Jupiter and Saturn are sometimes called the gas giants, whereas the more distant Uranus and Neptune have been nicknamed the ice giants. They're also mostly made of gases like hydrogen, helium and ammonia rather than of rocky surfaces, although astronomers believe some or all of them may have solid cores.
The four large outer worlds - Jupiter, Saturn, Uranus and Neptune - are sometimes called the Jovian or "Jupiter-like" planets because of their enormous size relative to the terrestrial planets. Pluto also has a rocky, albeit frozen, surface but has never been grouped with the four terrestrials. The inner four planets closest to the sun - Mercury, Venus, Earth and Mars - are often called the " terrestrial planets" because their surfaces are rocky. The planets: The many worlds of our solar system
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