Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003trgeo...1..461c&link_type=abstract
Treatise on Geochemistry, Volume 1. Editor: Andrew M. Davis. Executive Editors: Heinrich D. Holland and Karl K. Turekian. pp. 71
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Scientific paper
Modern theories for the origin of the planets are based on observations of the solar system and star-forming regions elsewhere in the galaxy, together with the results of numerical models. Some key observations are: - The solar system contains eight large planets with roughly circular, coplanar orbits lying 0.4-30 AU from the Sun. There are few locations between the planets where additional large objects could exist on stable orbits. - The major planets are grouped: small volatile-poor planets lie close to the Sun, with large volatile-rich planets further out. The main asteroid belt (2-4 AU from the Sun) is substantially depleted in mass with respect to other regions. - The planets and asteroids are depleted in volatile elements compared to the Sun. The degree of fractionation decreases with distance: the terrestrial planets and inner-belt asteroids are highly depleted in volatiles, the outer-belt asteroids are less so, while many satellites in the outer solar system are ice rich. Primitive CI meteorites (probably from the outer asteroid belt) have elemental abundances very similar to the Sun except for highly volatile elements. - Ancient solid surfaces throughout the solar system are covered in impact craters (e.g., the Moon, Mercury, Mars, Callisto). Most of the planets have large axial tilts with respect to their orbits. Earth possesses a large companion with a mass ˜1% that of the planet itself. - The terrestrial planets and many asteroids have undergone differentiation. There is strong evidence that Saturn is highly centrally condensed, with a core of mass ˜10M⊕, and weaker evidence that Jupiter has a core of similar mass. These cores have masses comparable to Uranus and Neptune. - Meteorites from the main asteroid belt show evidence that they once contained short-lived radioactive isotopes with half-lives <10 Myr. The main components of primitive meteorites (chondrules and refractory inclusions) have sizes clustered around 1 mm. These components appear to have undergone rapid melting and cooling. - Young stars generally exist in gas- and dust-rich environments. Many young stars possess massive, optically thick disks with diameters of 10-1,000 AU. These disks are inferred to have lifetimes of ˜1-10 Myr. - At least 4% of main sequence (ordinary) stars have planetary-mass companions. The companions have masses of 0.1-10 Jupiter masses (the lower limit is the current detection threshhold), and orbital distances from 0.05 AU to 5 AU (the upper limit is the current detection threshhold).These observations have led to the development and refinement of a theory in which the planets formed from a disk-shaped protoplanetary nebula (Laplace) by pairwise accretion of small solid bodies (Safranov, 1969). A variant of the standard model invokes the gravitational collapse of portions of this disk to form gas giant planets directly. It should be pointed out that the standard model is designed to explain the planets observed in the solar system. Attempts to account for planetary systems recently discovered orbiting other stars suggest that planet formation is likely to differ in several respects from one system to another.
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