Gas giant

Jupiter photographed by New Horizons in January 2007
Saturn at equinox, photographed by Cassini in August 2009

A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are sometimes known as failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. 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.[1]

Jupiter and Saturn consist mostly of hydrogen and helium, with heavier elements making up between 3 and 13 percent of the mass.[2] They are thought to consist of an outer layer of molecular hydrogen surrounding a layer of liquid metallic hydrogen, with probably a molten rocky core. The outermost portion of their hydrogen atmosphere is characterized by many layers of visible clouds that are mostly composed of water and ammonia. The layer of metallic hydrogen makes up the bulk of each planet, and is referred to as "metallic" because the very large 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) and pressures that their properties are poorly understood.[2]

The defining differences between a very low-mass brown dwarf and a gas giant (estimated at about 13 Jupiter masses) are debated.[3] One school of thought is based on formation; the other, on the physics of the interior.[3] Part of the debate concerns whether "brown dwarfs" must, by definition, have experienced nuclear fusion at some point in their history.


The term gas giant was coined in 1952 by the science fiction writer James Blish[4] and was originally used to refer to all giant planets. It is, arguably, something of a misnomer because throughout most of the volume of all giant planets, the pressure is so high that matter is not in gaseous form.[5] Other than solids in the core and the upper layers of the atmosphere, all matter is above the critical point, where there is no distinction between liquids and gases. The term has nevertheless caught on, because planetary scientists typically use "rock", "gas", and "ice" as shorthands for classes of elements and compounds commonly found as planetary constituents, irrespective of what phase the matter may appear in. In the outer Solar System, hydrogen and helium are referred to as "gases"; water, methane, and ammonia as "ices"; and silicates and metals as "rock". Because Uranus and Neptune are primarily composed of, in this terminology, ices, not gas, they are increasingly referred to as ice giants and separated from the gas giants.

Other Languages
Afrikaans: Gasreus
Alemannisch: Gasriese
العربية: عملاق غازي
asturianu: Xigante gaseosu
беларуская: Газавыя планеты
беларуская (тарашкевіца)‎: Газавыя плянэты
български: Газов гигант
català: Gegant gasós
čeština: Plynný obr
Cymraeg: Cawr nwy
dansk: Gaskæmpe
Deutsch: Gasplanet
español: Gigante gaseoso
Esperanto: Gasgiganto
فارسی: غول گازی
Frysk: Gasreus
Gaelg: Foawr gas
հայերեն: Հսկա մոլորակ
हिन्दी: गैस दानव
hrvatski: Plinoviti div
Bahasa Indonesia: Raksasa gas
íslenska: Gasrisi
italiano: Gigante gassoso
עברית: ענק גזים
kurdî: Dêwê gazê
latviešu: Gāzu planēta
Lëtzebuergesch: Gasplanéit
lumbaart: Gigante gasùs
македонски: Гасовит џин
Bahasa Melayu: Gergasi gas
монгол: Хийн гариг
Nederlands: Gasreus
norsk: Gasskjempe
ਪੰਜਾਬੀ: ਗੈਸ ਦਿਓ
پنجابی: گیس جن
Piemontèis: Gigant gaseus
português: Planeta gasoso
română: Gigant gazos
Simple English: Gas giant
slovenčina: Joviálna planéta
slovenščina: Plinasti orjak
Soomaaliga: Neefaha Waawayn
srpskohrvatski / српскохрватски: Plinoviti div
svenska: Gasjätte
Taqbaylit: Imetiwgen n gaz
татарча/tatarça: Газлы планета
Türkçe: Gaz devi
українська: Газові планети
اردو: گیسی دیو