formation and evolution of the Solar System

Main articles: Solar System and Stellar evolution

The theories concerning the formation and evolution of the Solar System are complex and varied, interweaving various scientific disciplines, from astronomy and physics to geology and planetary science. Over the centuries, many theories have been advanced as to its formation, but it was not until the eighteenth century that the development of the modern theory took shape. With the dawn of the space age, the images and structures of other worlds in the solar system refined our understanding, while advances in nuclear physics gave us our first glimpse of the processes which underpinned stars, and led to the first theories of their formation and ultimate destruction.

Artist's conception of a protoplanetary disc

History of solar system formation hypotheses

The current hypothesis of the Solar System's formation is the nebular hypothesis, first proposed in 1734 by Emanuel Swedenborg.^[1] A similar theory was independently formulated by Pierre-Simon Laplace in 1796.^[2] During the late 19th century the Kant-Laplace nebular hypothesis was criticized by James Clerk Maxwell, who showed that if the matter of the known planets had once been distributed around the Sun in the form of a disk, forces of differential rotation would have prevented the condensation of individual planets. Another objection was that the Sun possesses less angular momentum than the Kant-Laplace model indicated. For several decades, most astronomers preferred the near-collision hypothesis, in which the planets were considered to have been formed due to the approach of some other star to the Sun. This near-miss would have drawn large amounts of matter out of the Sun and the other star by their mutual tidal forces, which could have then condensed into planets.

Objections to the near-collision hypothesis were also raised and, during the 1940s, the nebular model was improved such that it became broadly accepted. In the modified version, the mass of the original protoplanet was assumed to be larger, and the angular momentum discrepancy was attributed to magnetic forces. That is, the young Sun transferred some angular momentum to the protoplanetary disk and planetesimals through Alvén waves, as is understood to occur in T Tauri stars.

The refined nebular model was developed entirely on the basis of observations of our own solar system, because it was the only one known until the mid 1990's. It was not confidently assumed to be widely applicable to other planetary systems, although scientists were anxious to test the nebular model by finding of protoplanetary disks or even planets around other stars, so-called extrasolar planets.

Stellar nebula or protoplanetary disks have now been observed in the Orion nebula, and other star-forming regions, by astronomers using the Hubble Space Telescope. Some of these are as large as 1000 AU in diameter.

As of November 2006, the discovery of over 200 exoplanets^[3] has turned up many surprises, and the nebular model must be revised to account for these discovered planetary systems, or new models considered. There is no consensus on how to explain the observed 'hot Jupiters,' but one leading idea is that of planetary migration. This idea is that planets must be able to migrate from their initial orbit to one nearer their star, by any of several possible physical processes, such as orbital friction while the protoplanetary disk is still full of hydrogen and helium gas.

In recent years, an alternative model for the formation of the solar system, the Capture Theory, has been developed. This theory holds that the gravity of a passing object drew material out of the Sun, which then cooled and condensed to form the planets. It is claimed that this model explains features of the solar system not explained by the Solar Nebula Theory. However, the Capture theory has been criticised as it predicts a different age for the Sun than for the planets, whereas evidence indicates that the Sun and the rest of the Solar System formed at roughly the same time, in line with the more widely accepted models.

Estimation of age

Using evidence provided by radiometric dating, scientists estimate that the solar system is 4.6 billion years old. The oldest rocks on Earth are approximately 3.9 billion years old. Rocks this old are rare, as the Earth's surface is constantly being reshaped by erosion, volcanism and plate tectonics. To estimate the age of the solar system scientists must use meteorites, which were formed during the early condensation of the solar nebula. The oldest meteorites (such as the Canyon Diablo meteorite) are found to have an age of 4.6 billion years, hence the solar system must be at least 4.6 billion years old.^[4]

Nebular hypothesis

Main article: nebular hypothesis

Pre-solar nebula

Hubble image of protoplanetary discs in the Orion nebula, a light years-wide "stellar nursery" likely very similar to the primordial nebula from which our Sun formed

The nebular theory maintains that 4.6 billion years ago, the Solar System formed from the gravitational collapse of a giant molecular cloud. This initial cloud was likely several light-years across and played host to the birth of several stars.^[5] Although the process was initially viewed as relatively tranquil, recent studies of ancient meteorites reveal traces of elements only formed in the hearts of very large exploding stars, indicating that the environment in which the Sun formed was within range of a number of nearby supernovas. The shock wave from these supernovas may have triggered the formation of the Sun by creating regions of overdensity in the surrounding nebula, causing them in turn to collapse,^[6] and may have altered the composition of the early Solar System.^[7]

One of these regions of collapsing gas (known as the pre-solar nebula)^[8] would form what became the Sun. This region had a diameter of between 7000 and 20,000 AU^[5][9] and a mass just over that of the Sun (between 1.001 and 1.1 solar masses).^[10] Its composition was believed to be about the same as the Sun today: about 98% (by mass) hydrogen and helium present since the Big Bang, and 2% heavier elements created by earlier generations of stars which died and ejected them back into interstellar space (see nucleosynthesis).

As the nebula collapsed, conservation of angular momentum meant that it spun faster. As the material within the nebula condensed, the atoms within it began to collide with increasing frequency, causing them to release energy as heat. The centre, where most of the mass collected, became increasingly hotter than the surrounding disc.^[5] As the competing forces associated with gravity, gas pressure, magnetic fields, and rotation acted on it, the contracting nebula began to flatten into a spinning protoplanetary disk with a diameter of roughly 200 AU^[5] and a hot, dense protostar at the center.^[12]

Studies of T Tauri stars, young, pre-fusing solar mass stars believed to be similar to the Sun at this point in its evolution, show that they are often accompanied by discs of pre-planetary matter.^[10] These discs extend to several hundred AU and are rather cool, reaching only a thousand kelvins at their hottest.^[13] After 100 million years, the temperature and pressure at the core of the Sun became so great that its hydrogen began to fuse, creating an internal source of energy which countered the force of gravitational contraction until hydrostatic equilibrium was achieved. At this point the Sun became a fully fledged star.^[14]

Formation of planets

See also: Protoplanetary_disk

From this cloud and its gas and dust (the "solar nebula"), the various planets are thought to have formed. The currently accepted method by which the planets formed is known as accretion, in which the planets began as dust grains in orbit around the central protostar, which initially formed by direct contact into clumps between one and ten kilometres in diameter, which in turn collided to form larger bodies (planetesimals), of roughly 5 km in size gradually increasing by further collisions by roughly 15 cm per year over the course of the next few million years.^[15]

The inner solar system was too warm for volatile molecules like water and methane to condense, so the planetesimals which formed there were relatively small (comprising only 0.6% the mass of the disc)^[5] and composed largely of compounds with high melting points, such as silicates and metals. These rocky bodies eventually became the terrestrial planets. Farther out, the gravitational effects of Jupiter made it impossible for the protoplanetary objects present to come together, leaving behind the asteroid belt.^[16]

Farther out still, beyond the frost line, where more volatile icy compounds could remain solid, Jupiter and Saturn were able to gather more material than the terrestrial planets, as those compounds were more common. They became the gas giants, while Uranus and Neptune captured much less material and are known as ice giants because their cores are believed to be made mostly of ices (hydrogen compounds).^[17][18]

The young Sun's solar wind then cleared away all the gas and dust in the protoplanetary disk, blowing it into interstellar space, thus ending the growth of the planets. T-Tauri stars have far stronger stellar winds than more stable, older stars.^[19][20]

Problems with the solar nebula model

One problem with this hypothesis is that of angular momentum. With the vast majority of the system's mass accumulating at the center of the rotating cloud, the hypothesis predicts that the vast majority of the system's angular momentum should accumulate there as well. However, the Sun's rotation is far slower than expected, and the planets, despite accounting for less than 1 percent of the system's mass, thus account for more than 90 percent of its angular momentum. One resolution of this problem is that dust grains in the original disc created drag which slowed down the rotation in the center.^[21]

Planets in the "wrong place" are a problem for the solar nebula model. Uranus and Neptune exist in a region where their formation is highly implausible due to the reduced density of the solar nebula and the longer orbital times in their region. Furthermore, the hot Jupiters now observed around other stars cannot have formed in their current positions if they formed from a "solar nebula" too. These issues are dealt with by assuming that interactions with the nebula itself and leftover planetesimals can result in planetary migrations.

The detailed features of the planets are yet another problem. The solar nebula hypothesis predicts that all planets will form exactly in the ecliptic plane. Instead, the orbits of the classical planets have various (but admittedly small) inclinations with respect to the ecliptic. Furthermore, for the gas giants it is predicted that their rotations and moon systems will also not be inclined with respect to the ecliptic plane. However most gas giants have substantial axial tilts with respect to the ecliptic, with Uranus having a 98° tilt. The Moon being relatively large with respect to the Earth and other moons which are in irregular orbits with respect to their planet is yet another issue. It is now believed these observations are explained by events which happened after the initial formation of the solar system.

Subsequent evolution

The planets were originally believed to have formed in or near the orbits at which we see them now. However, this view has been undergoing radical change during the late 20th century and the beginning of the 21st century. Currently, it is believed that the solar system looked very different after its initial formation, with five objects at least as massive as Mercury being present in the inner solar system (instead of the current four), the outer solar system being much more compact than it is now, and the Kuiper belt starting much farther in than it does now.

Impacts are currently believed to be a regular (if infrequent) part of the development and evolution of the solar system. In addition to the Moon-forming impact, the Pluto-Charon system is believed to be the result of a collision between Kuiper Belt objects. Other cases of moons around asteroids and other Kuiper Belt objects are also believed to be the result of collisions. That collisions continue to happen is evidenced by the collision of Comet Shoemaker-Levy 9 with Jupiter in 1994, and the impact feature Meteor Crater in the US state of Arizona.

Inner solar system

See also: Giant impact hypothesis

According to the currently accepted view, the inner solar system was "completed" by a giant impact in which the young Earth collided with a Mars-sized object (being the "fifth" inner solar system object alluded to above). This impact resulted in the formation of the Moon. The current speculation is that this Mars-sized object formed at one of the stable Earth-Sun Lagrangian points (either L₄ or L₅) and later drifted away from that position.

In the solar nebula model, the only other way terrestrial planets can get moons is by capturing them. Mars' two tiny low-altitude moons are clearly asteroids, and other examples of captured satellites abound in the jovian systems.

Asteroid belt

Main article: Asteroid belt

Under the solar nebula hypothesis, the asteroid belt initially contained more than enough matter to form a planet, and, indeed, a large number of planetesimals formed there. However, Jupiter formed before a planet could form from these planetesimals. Because of the large mass of Jupiter, orbital resonances with Jupiter govern orbits in the asteroid belt. These resonances either scattered the planetesimals away from the asteroid belt or held them in narrow orbital bands and prevented them from consolidating. What remains are the last of the planetesimals created initially during the formation of the solar system.

The effects of Jupiter have scattered most of the original contents of the asteroid belt, leaving less than the equivalent of 1/10th of the mass of the Earth. The loss of mass is the chief factor that prevents the asteroid belt from consolidating into a planet. Objects with very large mass have a gravitational field great enough to prevent the loss of large amounts of material as a result of a violent collision. In the asteroid belt this usually is not the case. As a result, many larger objects have been broken apart, and sometimes newer objects have been forced out of the remnants in less violent collisions. Evidence of collisions can be found in the moons around some asteroids, which currently can only be explained as being consolidations of material flung away from the parent object without enough energy to escape it.

Outer planets

See also: Gas giant

The larger protoplanets were sufficiently massive to accrete gas from the protoplanetary disk, and it is believed their mass distribution may be understood from their positions in the disk, although such an explanation is too simple to account for many other planetary systems. In essence, the first jovian planetesimal to reach the critical mass required to capture helium gas and subsequently hydrogen gas is the most interior one, because - compared to orbits farther from the Sun - here orbital speeds are higher, the density in the disk is higher, and collisions happen more frequently. Thus Jupiter is the largest jovian because it swept up hydrogen and helium gas for the longest period of time, and Saturn is next. The composition of these two is dominated by the captured hydrogen and helium gases (about 97% and 90% by mass, respectively).

The Uranus and Neptune protoplanets reach the critical size for collapse significantly later, and thus captured less hydrogen and helium, which presently makes up about only about 1/3 of their total mass.

Image showing Outer Planets and Kuiper Belt: a)Before Jupiter/Saturn 2:1 resonance b)Scattering of Kuiper Belt objects into the solar system after the orbital shift of Neptune c)After ejection of Kuiper Belt bodies by Jupiter

Following gas capture, the outer solar system is now believed to have been shaped by planetary migrations. As the gravity of the planets perturbed the orbits of the Kuiper belt objects, many were scattered inwards by Saturn, Uranus, and Neptune, while Jupiter often kicked those objects out of the solar system altogether. As a result, Jupiter migrated inwards while Saturn, Uranus, and Neptune migrated outwards. A major breakthrough in the understanding of how this led to the current solar system structure occurred in 2004. In that year, new computer models showed that if Jupiter started out taking fewer than two orbits around the Sun for every time that Saturn orbited the Sun once, this migration pattern would put Jupiter and Saturn into a 2:1 resonance when the orbital period of Jupiter became exactly half that of Saturn's. This resonance would have put Uranus and Neptune into highly elliptical orbits, with there being a 50% chance that they would have exchanged places. The object which ended up being outermost (Neptune) would then be forced outwards into the Kuiper belt as it initially existed.

The subsequent interaction between the planets and the Kuiper belt after Jupiter and Saturn passed through the 2:1 resonance can explain the orbital charactertistics and axial tilts of the giant outer planets. Uranus and Saturn end up where they are due to interactions with Jupiter and each other, while Neptune ended up at its current location because that is where the Kuiper Belt initially ended. The scattering of Kuiper belt objects could explain the late heavy bombardment which occurred approximately 4 billion years ago.^[22]

Kuiper belt, Oort cloud and Late Heavy Bombardment

Main article: Late Heavy Bombardment

The Kuiper Belt was initially an outer region of icy bodies which lacked enough of a mass density to consolidate. Originally its inner edge would have been just beyond the outermost of Uranus and Neptune when they formed. (This is most likely in the range of 15 - 20 A.U..) The outer edge was at approximately 30 A.U. The Kupier Belt initially "leaked" objects into the outer solar system, and caused the initial planetary migrations.

Long after the solar wind cleared the gas out of the disk, a large population of planetesimals remained behind, as yet unnaccreted by any planetary body. This population was thought to exist primarily beyond the outer planets, where planetesimal accretion times were so long that a planet was unable to form before gas dispersal. The outermost giant planet interacted with this 'planetesimal sea', scattering these small rocky bodies inwards, while itself moving outwards. These planetesimals then scattered off the next planet they encountered in a similar manner, and the next, moving the planets' orbits outwards while the planetesimals moved inwards.

Eventually the Jupiter-Saturn 2:1 orbital resonance described above caused Uranus and Neptune to plow into the Kupier belt, scattering most of the objects. Many of these objects were scattered inwards, until they interacted with Jupiter and most often were placed into highly elliptical orbits or even ejected outright from the solar system. The objects which ended up in highly elliptical orbits form the Oort cloud. Closer in, some objects were scattered outwards by Neptune, and those form the scattered disc, accounting for the Kuiper belt's present low mass. However, a large number of KBOs, including Pluto, became gravitationally tied to Neptune's orbit, forcing them into resonant orbits.^[23] Planetesimals impacting Earth are thought to have brought the Earth its water and other hydrogen compounds^{[citation needed]}. Although not widely accepted, some believe life itself may have been deposited on Earth in this way (known as the panspermia hypothesis). The current locations and contents of the Kuiper and Asteroid Belts may be largely dependent on the Late Heavy Bombardment for transporting large quantities of mass throughout the solar system.

This period of heavy bombardment lasts several hundred million years, and is evident in the cratering still visible on geologically dead bodies of the solar system. Importantly, the bombardment and collisions of planetesimals and protoplanets can explain unusual moons, moon orbits, axial tilts, and other discrepancies from the originally very orderly motions. Excessive cratering of the Moon and other large bodies, dated to this era of the Solar System, is also naturally explained by the process.

The evolution of the outer solar system appears to have been influenced by nearby supernovae and possibly also passage through interstellar clouds. The surfaces of bodies in the outer solar system would experience space weathering from the Solar Wind, micrometeorites, as well as the neutral components of the interstellar medium, and more momentary influences like supernovae and magnetar eruptions (also called starquakes).^[24]

The Stardust sample return from Comet Wild 2 has also revealed some evidence that materials from the early formation of the solar system migrated from the warmer inner Solar System to the region of the Kuiper Belt as well as some of the dust that existed before the solar system formed.^[25]

Moons

Moons have come to exist around most planets and many other Solar System bodies. These natural satellites have come into being from one of three possible causes:

• co-formation from a proto-planetary disk (peculiar to the gas giants),
• formation from impact debris (given a large enough impact at a shallow angle), and
• capture of a passing object.

The gas giants tend to have inner moon systems which originated from the proto-planetary disk. This is indicated by the large sizes of the moons and their proximity to the planet. (These attributes are impossible to achieve via capture, while the gaseous nature of the primaries make formation from collision debris another impossibility.) The outer moons of the gas giants tend to be small and have orbits which are elliptical and have arbitrary inclinations. These features are appropriate for captured bodies.

For the inner planets and other solid solar system bodies, collisions appear to be the main creator of moons, with a percentage of the material kicked up by the collision ending up in orbit and coalescing into one or more moons. The Moon is believed to have formed in this way. ^[26]

Future

Artist's conception of the future evolution of our Sun. Left: main sequence; middle: red giant; right: white dwarf

Barring some unforeseeable accident, such as the arrival of a rogue black hole or star into its territory, astronomers estimate that the solar system as we know it today will last until the Sun begins its journey off of the main sequence. Even so, it will continue to evolve as time goes on.

Long term stability

The solar system is known to be chaotic,^[27] with the orbits of the planets open to long term variations. One notable example of this chaos is the Neptune-Pluto system, which lies in a 3:2 orbital resonance. Although the resonance itself will remain stable, it becomes impossible to predict the position of Pluto with any degree of accuracy more than 10-20 million years (the Lyapunov time) into the future.^[28] The planets' orbits are chaotic over longer timescales, such that the whole solar system possesses a Lyapunov time in the range of 2 - 230 million years.^[29] In all cases this means that the position of a planet along its orbit ultimately becomes impossible to predict with any certainty (so, for example, the timing of winter and summer become uncertain); but in some cases the orbits themselves may change dramatically. Such chaos manifests most strongly as changes in eccentricity, with some planets' orbits becoming significantly more — or less — elliptical.^[30]

Ultimately, the solar system is understood to be "practically stable",^[29] such that none of the planets will be ejected from the solar system, or suffer a mutual collision, within the next few billion years. Beyond this, within 5 billion years or so Mars' eccentricity may grow to around 0.2, such that it lies on an Earth-crossing orbit, leading to a potential collision. In the same timescale, Mercury's eccentricity may grow even further, and a close encounter with Venus could theoretically eject it from the solar system altogether.^[27] (Alternatively, neither of these events may occur; the inherent chaos means only future possibilities can be determined.) However, 5 billion years is the estimated remaining time the Sun will spend on the main sequence, and it may be that the ensuing red giant phase destroys the inner planets before any large dynamical instabilities can manifest.

Evolution of the Sun and planetary environments

The Sun is growing brighter at a rate of roughly ten percent every billion years. In one billion years time, as the Sun's radiation output increases, its circumstellar habitable zone will move outwards, and the Earth's surface will be seared by solar radiation until it becomes uninhabitable. At this point all life on land will become extinct, though life could still survive in the deeper oceans. Within 3 billion years, Earth may attain surface conditions similar to Venus's today; the oceans may gradually evaporate, and all life (in known forms) will probably be impossible. By then this is possible that when Mars surface temperature also gradually rises, it may potentially achieve surface conditions akin to the Earth today.

In about 4.5 billion years from now, the hydrogen reserves within the Sun's core will be spent, and it will begin to use those in its less dense upper layers. This will require it to expand to roughly 80 times its current diameter, and become a red giant. Within 7.7 billion years from now, the sun will have cooled and dulled by its vastly increased surface area. As the Sun expands, it will swallow up the planet Mercury. On the other hand, Earth and Venus are expected to survive.^[31] Earth will be left a scorched cinder, with surface temperature of 1273° K (1000° Celsius, or 1700° F); its land surface reduced to the consistency of hot clay by sunlight a thousand times more powerful than today's, and its atmosphere stripped away by a now-ferocious solar wind. The Sun is expected to remain in a red giant phase for about 100 million years. During this time, it is possible that the icy moons around Jupiter and Saturn, such as Europa and Titan, could achieve surface temperatures and atmospheres akin to those currently required to support life.^[32]

Eventually, the helium produced in the shell will fall back into the core, increasing the density until it reaches the levels needed to fuse helium into carbon. A helium flash will then occur; the Sun will shrink abruptly to slightly larger than its original radius, as its energy source has fallen back to its core. The helium-fusing stage will last only 100 million years. Eventually it will have to again resort to its reserves in its outer layers, and will expand again turning into Asymptotic Giant Branch star (AGB star). This phase lasts a further 100 million years, after which, over the course of a further 100,000 years, the Sun's outer layers will fall away, ejecting a vast stream of matter into space and forming a halo known (misleadingly) as a planetary nebula.^[33]

The Ring nebula, a planetary nebula similar to what the Sun will eventually become

This is a relatively peaceful event; nothing akin to a supernova, which our Sun is too small to ever undergo. Earthlings, if we are still alive to witness this occurrence, would observe a massive increase in the speed of the solar wind, but not enough to destroy the Earth completely. Eventually, all that will remain of the Sun is a white dwarf, a hot, dim and extraordinarily dense object; half its original mass but only the size of the Earth. ^[34]

As the Sun dies, its gravitational pull on the orbiting planets, comets and asteroids will weaken. All the other planets' orbits, except Mercury (which will have long since been destroyed) will expand. When the sun becomes a white dwarf, the solar system's final configuration will be reached: Venus' orbit will lie roughly at 1.35 AU; and Earth's orbit will lie roughly at 1.85 AU; and Mars' orbit will lie roughly at 2.8 AU. They and the other remaining planets and moons will be frozen, as bleak, frigid husks, with life (in all known forms), becoming impossible in our solar system.^[35] They will continue to orbit their star, their speed slowed due to their increased distance from the Sun and the Sun's reduced gravity. Two billion years farther on, the carbon in the Sun's core will crystallize, transforming it into a giant diamond. Eventually, after trillions more years, it will fade and die, finally ceasing to shine altogether.^[31][36]

Moon-ring systems

The evolution of moon systems is driven by tidal forces. A moon will raise a bulge in its primary due to its own gravity. If a moon is revolving in the same direction as the planet's rotation and the planet is rotating faster than the orbital period of the moon, the bulge will constantly be pulled ahead of the moon. In response, the moon will gain energy and slowly spiral outward. The same situation will also cause the primary to rotate more slowly over time. The Earth and its moon are just one example of this configuration. Other examples are the Galilean moons of Jupiter (as well as many of Jupiter's smaller moons), and most of the larger moons of Saturn and Uranus. ^[37]

Another situation is when the moon is either revolving around the primary faster than the primary rotates, or is revolving in the opposite direction as the planet's rotation. In these cases, the tidal bulge ends up being behind the moon in its orbit. This causes the moon to spiral in towards the primary until it either is torn apart by tidal stresses or plows into the planet's surface or atmosphere. Such a fate awaits the moons Phobos of Mars, Metis and Adrastea of Jupiter, and Triton of Neptune.^[38][39]

A third possibility is where the primary and moon are tidally locked to each other. In that case, the tidal bulge stays directly under the moon and the orbital period will not change. Pluto and Charon are an example of this type of configuration. ^[40]

Saturn's rings are quite young, and are not expected to survive beyond 300 million years. The gravity from Saturn's moons will gradually sweep the rings' outer edge toward the planet, and, eventually, abrasion by meteorites and Saturn's gravity will take the rest, leaving Saturn unadorned.^[41]

References

• William K. Hartmann and Donald R. Davis, Satellite-sized planetesimals and lunar origin, (International Astronomical Union, Colloquium on Planetary Satellites, Cornell University, Ithaca, N.Y., Aug. 18-21, 1974) Icarus, vol. 24, Apr. 1975, p. 504-515
• Alfred G. W. Cameron and William R. Ward, The Origin of the Moon, Abstracts of the Lunar and Planetary Science Conference, volume 7, page 120, 1976
• K. Tsiganis, R. Gomes, A. Morbidelli and H. F. Levison (May 2005). "Origin of the orbital architecture of the giant planets of the Solar System". Nature 435 (7041): 459-461. DOI:10.1038/nature03539. 
• Adrián Brunini (April 2006). "Origin of the obliquities of the giant planets in mutual interactions in the early Solar System". Nature 440 (7088): 1163-1165. DOI:10.1038/nature04577. 

Capture theory references

• M M Woolfson 1969, Rep. Prog. Phys. 32 135-185
• M M Woolfson 1999, Mon. Not. R. Astr. Soc.304, 195-198.

Further references

See also

• History of Earth
• Age of the Earth
• Giant impact hypothesis
• Planetary migration
• Solar System
• Solar nebula
• Stellar evolution
• Tidal locking

External links

• 7M animation from skyandtelescope.com showing the early evolution of the outer solar aystem.be-x-old:Паходжаньне Сонечнай сыстэмы

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