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Formation and evolution of the Solar System

The theories concerning the origin 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.

Initial formation

Solar nebula

The current hypothesis of Solar System formation is the nebular hypothesis, first proposed in 1755 by Immanuel Kant and independently formulated by Pierre-Simon Laplace.[1] The nebular theory maintains that 4.6 billion years ago, the Solar System formed from the gravitational collapse of a gaseous cloud. This initial cloud was likely several light-years across and played host to the birth of several stars.[2] 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. [3]

One of these regions of collapsing gas (known as the solar nebula) would form what became the Sun. This region had a diameter of between 7000 and 20,000 AU[2][4] and a mass just over that of the Sun (by between 0.1 and 0.001 solar masses).[5] 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. [2] 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[2] and a hot, dense protostar at the center.[6] 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.[5] These discs extend to several hundred AU and are rather cool, reaching only a thousand kelvins at their hottest.[7] Eventually, the temperature and pressure at the core of the Sun became so great that the hydrogen began to fuse, creating an internal source of energy which countered the force of gravitational contraction. At this point the Sun became a fully fledged star.

From this cloud and its gas and dust, 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.[8]

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) [2] 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. [9]

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).[10] [11]

After 100 million years, the pressure and density of hydrogen in the centre of the collapsing nebula became great enough for the protosun to begin thermonuclear fusion, which increased until hydrostatic equilibrium was achieved.[12]

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. [13] [14]

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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. [15]

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.

Estimation of age

Using radiometric dating, scientists can 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. [16]

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 obejcts 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

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 fomation 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.

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

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 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. These composition of these two are 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.

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 interations 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.[17]

Heavy Bombardment

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 though 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 this planetary motion resulted in the 2:1 resonance crossing of Jupiter and Saturn described above, and (so it is believed) Neptune and Uranus were rapidly moved outwards to interact heavily with the sea of planetesimals. The quantity of planetesimals being scattered inwards to reach the rest of the Solar System increased hugely, and with many impacts upon all observed planetary and lunar bodies. This period is known as the Late Heavy Bombardment.

In this manner, the jovian planets (particularly Jupiter and Neptune) gradually swept the disk clean of leftover planetesimals, "clearing the neighbourhood" either by slinging them in the distant outer reaches of the Oort Cloud (as far as 50,000 AU), or continually nudging their orbits into collisions with other planets (or into more stable orbits like the asteroid belt). 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. Planetesimals impacting Earth are thought to have brought the Earth its water and other hydrogen compounds. 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.

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. A giant impact of a Mars-sized protoplanet is suspected of being responsible for Earth's unusually large moon, whose composition and density is similar to the Earth's mantle, and could simultaneously have altered Earth's rotation axis to its present 23.5° from its orbital plane.

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.

Jupiter's regular gravitational interactions (see orbital resonance) are also responsible for preventing the material which once inhabited the asteroid belt from accreting into another probably sizable terrestrial planet. Most of that material has long since been thrown into eccentric orbits and collided with something else; the total mass of the asteroid belt is now less than a tenth of Earth's Moon.

Kuiper belt and Oort cloud

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.

The Jupiter-Saturn 2:1 orbital resonance caused 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 gratitationally tied to Neptune's orbit, forcing them into resonant orbits.[18]

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 a 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.

After forming, moon systems will continue to evolve. The most usual effect is orbital modification due to tides. This occurs due the tidal bulge that a moon creates in the atmosphere and oceans of a planet, and to a lesser extent in the primary itself. If the planet rotates faster than the moon orbits, the tidal bulge will constantly be displaced ahead of the satellite. In this case, the gravity of the bulge will cause the satellite to accelerate and slowly move away from the planet (as is the case for the Moon). On the other hand, if the moon orbits faster than the primary spins (or orbits against the spin), the bulge will stay behind the moon, and the bulge's gravity will cause the moon's orbit to decay over time. (The Martian moon Phobos is slowly spiraling in towards Mars for this reason.)

A planet can also create a tidal bulge in a moon, and this will slow down the moon's rotation until its rotation period becomes the same as its revolution period. Thus, the moon will keep one side of itself facing the planet, as is the case for the Moon. This is called tidal locking and is present in many other moons in the Solar System such as Jupiter's satellite Io. In the case of Pluto and Charon, both the primary and the satellite are tidally locked to each other.

Future

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 another billion years or so, whereupon the Sun will claim its first casualty, the Earth. As the Sun brightens a further ten percent beyond today's levels, its radiation output will increase, gradually searing the Earth until its land surface becomes uninhabitable, though life could still survive in the deeper oceans. Within 3.5 billion years, Earth will attain surface conditions similar to Venus's today; the oceans will boil, and all life (in known forms) will be impossible.

Five billion or so 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 eighty times its current diameter, and, about 7.5 billion years from now, to become a red giant, cooled and dulled by its vastly increased surface area. As the Sun expands, it will swallow the planet Mercury. Earth and Venus, however, are expected to survive, since the Sun will lose about 28 percent of its mass, and its lower gravity will send them into higher orbits. Earth will be left a scorched cinder, 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 a hundred million years.

During this time, it is possible that the watery worlds around Jupiter and Saturn, such as Titan and Europa, might achieve conditions similar to those required for current human life.

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. The Sun will then shrink to slightly larger than its original radius, as its energy source has fallen back to its core, however, due to the relative rarity of helium as opposed to hydrogen, the helium-fusing stage will only last about 100 million years. Eventually it will have to again resort to its reserves in its outer layers, and will regain its red giant form. This phase lasts only 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.

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. Were it viewed from Earth's surface, it would be a point of light the size of Venus with the brightness of a hundred current Suns.[19][20]

As the Sun dies, its gravitational pull on the orbiting planets, comets and asteroids will weaken. Earth and the other planets' orbits will expand. When the sun becomes a white dwarf, the solar system's final configuration will be reached: Mercury will have long since ceased to exist; Venus will lie roughly a third again farther out than Earth is now, and Earth's orbit will roughly equal that of Mars today. 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. [21] [22] [23] [24]

History of solar system formation hypotheses

During the late-19th century the Kant-Laplace nebular hypothesis was criticized by James Clerk Maxwell, who showed that if 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 based entirely on 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[25] 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. It is claimed that this model explains features of the solar system not explained by the Solar Nebula Theory.

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