# Ice giant <!-- SOLSIM:BEGIN g31 — Solar System explorer state (hand-built on wt-core, specs/solar/); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *Rock inside, gas and ice outside (Solar System explorer)* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?view=composition&embed=1" data-title="Rock inside, gas and ice outside (Solar System explorer)"></div> *The Solar System explorer locked on this article's state (`?view=composition`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.* <!-- SOLSIM:END --> *Try: press l to show the labels and find Uranus and Neptune, the two ice giants, at the outer edge of the planetary system; drag to a view from above to compare their distance with Jupiter's and Saturn's, the two gas giants; then set the speed to 100 years/s and watch Neptune take about 165 years for a single orbit while the inner planets blur.* An **ice giant** is a giant planet made mostly of elements heavier than [[Hydrogen|hydrogen]] and [[Helium|helium]], chiefly oxygen, carbon, nitrogen and sulfur, rather than of hydrogen and helium themselves.[^hofstadter2011] The Solar System has two, [[Uranus]] and [[Neptune]]. In planetary science an "ice" is a volatile compound with a freezing point above about 100 K, such as [[Water|water]] (273 K), ammonia (195 K) or methane (91 K). These compounds entered the planets as solids when they formed, but inside the planets today most of the water is not ice at all: it exists as a hot, dense supercritical fluid.[^hofstadter2011] Uranus and Neptune contain only about 20% hydrogen and helium by mass, compared with more than 90% in the [[Gas_giant|gas giants]] [[Jupiter]] and [[Saturn]], and after the Voyager 2 flybys they came to be treated as a distinct class of giant planet.[^hofstadter2011][^marley2019] A 2026 study has proposed that much of their interiors may instead be hydrogen-rich magma, which would make them relatives of the sub-Neptunes common around other stars.[^young2026] The explorer at the top of this page opens on the composition view, which colours the ice giants apart from the gas giants and from the rocky planets inside the [[Frost_line_(astrophysics)|frost line]]; the two ice giants are the outermost planets, at about 19 and 30 [[Astronomical_unit|AU]].[^jpl-t1] ## Terminology The term "gas giant" appeared in 1952, in a science-fiction story by James Blish, and was first applied to all four of the large outer planets.[^hdsf] Yet the compositions of Uranus and Neptune have been recognised as different from Jupiter's and Saturn's since the late 1940s: their densities require large amounts of material heavier than hydrogen and helium.[^marley2019] Because that material was incorporated as ice, or as gas trapped in water ice, when the planets formed, the name "ice giant" came into use.[^marley2019][^hofstadter2011] The phrase became popular first in science fiction, for example in a 1971 anthology edited by Ben Bova, and its earliest scientific use was probably in a 1978 NASA report by James Dunne and Eric Burgess on the Mariner 10 mission.[^molaverdikhani2019] The name is a statement about composition, not about the state of the material. By the convention planetary scientists use, "gas" means hydrogen and helium, "ice" means water, ammonia and methane, and "rock" means silicates and metals, whatever the temperature and pressure.[^marley2019] The difference from the gas giants shows in mass and density. Uranus has 14.5 Earth masses and Neptune 17.1, against Saturn's 95 and Jupiter's 318 (derived from the fact-sheet masses). Neptune's mean density, 1,638 kg/m³, is more than twice Saturn's 687 kg/m³, although Neptune is less than a fifth as massive; a hydrogen planet of Neptune's mass would be far less dense, so the difference must be made up by heavier material.[^nasa-fs] ## Formation The [[Terrestrial_planet|terrestrial planets]] are understood to have formed by the collisional accumulation of planetesimals. The gas giants are thought to have formed solid cores of about ten Earth masses in the same way and then drawn in gas envelopes from the [[Protoplanetary_disk|protoplanetary disk]] over a few million years.[^lissauer2009][^dangelo2010] Pebble accretion, in which cores grow by sweeping up centimetre-sized particles drifting through the gas, has been proposed as a faster route.[^levison2015] Some giant planets around other stars may instead have formed through gravitational instability in their discs.[^dangelo2010][^boss2003] Growing Uranus and Neptune by core accretion at their present distances is harder. Near 20 AU and beyond, the escape velocity of a small protoplanet is comparable to the speeds at which planetesimals move relative to it, so encounters scatter material rather than adding it, and bodies crossing the orbits of Saturn or Jupiter tend to be ejected, swallowed by the gas giants or thrown onto cometary orbits.[^boss2003] Orbits there are also slow: by [[Kepler's_laws_of_planetary_motion|Kepler's third law]] a planet at 30 AU takes 30^1.5 ≈ 164 years to go round the Sun, about 14 times Jupiter's 11.9 years, so encounters happen much less often (derived).[^jpl-t1] Despite these difficulties, many planets of similar size have been found around other stars since 2004, suggesting that ice giants are common.[^hofstadter2011] ### Migration One solution is that the ice giants did not form where they are. They may have grown between or near the orbits of Jupiter and Saturn, where material was denser and orbits faster, and then been scattered outward to their present, more distant orbits, as in the [[Nice_model|Nice model]].[^boss2003][^thommes2002] ### Disk instability Alternatively, gravitational instability could have produced giant protoplanets out to about 30 AU. In a disc massive enough to be marginally unstable, slightly denser regions collapse into clumps within about a thousand years, far faster than the 100,000 to 1,000,000 years core accretion needs, so the process could work even in short-lived discs.[^boss2003] What keeps such a disc stable until it collapses is unclear; possible triggers include a close pass by another young star, mass piling up in magnetically inactive dead zones, and bursts of accretion that briefly raise the disc's density.[^boss2003] A variant uses photoevaporation. In the Orion Nebula's Trapezium cluster, extreme ultraviolet light from the massive star θ¹ Orionis C is evaporating the discs of nearby young stars. Giant protoplanets formed quickly by disc instability in such a setting could lose most of their hydrogen envelopes to that radiation, leaving planets rich in heavier material; in the Carina Nebula the ultraviolet flux is about 100 times higher, making the process even more effective.[^boss2003] ## Characteristics The two kinds of giant planet differ throughout. In the gas giants hydrogen extends all the way down to the core, turning into metallic hydrogen at pressures of hundreds of gigapascals. The ice giants' hydrogen envelopes are much thinner, less than about 20% of their mass, and never reach pressures high enough to make metallic hydrogen.[^hofstadter2011] Beneath their atmospheres a hot, dense fluid of water and ammonia is thought to make up about two-thirds of each planet's mass.[^jpl2017][^reh2017] ### Atmosphere and weather The ice giants' atmospheres share features with the gas giants': long-lived, fast equatorial winds, polar vortices, large-scale circulation, and chemistry driven by ultraviolet light from above and mixing from below.[^hofstadter2011] They also differ in ways that test atmospheric physics: their compositions allow other chemistry, and they receive less sunlight than any other planet, so internal heat matters more for their weather. Neptune's largest visible feature, the Great Dark Spot, forms and disappears over a few years, unlike Jupiter's Great Red Spot, which has lasted for centuries. Neptune emits more internal heat relative to absorbed sunlight than any other giant, a ratio of about 2.6, against about 1.8 for Saturn, while Uranus emits about a tenth as much as Neptune, perhaps because of its extreme tilt.[^hofstadter2011] Uranus's rotation axis is tilted 97.77° to its orbit, so its poles take turns facing the Sun over its 84-year year.[^nasa-uranus] ### Interior Because the ice giants are large and conduct heat poorly, their interiors reach pressures of several hundred gigapascals and temperatures of several thousand kelvins.[^nellis2012] Laboratory work in 2012 found that the compressibility of water used in ice-giant models could be wrong by as much as a third, with consequences for every model of their structure.[^astrobio2012] A 2026 study proposes that Uranus and Neptune may consist largely of hydrogen-rich magma rather than of volatiles, and that they may have formed much as sub-Neptune exoplanets are thought to.[^young2026] ### Magnetic fields The magnetic fields of Uranus and Neptune are unusual. Their dipoles are tilted steeply from the rotation axis, by 58.6° for Uranus and 46.9° for Neptune, and offset from the planets' centres by about 0.35 and 0.49 planetary radii.[^nasa-uranus][^nasa-neptune] At the surface their equatorial dipole fields are 0.228 and 0.142 gauss, about 75% and 46% of Earth's 0.306 gauss (derived).[^nasa-uranus][^nasa-neptune][^nasa-earth] The fields are thought to be generated not in a deep core but in an electrically conducting, convecting layer of fluid ices, a thin shell that would explain both the tilt and the offset.[^thomas1994] ## Exploration ### Past Only one spacecraft has visited the ice giants. Voyager 2, launched on 20 August 1977, passed about 81,500 km from Uranus on 24 January 1986 and about 4,800 km from Neptune on 25 August 1989, and remains the only source of close-up data on either planet, their rings and their moons, including Neptune's large moon [[Triton_(moon)|Triton]].[^nasa-voyager] Its measurements established the tilted magnetic fields, the fast winds and the Great Dark Spot described above. ### Proposals A return to the ice giants has been proposed many times, as an orbiter, a probe, or a mission combining a flyby of Neptune and [[Triton_(moon)|Triton]] with a passage into the [[Kuiper_belt|Kuiper belt]], such as the Outer Solar System mission proposed in 2012.[^christophe2012] In 2017 a NASA study of ice-giant mission concepts, carried out ahead of the next decadal survey, compared flybys, orbiters and probes and concluded that an orbiter carrying an atmospheric probe would return the most science; it also noted that the two planets' interiors, including the fluid water–ammonia layer, are among the least understood in the Solar System.[^jpl2017][^reh2017] The 2023–2032 planetary science decadal survey of the US National Academies then recommended a Uranus orbiter and probe as the highest-priority new flagship mission.[^nasem2022] The distances are the main obstacle: Uranus and Neptune orbit about 18 and 29 AU beyond Earth's orbit, several times as far as [[Jupiter]] (derived from the semi-major axes).[^jpl-t1] ## See also - [[Gas_giant]] - [[Terrestrial_planet]] - [[Uranus]] · [[Neptune]] - [[Planet_Nine]] - Mini-Neptune · Neptunian exoplanet ## References [^hofstadter2011]: Hofstadter, M. (2011). "The atmospheres of the ice giants, Uranus and Neptune". White paper for the Planetary Science Decadal Survey. https://www.lpi.usra.edu/decadal/opag/IceGiantAtmospheres_v7.pdf [^marley2019]: Marley, M. (2 April 2019). "Not a heart of ice". The Planetary Society. https://www.planetary.org/articles/not-a-heart-of-ice [^young2026]: Young, E. D.; Marcum, S. P.; Werlen, A.; Wulff, P. N. (2026). "Ice giants revisited: Uranus and Neptune as magma ocean worlds". arXiv preprint. https://doi.org/10.48550/arXiv.2606.18219 [^jpl-t1]: JPL Solar System Dynamics. "Approximate positions of the planets", Table 1. https://ssd.jpl.nasa.gov/planets/approx_pos.html [^hdsf]: Sheidlower, J. (ed.). "Gas giant". *Historical Dictionary of Science Fiction*. https://sfdictionary.com/view/52/gas-giant [^molaverdikhani2019]: Molaverdikhani, K.; Henning, T.; Mollière, P. (2019). "From cold to hot irradiated gaseous exoplanets: toward an observation-based classification scheme". *The Astrophysical Journal* 873: 32. https://doi.org/10.3847/1538-4357/aafda8 [^nasa-fs]: NASA NSSDCA. "Planetary Fact Sheet". https://nssdc.gsfc.nasa.gov/planetary/factsheet/ (fetched 2026-09-18). [^lissauer2009]: Lissauer, J. J.; Hubickyj, O.; D'Angelo, G.; Bodenheimer, P. (2009). "Models of Jupiter's growth incorporating thermal and hydrodynamic constraints". *Icarus* 199: 338–350. https://doi.org/10.1016/j.icarus.2008.10.004 [^dangelo2010]: D'Angelo, G.; Durisen, R. H.; Lissauer, J. J. (2010). "Giant planet formation". In Seager, S. (ed.), *Exoplanets*. University of Arizona Press, pp. 319–346. ISBN 978-0-8165-2945-2. [^levison2015]: Levison, H. F.; Kretke, K. A.; Duncan, M. J. (2015). "Growing the gas-giant planets by the gradual accumulation of pebbles". *Nature* 524: 322–324. https://doi.org/10.1038/nature14675 [^boss2003]: Boss, A. P. (2003). "Rapid formation of outer giant planets by disk instability". *The Astrophysical Journal* 599: 577–581. https://doi.org/10.1086/379163 [^thommes2002]: Thommes, E. W.; Duncan, M. J.; Levison, H. F. (2002). "The formation of Uranus and Neptune among Jupiter and Saturn". *The Astronomical Journal* 123: 2862–2883. https://doi.org/10.1086/339975 [^jpl2017]: NASA Jet Propulsion Laboratory (20 June 2017). "NASA completes study of future 'ice giant' mission concepts". https://www.jpl.nasa.gov/news/nasa-completes-study-of-future-ice-giant-mission-concepts/ [^reh2017]: Reh, K.; Hofstadter, M.; Elliott, J.; Simon, A. (2017). "On to the ice giants: pre-decadal study summary". NASA Jet Propulsion Laboratory. https://dataverse.jpl.nasa.gov/file.xhtml?fileId=59342&version=2.0 [^nasa-uranus]: NASA NSSDCA. "Uranus Fact Sheet". https://nssdc.gsfc.nasa.gov/planetary/factsheet/uranusfact.html (fetched 2026-09-18). [^nasa-neptune]: NASA NSSDCA. "Neptune Fact Sheet". https://nssdc.gsfc.nasa.gov/planetary/factsheet/neptunefact.html (fetched 2026-09-18). [^nasa-earth]: NASA NSSDCA. "Earth Fact Sheet". https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html (fetched 2026-09-18). [^nellis2012]: Nellis, W. (2012). "Seeing deep inside icy giant planets". *Physics* 5: 25. https://doi.org/10.1103/Physics.5.25 [^astrobio2012]: *Astrobiology Magazine* (23 March 2012). "The interiors of ice giant planets". http://www.astrobio.net/pressrelease/4641/the-interiors-of-ice-giant-planets [^thomas1994]: Thomas, C. (1994). "The nature and origin of magnetic fields". http://evildrganymede.net/work/magfield.htm [^nasa-voyager]: NASA Science. "Voyager 2". https://science.nasa.gov/mission/voyager/voyager-2/ [^christophe2012]: Christophe, B.; Spilker, L. J.; Anderson, J. D.; et al. (2012). "OSS (Outer Solar System): a fundamental and planetary physics mission to Neptune, Triton and the Kuiper belt". *Experimental Astronomy* 34: 203–242. https://doi.org/10.1007/s10686-012-9309-y [^nasem2022]: National Academies of Sciences, Engineering, and Medicine (2022). *Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023–2032*. The National Academies Press. https://doi.org/10.17226/26522 ## External links - NASA Science: Uranus. https://science.nasa.gov/uranus/ - NASA Science: Neptune. https://science.nasa.gov/neptune/ - NASA Science: Voyager 2. https://science.nasa.gov/mission/voyager/voyager-2/ ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Ice_giant) : [Wikitube](https://en.wikitube.io/wiki/Ice_giant) · pinned revision [1370403295](https://en.wikipedia.org/w/index.php?oldid=1370403295) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. 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