coal beds that could not have existed at the assumed time of creation because they formed on Earth's surface after creation of the planet when forests and swamps were exposed to solar energy. The coal beds were subsequently buried under overburden accreted in later millennia, steadily increasing Earth's mass and diameter. The coal beds and layers of overburden are proof Earth was not created 4.5 Ga but is growing and expanding by accretion of extraterrestrial mass and core expansion-a process termed "Accreation" (creation by accretion). Each process accelerates over time, but internal expansion exceeds the rate of external accretion. Because the nebular hypothesis is erroneous researchers assumed Earth's diameter never changes, and, faced with the possibility the Earth might be expanding after the Atlantic basin was discovered to be widening, this assumption led to the unworkable concept of subduction to maintain a constant diameter Earth. Subduction will prove to be one of the greatest errors in the history of science. Nullification of the nebular hypothesis also nullifies subduction and rejuvenates Carey's earth expansion theory. Accreation provides Carey's missing energy source and mechanism of expansion. Expansion is proved by morphologic evidence today's continents were once a single planetary landmass on a smaller Earth when today's oceans, covering 70% of the planet, did not exist 200-250 Ma. Despite hundreds of tons of meteorites and dust known to accrete daily, its cumulative effect has been ignored in the belief this comparatively small volume is insignificant relative to Earth's total mass and gravity. This misconception led to outdated gravitational constants and trajectories for "slingshotted" space missions that approached Earth closer than anticipated because the daily increase in mass increases Earth's gravitational pull. Today's philosophy assumes comets, meteoroids, asteroids and planets are different types of objects because of their varied sizes and appearances, but when all solar bodies are arranged by size they form a continuum from irregular meteoroids (remnants of comets) to spherical asteroids and planets. When meteoroids reach diameters of 500-600 kilometers, they become spherical-the critical threshold at which gravity can focus total molecular weight of any body omnidirectionally onto its exact center to initiate compressive heating and melting of originally cold rock core, producing magma, H2O and other gases. The Accreation concept assumes all solar bodies are different-sized objects of the same species, each having reached its present size and chemical composition by amalgamation and accretion. Each is at a different stage of growth but destined to become larger until it reaches the size of another sun (star). This is universal planetary growth controlled by gravity, but initiated by the trajectory imparted at its supernova birth and chance capture by some larger body elsewhere in the Universe. Like the paradigm shift from geocentrism to heliocentrism sparked by Copernicus in 1543, the time has come for a new paradigm to put scientific research on a more productive course toward TRUTH. The new concept of Accreation (creation by accretion) is offered as a replacement for the now defunct nebular hypothesis.

• 1200 GEODESY AND GRAVITY;
• 3010 Gravity;
• 3040 Plate tectonics (8150;
• 5455 Origin and evolution;
• 8125 Evolution of the Earth

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