A gas giant is a giant planet composed mainly of hydrogen and helium. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" was originally synonymous with "giant planet". However, in the 1990s, it became known that Uranus and Neptune are really a distinct class of giant planets, being composed mainly of heavier volatile substances. For this reason, Uranus and Neptune are now often classified in the separate category of ice giants.
Jupiter photographed by New Horizons in January 2007
Saturn at equinox, photographed by Cassini in August 2009
Artist's impression of the formation of a gas giant around the star HD 100546
A gas giant exoplanet [right] with the density of a marshmallow has been detected in orbit around a cool red dwarf star [left] by the NASA-funded NEID radial-velocity instrument on the 3.5-meter WIYN Telescope at Kitt Peak National Observatory.
A giant planet, sometimes referred to as a jovian planet, is a diverse type of planet much larger than Earth. Giant planets are usually primarily composed of low-boiling point materials (volatiles), rather than rock or other solid matter, but massive solid planets can also exist. There are four such planets in the Solar System: Jupiter, Saturn, Uranus, and Neptune. Many extrasolar giant planets have been identified.
Cut-away illustrations of the interior of the giant planets. Jupiter is shown with a rocky core overlaid by a deep layer of metallic hydrogen.
Saturn's north polar vortex
An artist's conception of 79 Ceti b, the first extrasolar giant planet found with a minimum mass less than Saturn.
Image: Jupiter and its shrunken Great Red Spot
![A gas giant exoplanet [right] with the density of a marshmallow has been detected in orbit around a cool red dwarf star [left] by the NASA-funded NEID](https://upload.wikimedia.org/wikipedia/commons/thumb/b/b3/Artist_impression_of_ultra_fluffy_gas_giant_planet_orbiting_a_red_dwarf_star.jpg/640px-Artist_impression_of_ultra_fluffy_gas_giant_planet_orbiting_a_red_dwarf_star.jpg)
