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21 Apr 08
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The most widely accepted theory of planetary formation, known as the nebular hypothesis, was first developed in the eighteenth century by Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace. The nebular hypothesis maintains that 4.6 billion years ago, the Solar System formed from the gravitational collapse of a giant molecular cloud which was light years across. Several stars, including the Sun, formed within the collapsing cloud. The gas that formed the Solar System was slightly more massive than the Sun itself. Most of the mass collected in the centre, forming the Sun; the rest of the mass flattened into a protoplanetary disc, out of which the planets and other bodies in the Solar System formed.
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The nebular hypothesis maintains that the Solar System formed from the gravitational collapse of a giant molecular cloud. The timeframe of these events has been determined using 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 Earth's surface is constantly being reshaped by erosion, volcanism and plate tectonics. To estimate the age of the Solar System, scientists 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, suggesting that the Solar System must be at least this old.[1]
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One of these regions of collapsing gas (known as the pre-solar nebula)[5] would form what became the Solar System. This region had a diameter of between 7000 and 20,000 AU[2][6] and a mass just over that of the Sun (between 1.001 and 1.1 solar masses).[7] Its composition was 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 nucleosynthesis in earlier generations of stars which died, ejecting these heavier elements into the interstellar medium.
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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, converting their kinetic energy into heat (random motions). 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 disc with a diameter of roughly 200 AU[2] and a hot, dense protostar (a star in which hydrogen fusion has not yet begun) at the centre.[8]
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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.[13]
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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.[14]
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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).[15][16
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