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.