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.[1] 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 (which are referred to as "ices"). For this reason, Uranus and Neptune are now often classified in the separate category of ice giants.[2]
Jupiter and Saturn consist mostly of elements such as hydrogen and helium, with heavier elements making up between 3 and 13 percent of their mass.[3] They are thought to consist of an outer layer of compressed molecular hydrogen surrounding a layer of liquid metallic hydrogen, with probably a molten rocky core inside. The outermost portion of their hydrogen atmosphere contains many layers of visible clouds that are mostly composed of water (despite earlier certainty that there was no water anywhere else in the Solar System) 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 large 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.[3]
The defining differences between a very low-mass brown dwarf (which can have a mass as low as roughly 13 times that of Jupiter[4]) and a gas giant are debated.[5] One school of thought is based on formation; the other, on the physics of the interior.[5] Part of the debate concerns whether brown dwarfs must, by definition, have experienced nuclear fusion at some point in their history.
^D'Angelo, G.; Lissauer, J. J. (2018). "Formation of Giant Planets". In Deeg H., Belmonte J. (ed.). Handbook of Exoplanets. Springer International Publishing AG, part of Springer Nature. pp. 2319–2343. arXiv:1806.05649. Bibcode:2018haex.bookE.140D. doi:10.1007/978-3-319-55333-7_140. ISBN 978-3-319-55332-0. S2CID 116913980.
^National Aeronautics and Space Administration website, Ten Things to Know About Neptune
^ abThe Interior of Jupiter, Guillot et al., in Jupiter: The Planet, Satellites and Magnetosphere, Bagenal et al., editors, Cambridge University Press, 2004
^Bodenheimer, Peter; D'Angelo, Gennaro; Lissauer, Jack J.; Fortney, Jonathan J.; Saumon, Didier (2013). "Deuterium Burning in Massive Giant Planets and Low-mass Brown Dwarfs Formed by Core-nucleated Accretion". The Astrophysical Journal. 770 (2): 120. arXiv:1305.0980. Bibcode:2013ApJ...770..120B. doi:10.1088/0004-637X/770/2/120. S2CID 118553341.
^ abBurgasser, Adam J. (June 2008). "Brown dwarfs: Failed stars, super Jupiters" (PDF). Physics Today. Archived from the original (PDF) on 8 May 2013. Retrieved 11 January 2016.
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