Geochemistry source for chemical abundance, despite their rareness (only five have been recovered on Earth). The planets of the Solar System are divided into two groups: the four inner planets are the terrestrial planets (Mercury, Venus, Earth and Mars), with relatively small sizes and rocky surfaces. The four outer planets are the giant planets, which are dominated by hydrogen and helium and have lower mean densities. These can be further subdivided into the gas giants (Jupiter and Saturn) and the ice giants (Uranus and Neptune) that have large icy cores. Most of our direct information on the composition of the giant
Planets in science fiction and it is extremely hostile to life. Various planets of the Solar System were used as settings for science fiction stories in the first half of the 20th century; but dissatisfaction with the limits imposed by science led many writers early on to forsake the Solar System for fictional planets around distant stars. As increasing knowledge of the Solar System made the prospects of life in the vicinity of Earth marginal at best, the extrasolar planet has become almost the only venue for contemporary science fiction. In many works of science fiction, planets are only described casually, as points of