Gas giant

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 a distinct class of giant planets composed mainly of heavier volatile substances (referred to as "ices"). For this reason, Uranus and Neptune are often classified in the separate category of ice giants.

Jupiter and Saturn consist mostly of hydrogen and helium, with heavier elements making up between 3 and 13 percent of their mass. They are thought to have an outer layer of compressed molecular hydrogen surrounding a layer of liquid metallic hydrogen, with a molten rocky core inside. The outermost portion of their hydrogen atmosphere contains many layers of visible clouds that are mostly composed of water and ammonia. The layer of metallic hydrogen located in the mid-interior makes up the bulk of every gas giant and is referred to as "metallic" because the very high atmospheric pressure turns hydrogen into an electrical conductor. The gas giants' cores are thought to consist of heavier elements at such high temperatures (20,000 K [19,700 °C; 35,500 °F]) and pressures that their properties are not yet completely understood. The placement of the solar system's gas giants can be explained by the grand tack hypothesis.

The defining differences between a very low-mass brown dwarf (which can have a mass as low as roughly 13 times that of Jupiter) and a gas giant are debated. One school of thought is based on formation, the other, on the physics of the interior. Part of the debate concerns whether brown dwarfs must by definition have experienced nuclear fusion at some point in their history.