G I A N T P L A N E T S
Comparative sizes of the Solar System's four giant planets: courtesy NASA/JPL-Caltech
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The four giant planets are composed principally of hydrogen, like the Sun and other stars, but they vary considerably both in their internal structure and in their relative proportions of heavier elements. The two gas giant planets, Jupiter and Saturn, probably formed quite rapidly during the first few million years of the Solar System's existence, just outside the ice line where rocky and icy planetesimals and cool gases were abundant. After assembling silicate cores, both accreted enormous amounts of gas to reach their present masses. Uranus and Neptune formed more slowly at greater distances from the ice line. Their internal compositions include higher proportions of ices than do Jupiter and Saturn, and their overall masses are much smaller. As a result they are classified as ice giants to distinguish them from the two larger planets. Neptune, at least, has evidently migrated some distance into the outer Solar System to reach its present semimajor axis of about 30 AU. All four giant planets have families of satellites that formed in each planet's accretion disk, probably at the very end of the evolutionary process (Canup & Ward 2006, Dixon et al. 2007). The aggregate mass of each planet's satellite family is equal to about 0.0001 of the parent planet's mass. Most extrasolar planets so far detected are likely to resemble the Solar System' giant planets, in overall composition if not in physical appearance. |
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