Inner planets. Computer artwork of the rocky inner planets of the solar system. These are Earth (upper left), Mars (red, lower right), Venus (dark, behind Mars) and Mercury (brown, behind Mars and Venus). The Sun is seen emerging from behind Earth. These planets are known as the rocky (or terrestrial) planets, as they mainly comprise iron and silicon-rich rocks. This differentiates them from the much larger gaseous planets (Jupiter, Saturn, Uranus and Neptune) further out from the Sun, which mainly comprise hydrogen and helium. Pluto, the furthest planet from the Sun, is thought to have a rocky core surrounded by water ice.