--- title: Ice giant summary: A giant planet made mostly of water, methane and ammonia rather than hydrogen and helium, like Uranus and Neptune. Their interiors are hot, dense fluids, their magnetic fields are lopsided, and planets of about their mass may be among the commonest in the Galaxy. science_status: [observed, model, sim] categories: [Ice giants, Planet classes, Giant planets] aliases: [Ice giants, IGC, IGH, IG1-C, IG2-H, Cold ice giant, Hot ice giant, Neptune-like planet, Uranus-like planet, Neptunian planet, Active ice giant, Quiescent ice giant, IGC-CA, IGC-CQ, IGH-WV, IGH-RB, IGH-DS, Water-veiled ice giant, Rayleigh-blue ice giant, Dark-stripped ice giant] infobox: type: planet_class name: Ice giant image: File:Ice_giant_IGC-CQ_sim.png caption: "Sim render: the sim's quiescent, Uranus-like cold ice giant (IGC-CQ), a pale blue-green disc with gentle limb darkening" code: "IGC, IGH" legacy_code: "IG1-C, IG2-H" level: "Series (two sim types)" series: IG (ice giant) science_status: "observed (Uranus, Neptune); model (interiors); sim (subtypes)" literature_equivalent: "Ice giant; Neptune-mass planet ('Neptune-like')" defining_criteria: "Mostly elements heavier than hydrogen and helium by mass, with an H/He envelope of perhaps 10 to 20%. Sim: cold (IGC) split by internal heat into Active and Quiescent; hot (IGH) split by temperature, 350 to 700 K Water-Veiled, 700 to 1,200 K Rayleigh-Blue, above 1,200 K Dark-Stripped" mass: {value: "Uranus 14.5, Neptune 17.1; about 10 to 50 for the class", unit: M_Earth, source: observed, note: "Sim: 8 to 50 (cold), 6 to 50 (hot)"} radius: {value: "Uranus 25,559; Neptune 24,764 (about 4 Earth radii)", unit: km, source: observed} bulk_density: {value: "Uranus 1.27, Neptune 1.64", unit: g/cm³, source: observed} gravity_at_1_bar: {value: "Uranus 8.7, Neptune 11.0 (equatorial)", unit: m/s², source: observed} escape_velocity: {value: "Uranus 21.3, Neptune 23.5", unit: km/s, source: observed} temperature_at_1_bar: {value: "Uranus 76, Neptune 72", unit: K, source: observed} bond_albedo: {value: "Uranus 0.30, Neptune 0.29", unit: dimensionless, source: observed} dominant_gases: "H2 about 80%, He 15 to 19%, CH4 1.5 to 2.3% by volume in the upper atmosphere" cloud_and_haze_species: "Methane ice near 1 to 2 bar; hydrogen sulfide and haze below; NH4SH and water far deeper; photochemical haze above" wind_speeds: {value: "Neptune's equatorial winds about 400 westward; Uranus's jets up to about 250 eastward at mid-latitudes", unit: m/s, source: observed} magnetic_field: "Multipolar and offset: dipoles tilted about 59 (Uranus) and 47 (Neptune) degrees from the spin axis" interior_structure: "H/He envelope over a hot, electrically conducting fluid of water, ammonia and methane, possibly superionic at depth, over a rocky core; proportions uncertain" tidal_state: "Cold ice giants not locked (Uranus 17.2 h, Neptune 16.1 h); hot ones locked" typical_orbit: "Uranus 19.2 AU, Neptune 30.2 AU; Neptune-mass exoplanets from days to many AU" rings_moons: "Both have narrow dark rings and families of moons; Neptune's Triton is a captured body" frequency_in_sim: "752 of the 8,742 planets in the 5,159 generated systems (709 cold, 46 hot, including 3 legacy-coded bodies)" real_examples: "Uranus, Neptune; GJ 436 b (warm Neptune); LTT 9779 b (ultra-hot Neptune)" subtypes: "IGC-CA Active, IGC-CQ Quiescent; IGH-WV Water-Veiled, IGH-RB Rayleigh-Blue, IGH-DS Dark-Stripped (sim names)" sim_source: "Ice giant physics engine and properties module; the ice giant science reference" rendered_example: "IGC-CQ-AQ and IGC-CA-DP-1 (sim renders); 17 ice giant looks in the procedural generator" last_verified: "2026-09-27, writer B" sim: entity: planet_series.IG refs: - id: nasa_uranus type: webpage author: [{literal: "NASA Space Science Data Coordinated Archive"}] title: "Uranus Fact Sheet" URL: https://nssdc.gsfc.nasa.gov/planetary/factsheet/uranusfact.html accessed: 2026-09-27 - id: nasa_neptune type: webpage author: [{literal: "NASA Space Science Data Coordinated Archive"}] title: "Neptune Fact Sheet" URL: https://nssdc.gsfc.nasa.gov/planetary/factsheet/neptunefact.html accessed: 2026-09-27 - id: nasa_neptune_facts type: webpage author: [{literal: "NASA Science"}] title: "Neptune Facts" URL: https://science.nasa.gov/neptune/facts/ accessed: 2026-09-27 - id: helled2020 type: article-journal author: [{family: Helled, given: Ravit}, {family: Fortney, given: Jonathan J.}] title: "The interiors of Uranus and Neptune: current understanding and open questions" container-title: "Philosophical Transactions of the Royal Society A" volume: 378 page: 20190474 issued: 2020 DOI: 10.1098/rsta.2019.0474 - id: millot2019 type: article-journal author: [{family: Millot, given: Marius}, {family: Coppari, given: Federica}, {family: Rygg, given: J. Ryan}, {literal: "et al."}] title: "Nanosecond X-ray diffraction of shock-compressed superionic water ice" container-title: Nature volume: 569 page: 251-255 issued: 2019 DOI: 10.1038/s41586-019-1114-6 - id: pearl1991 type: article-journal author: [{family: Pearl, given: J. C.}, {family: Conrath, given: B. J.}] title: "The albedo, effective temperature, and energy balance of Neptune, as determined from Voyager data" container-title: "Journal of Geophysical Research: Space Physics" volume: 96 page: 18921-18930 issued: 1991 DOI: 10.1029/91JA01087 - id: pearl1990 type: article-journal author: [{family: Pearl, given: J. C.}, {family: Conrath, given: B. J.}, {family: Hanel, given: R. A.}, {literal: "et al."}] title: "The albedo, effective temperature, and energy balance of Uranus, as determined from Voyager IRIS data" container-title: Icarus volume: 84 page: 12-28 issued: 1990 DOI: 10.1016/0019-1035(90)90155-3 - id: wang2025 type: article-journal author: [{family: Wang, given: Xinyue}, {family: Li, given: Liming}, {family: Roman, given: Michael}, {literal: "et al."}] title: "Internal Heat Flux and Energy Imbalance of Uranus" container-title: Geophysical Research Letters volume: 52 page: e2025GL115660 issued: 2025 DOI: 10.1029/2025GL115660 - id: irwin2022 type: article-journal author: [{family: Irwin, given: P. G. J.}, {family: Teanby, given: N. A.}, {family: Fletcher, given: L. N.}, {literal: "et al."}] title: "Hazy Blue Worlds: A Holistic Aerosol Model for Uranus and Neptune, Including Dark Spots" container-title: "Journal of Geophysical Research: Planets" volume: 127 page: e2022JE007189 issued: 2022 DOI: 10.1029/2022JE007189 - id: irwin2024 type: article-journal author: [{family: Irwin, given: Patrick G. J.}, {family: Dobinson, given: Jack}, {family: James, given: Arjuna}, {literal: "et al."}] title: "Modelling the seasonal cycle of Uranus's colour and magnitude, and comparison with Neptune" container-title: Monthly Notices of the Royal Astronomical Society volume: 527 page: 11521-11538 issued: 2024 DOI: 10.1093/mnras/stad3761 - id: limaye1991 type: article-journal author: [{family: Limaye, given: Sanjay S.}, {family: Sromovsky, given: Lawrence A.}] title: "Winds of Neptune: Voyager observations of cloud motions" container-title: "Journal of Geophysical Research: Space Physics" volume: 96 page: 18941-18960 issued: 1991 DOI: 10.1029/91JA01701 - id: sromovsky2015 type: article-journal author: [{family: Sromovsky, given: L. A.}, {family: de Pater, given: I.}, {family: Fry, given: P. M.}, {literal: "et al."}] title: "High S/N Keck and Gemini AO imaging of Uranus during 2012-2014: New cloud patterns, increasing activity, and improved wind measurements" container-title: Icarus volume: 258 page: 192-223 issued: 2015 DOI: 10.1016/j.icarus.2015.05.029 - id: hammel1995 type: article-journal author: [{family: Hammel, given: H. B.}, {family: Lockwood, given: G. W.}, {family: Mills, given: J. R.}, {literal: "et al."}] title: "Hubble Space Telescope Imaging of Neptune's Cloud Structure in 1994" container-title: Science volume: 268 page: 1740-1742 issued: 1995 DOI: 10.1126/science.268.5218.1740 - id: ness1986 type: article-journal author: [{family: Ness, given: Norman F.}, {family: Acuña, given: Mario H.}, {family: Behannon, given: Kenneth W.}, {literal: "et al."}] title: "Magnetic Fields at Uranus" container-title: Science volume: 233 page: 85-89 issued: 1986 DOI: 10.1126/science.233.4759.85 - id: ness1989 type: article-journal author: [{family: Ness, given: Norman F.}, {family: Acuña, given: Mario H.}, {family: Burlaga, given: Leonard F.}, {literal: "et al."}] title: "Magnetic Fields at Neptune" container-title: Science volume: 246 page: 1473-1478 issued: 1989 DOI: 10.1126/science.246.4936.1473 - id: stanley2004 type: article-journal author: [{family: Stanley, given: Sabine}, {family: Bloxham, given: Jeremy}] title: "Convective-region geometry as the cause of Uranus' and Neptune's unusual magnetic fields" container-title: Nature volume: 428 page: 151-153 issued: 2004 DOI: 10.1038/nature02376 - id: tsiganis2005 type: article-journal author: [{family: Tsiganis, given: K.}, {family: Gomes, given: R.}, {family: Morbidelli, given: A.}, {family: Levison, given: H. F.}] title: "Origin of the orbital architecture of the giant planets of the Solar System" container-title: Nature volume: 435 page: 459-461 issued: 2005 DOI: 10.1038/nature03539 - id: suzuki2016 type: article-journal author: [{family: Suzuki, given: D.}, {family: Bennett, given: D. P.}, {family: Sumi, given: T.}, {literal: "et al."}] title: "The Exoplanet Mass-Ratio Function from the MOA-II Survey: Discovery of a Break and Likely Peak at a Neptune Mass" container-title: The Astrophysical Journal volume: 833 page: 145 issued: 2016 DOI: 10.3847/1538-4357/833/2/145 - id: mazeh2016 type: article-journal author: [{family: Mazeh, given: T.}, {family: Holczer, given: T.}, {family: Faigler, given: S.}] title: "Dearth of short-period Neptunian exoplanets: A desert in period-mass and period-radius planes" container-title: Astronomy & Astrophysics volume: 589 page: A75 issued: 2016 DOI: 10.1051/0004-6361/201528065 - id: ehrenreich2015 type: article-journal author: [{family: Ehrenreich, given: David}, {family: Bourrier, given: Vincent}, {family: Wheatley, given: Peter J.}, {literal: "et al."}] title: "A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b" container-title: Nature volume: 522 page: 459-461 issued: 2015 DOI: 10.1038/nature14501 - id: hoyer2023 type: article-journal author: [{family: Hoyer, given: S.}, {family: Jenkins, given: J. S.}, {family: Parmentier, given: V.}, {literal: "et al."}] title: "The extremely high albedo of LTT 9779 b revealed by CHEOPS" container-title: Astronomy & Astrophysics volume: 675 page: A81 issued: 2023 DOI: 10.1051/0004-6361/202346117 images_wanted: - file: File:Ice_giant_IGC-CQ_sim.png subject: "The sim's quiescent (Uranus-like) cold ice giant: a pale blue-green, almost featureless disc darkening gently toward the limb" source: sim source_ref: "QA:gas_giant_limb_policy_stage_capped/aq_four_radii_sun20.png" credit: "Pax Abyssi (sim render, in-game look-development stage)" note: "1600 x 900 stage frame, no HUD." - file: File:Ice_giant_IGC-CA_sim.png subject: "The sim's active (Neptune-like) cold ice giant as a gibbous disc, deep blue with a bright streak of high cloud" source: sim source_ref: "QA:gas_giant_limb_policy_stage_capped/dp_four_radii_sun70.png" credit: "Pax Abyssi (sim render, in-game look-development stage)" note: "Caption must say the palette is bluer than Neptune's true colour (Irwin et al. 2024)." - file: File:Neptune_Webb_NIRCam_weic2214c.jpg subject: "Webb near-infrared image of Neptune with its rings and bright high-altitude methane-ice clouds" source: esa-webb page_url: https://esawebb.org/images/weic2214c/ credit: "NASA, ESA, CSA, and STScI" licence: "CC BY 4.0 (ESA/Webb usage terms)" --- An **ice giant** is a giant planet made mostly of elements heavier than hydrogen and helium: chiefly water, methane and ammonia, which planetary scientists call "ices" whatever their state. Uranus and Neptune are the two in the Solar System, each about 15 to 17 times Earth's mass and four times its width. Despite the name, almost nothing inside them is frozen: most of each planet is a hot, dense, electrically conducting fluid under a comparatively thin hydrogen-helium atmosphere :cite[helled2020]. Planets of about Neptune's mass appear to be among the most common in the Galaxy, which makes the two least-explored planets of the Solar System templates for a very large class. ## Characteristics ### Built differently from Jupiter Jupiter and Saturn are mostly hydrogen and helium. Uranus and Neptune are not: at least 80% of Neptune's mass is a hot, dense fluid of water, methane and ammonia :cite[nasa_neptune_facts]. Their densities, 1.27 g/cm³ for Uranus and 1.64 for Neptune :cite[nasa_uranus] :cite[nasa_neptune], sit between those of the gas giants and the rocky planets. Their interiors are usually modelled as three layers: a hydrogen-helium envelope, a deep "mantle" of water-rich fluid, and a rocky core. The proportions are uncertain, because the same gravity field can be matched by different mixtures of rock and water, and the boundaries between layers may be gradual rather than sharp :cite[helled2020]. The pressures in an ice giant never become high enough in its thin hydrogen envelope to make metallic hydrogen, so the magnetic field must come from somewhere else. At the millions of atmospheres and thousands of degrees of the deep mantle, water is ionic and conducts electricity. Laboratory shock experiments have produced **superionic** water ice under such conditions: a crystal lattice of oxygen through which hydrogen ions flow like a liquid :cite[millot2019]. ### Tilted, lopsided magnetic fields Voyager 2, the only spacecraft to visit either planet, found magnetic fields unlike any other planet's. Uranus's dipole is tilted about 59 degrees from its spin axis and offset from the centre by about a third of the planet's radius :cite[ness1986]; Neptune's is tilted about 47 degrees and offset by more than half its radius :cite[ness1989]. Both fields also have strong components more complex than a simple dipole. Models reproduce this if the field is generated in a thin convecting shell of conducting fluid above a stable interior, rather than in a deep, fully convecting region :cite[stanley2004]. ### Hot Neptune, cold Uranus The two planets are near twins in size and mass but not in heat. Neptune radiates about 2.6 times as much energy as it absorbs from the Sun :cite[pearl1991]. Voyager found Uranus's output almost indistinguishable from the sunlight it absorbs :cite[pearl1990]; a 2025 reanalysis of Uranus's reflected light finds that it emits about 12.5% more than it absorbs, a small but real internal heat flow :cite[wang2025]. Why Uranus holds its heat in is unresolved; suggestions include a giant impact, which may also explain its 98-degree tilt, and layers in the interior that block convection :cite[helled2020]. ### Clouds, colour and wind The top clouds of an ice giant are methane ice, near 1 to 2 bar, with a thicker layer of haze and hydrogen sulfide ice below :cite[irwin2022]. Methane gas absorbs red light, which is why both planets are blue-green. Their true colours are more alike than the familiar Voyager images suggest. Both are a pale greenish blue; Neptune is only slightly bluer, because Uranus has a thicker layer of haze over its methane cloud. Voyager's images of Neptune were contrast-stretched to bring out its clouds, which made it look a much deeper blue than it is :cite[irwin2024]. Neptune, the planet farthest from the Sun, has the fastest winds measured on any planet: Voyager tracked clouds moving westward at about 400 m/s near the equator :cite[limaye1991]. Uranus's winds are gentler, with eastward jets near 60 degrees latitude of about 250 m/s :cite[sromovsky2015]. Neptune also makes large dark storms. The Great Dark Spot that Voyager photographed in 1989, about the size of Earth, was gone when the Hubble Space Telescope looked in 1994 :cite[hammel1995]; new spots have appeared and faded since, each lasting a few years. ::figure{src="File:Neptune_Webb_NIRCam_weic2214c.jpg" size=wide alt="Neptune in near-infrared light, a dim disc ringed by thin bright rings, with bright clouds in its southern hemisphere" caption="Observation: Neptune and its rings in near-infrared light from the James Webb Space Telescope, 2022. The planet looks dark because methane absorbs these wavelengths; the bright patches are high methane-ice clouds. Credit: NASA, ESA, CSA, and STScI."} ## Formation Ice giants are a puzzle for formation models. At 20 to 30 AU from the Sun, solid material was sparse and orbits slow, so growing a core of more than ten Earth masses before the gas disc disappeared would have taken too long. One answer is that Uranus and Neptune formed closer to the Sun and were pushed outward later, when the orbits of the giant planets were rearranged; the Nice model reproduces the giants' present orbits this way :cite[tsiganis2005]. Their modest hydrogen-helium envelopes suggest that their cores reached the size for runaway gas capture only as the disc was fading, so they never became gas giants. ## Neptune-like planets around other stars Microlensing surveys, which can detect planets several AU from their stars, find that planets of about Neptune's mass-to-star ratio are the most common type in that region :cite[suzuki2016]. Close to their stars, however, Neptune-sized planets are rare: there is a **hot Neptune desert** at orbital periods shorter than two to four days, probably because such planets lose their atmospheres to starlight or never arrive there :cite[mazeh2016]. The warm Neptune GJ 436 b trails a comet-like cloud of escaping hydrogen that blocks 56% of its star's ultraviolet light in transit, against 0.7% of visible light for the planet itself :cite[ehrenreich2015]. LTT 9779 b, an ultra-hot Neptune inside the desert, is unexpectedly reflective, with a geometric albedo of about 0.8, similar to Venus's, best explained by silicate clouds in a very metal-rich atmosphere :cite[hoyer2023]. Whether such planets have ice-giant interiors like Uranus and Neptune, or rocky cores with thick envelopes, usually cannot be told from mass and radius alone; the smaller, commoner [[Mini-Neptune|mini-Neptunes]] are a separate class. ## How we know Voyager 2 flew past Uranus in 1986 and Neptune in 1989, and everything known about their magnetic fields, interiors and moons at close range comes from those two passes. Since then the Hubble Space Telescope, large ground-based telescopes with adaptive optics and JWST have tracked their clouds, storms, seasons and rings. Neptune-mass exoplanets are found by transits, radial velocities and microlensing. ## Notable examples | Planet | Mass | Radius | Orbit | Notes | |---|---|---|---|---| | Uranus | 14.5 Earth masses | 25,559 km | 19.2 AU, 84 years | Tilted 97.8 degrees; little internal heat :cite[nasa_uranus] :cite[wang2025] | | Neptune | 17.1 Earth masses | 24,764 km | 30.2 AU, 165 years | Fastest winds; emits 2.6 times what it absorbs :cite[nasa_neptune] :cite[pearl1991] | | GJ 436 b | about 22 Earth masses | about 4 Earth radii | 2.6 days | Warm Neptune losing hydrogen :cite[ehrenreich2015] | | LTT 9779 b | about 29 Earth masses | about 4.7 Earth radii | 0.8 days | Ultra-hot Neptune with an albedo near 0.8 :cite[hoyer2023] | ::figure{src="File:Ice_giant_IGC-CA_sim.png" size=wide alt="A deep blue planet seen gibbous, with a thin bright white cloud streak near its limb" caption="Sim render: the sim's active, Neptune-like ice giant. Its deep blue is the game's palette, bluer than Neptune's true pale greenish blue."} :::callout{type=sim title="In Pax Abyssi"} The sim has cold ice giants (IGC) and hot ice giants (IGH). Cold ice giants are split by internal heat, following the Uranus and Neptune pair: **Active** (IGC-CA, Neptune-like, emitting two to three times the sunlight it absorbs, with fast westward equatorial jets, thin haze and a chance of a dark spot) and **Quiescent** (IGC-CQ, Uranus-like, little internal heat, slower winds and thicker haze). Hot ice giants, for Neptune-mass planets closer to their stars, take names of the sim's own by temperature: **Water-Veiled** (350 to 700 K), **Rayleigh-Blue**, a cloudless blue planet (700 to 1,200 K), and **Dark-Stripped** (above 1,200 K). There are 752 ice giants among the 8,742 planets of the 5,159 generated systems, 709 of them cold. The procedural gas giant generator (BUILT, awaiting the owner's verdict) has 17 ice giant looks. It gives ice giants a westward equatorial jet, softer limb darkening and a pale haze at the limb. One of the first flyable stops takes the ship to a Neptune-class ice giant generated for Xi Bootis, a real Sun-like star 22 light years away. ::: ## See also - [[Gas giant]] - [[Mini-Neptune]] - [[Hot Neptune desert]] - [[Exotic worlds]] - [[Planetary rings]] - [[Sol]]