Gas Giant
A Gas Giant is a giant planet composed mainly of hydrogen and helium. Jupiter and Saturn are the Solar System's gas giants. The term "gas giant" was originally synonymous with "giant planet", but in the 1990s it became known that Uranus and Neptune are really a distinct class of giant planet, being composed mainly of heavier volatile substances (which are referred to as 'ices'). For this reason, Uranus and Neptune are now often classified in the separate category of ice giants.
Gas giants can, theoretically, be divided into five distinct classes according to their modeled physical atmospheric properties, and hence their appearance: ammonia clouds (I), water clouds (II), cloudless (III), alkali metals (IV), and silicate clouds (V). Jupiter and Saturn are both class I.
References==