Nine major planets are currently known. The inner planets are small and are composed primarily of rock and iron. Mercury is surprisingly dense, apparently because it has an unusually large iron core. With only a transient atmosphere, Mercury has a surface that still bears the record of bombardment by asteroidal bodies early in its history. Venus has a carbon dioxide atmosphere 90 times thicker than that of Earth, causing an efficient greenhouse effect by which the Venusian atmosphere is heated.
Earth is the only planet with abundant liquid water and known life.
Strong evidence exists that Mars once had water on its surface, but now its carbon dioxide atmosphere is so thin that the planet is dry and cold, with polar caps of solid carbon dioxide, or dry ice. Jupiter is the largest of the planets. Its hydrogen and helium atmosphere contains pastel-colored clouds, and its immense magnetosphere, rings, and satellites make it a planetary system unto itself. One of Jupiter's largest moons, Io , has volcanoes that produce the hottest surface temperatures in the solar system. At least four of Jupiter's moons have atmospheres, and at least three show evidence that they contain liquid or partially-frozen water.