--- title: Ice giant canonical_url: https://paxabyssi.com/wiki/Ice_giant markdown_url: https://paxabyssi.com/wiki/Ice_giant.md type: wiki-page revision_id: 165 revision_view: stable last_updated: 2026-09-27 license: CC BY-SA 4.0 license_url: https://creativecommons.org/licenses/by-sa/4.0/ science_status: - observed - model - sim 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. 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 code: IGC, IGH mass: note: "Sim: 8 to 50 (cold), 6 to 50 (hot)" unit: M_Earth value: Uranus 14.5, Neptune 17.1; about 10 to 50 for the class source: observed name: Ice giant image: File:Ice_giant_IGC-CQ_sim.png level: Series (two sim types) radius: unit: km value: Uranus 25,559; Neptune 24,764 (about 4 Earth radii) source: observed series: IG (ice giant) caption: "Sim render: the sim's quiescent, Uranus-like cold ice giant (IGC-CQ), a pale blue-green disc with gentle limb darkening" 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 bond_albedo: unit: dimensionless value: Uranus 0.30, Neptune 0.29 source: observed legacy_code: IG1-C, IG2-H rings_moons: Both have narrow dark rings and families of moons; Neptune's Triton is a captured body tidal_state: Cold ice giants not locked (Uranus 17.2 h, Neptune 16.1 h); hot ones locked wind_speeds: unit: m/s value: Neptune's equatorial winds about 400 westward; Uranus's jets up to about 250 eastward at mid-latitudes source: observed bulk_density: unit: g/cm³ value: Uranus 1.27, Neptune 1.64 source: observed last_verified: 2026-09-27, writer B real_examples: Uranus, Neptune; GJ 436 b (warm Neptune); LTT 9779 b (ultra-hot Neptune) typical_orbit: Uranus 19.2 AU, Neptune 30.2 AU; Neptune-mass exoplanets from days to many AU dominant_gases: H2 about 80%, He 15 to 19%, CH4 1.5 to 2.3% by volume in the upper atmosphere magnetic_field: "Multipolar and offset: dipoles tilted about 59 (Uranus) and 47 (Neptune) degrees from the spin axis" science_status: observed (Uranus, Neptune); model (interiors); sim (subtypes) escape_velocity: unit: km/s value: Uranus 21.3, Neptune 23.5 source: observed frequency_in_sim: 752 of the 8,742 planets in the 5,159 generated systems (709 cold, 46 hot, including 3 legacy-coded bodies) gravity_at_1_bar: unit: m/s² value: Uranus 8.7, Neptune 11.0 (equatorial) source: observed rendered_example: IGC-CQ-AQ and IGC-CA-DP-1 (sim renders); 17 ice giant looks in the procedural generator 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" 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 temperature_at_1_bar: unit: K value: Uranus 76, Neptune 72 source: observed literature_equivalent: Ice giant; Neptune-mass planet ('Neptune-like') cloud_and_haze_species: Methane ice near 1 to 2 bar; hydrogen sulfide and haze below; NH4SH and water far deeper; photochemical haze above related: - https://paxabyssi.com/wiki/Gas_giant.md - https://paxabyssi.com/wiki/Cold_gas_giant.md - https://paxabyssi.com/wiki/Hot_Jupiter.md - https://paxabyssi.com/wiki/Temperate_gas_giant.md - https://paxabyssi.com/wiki/Exotic_worlds.md - https://paxabyssi.com/wiki/Mini-Neptune.md --- # Ice giant > Source: https://paxabyssi.com/wiki/Ice_giant > > Licence: [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0/). Text by Pax Abyssi Wiki contributors; history at https://paxabyssi.com/wiki/Ice_giant/history > > Revision 165, 27 September 2026 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 [1]. 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 [2]. Their densities, 1.27 g/cm³ for Uranus and 1.64 for Neptune [3] [4], 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 [1]. 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 [5]. ### 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 [6]; Neptune's is tilted about 47 degrees and offset by more than half its radius [7]. 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 [8]. ### 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 [9]. Voyager found Uranus's output almost indistinguishable from the sunlight it absorbs [10]; 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 [11]. 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 [1]. ### 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 [12]. 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 [13]. 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 [14]. Uranus's winds are gentler, with eastward jets near 60 degrees latitude of about 250 m/s [15]. 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 [16]; new spots have appeared and faded since, each lasting a few years. ![Neptune in near-infrared light, a dim disc ringed by thin bright rings, with bright clouds in its southern hemisphere](https://media.paxabyssi.com/public/88aed0bf91c7773b7c6d488348f95ac0a156a45f2bad6b7428925c429b285953/1680.webp "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.") *Figure 1.* 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. Licence: CC BY 4.0. ## 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 [17]. 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 [18]. 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 [19]. 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 [20]. 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 [21]. 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-Neptunes](https://paxabyssi.com/wiki/Mini-Neptune.md) 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 [3] [11] | | Neptune | 17.1 Earth masses | 24,764 km | 30.2 AU, 165 years | Fastest winds; emits 2.6 times what it absorbs [4] [9] | | GJ 436 b | about 22 Earth masses | about 4 Earth radii | 2.6 days | Warm Neptune losing hydrogen [20] | | LTT 9779 b | about 29 Earth masses | about 4.7 Earth radii | 0.8 days | Ultra-hot Neptune with an albedo near 0.8 [21] | ![A deep blue planet seen gibbous, with a thin bright white cloud streak near its limb](https://media.paxabyssi.com/public/23feddbfce6132189389d35446ea20799e0d50e272190c631cfcb56941da83e2/1280.webp "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.") *Figure 2.* 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. Credit: Pax Abyssi. Licence: Pax Abyssi, all rights reserved. > **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](https://paxabyssi.com/wiki/Gas_giant.md) - [Mini-Neptune](https://paxabyssi.com/wiki/Mini-Neptune.md) - [Hot Neptune desert](https://paxabyssi.com/wiki/Hot_Neptune_desert.md) - [Exotic worlds](https://paxabyssi.com/wiki/Exotic_worlds.md) - [Planetary rings](https://paxabyssi.com/wiki/Planetary_rings.md) - [Sol](https://paxabyssi.com/wiki/Sol.md) ## References 1. Helled, R. and Fortney, J. J. (2020). The interiors of Uranus and Neptune: current understanding and open questions. Philosophical Transactions of the Royal Society A 378, 20190474. 2. NASA Science. Neptune Facts. 3. NASA Space Science Data Coordinated Archive. Uranus Fact Sheet. 4. NASA Space Science Data Coordinated Archive. Neptune Fact Sheet. 5. Millot, M. et al. (2019). Nanosecond X-ray diffraction of shock-compressed superionic water ice. Nature 569, 251-255. 6. Ness, N. F. et al. (1986). Magnetic Fields at Uranus. Science 233, 85-89. 7. Ness, N. F. et al. (1989). Magnetic Fields at Neptune. Science 246, 1473-1478. 8. Stanley, S. and Bloxham, J. (2004). Convective-region geometry as the cause of Uranus' and Neptune's unusual magnetic fields. Nature 428, 151-153. 9. Pearl, J. C. and Conrath, B. J. (1991). The albedo, effective temperature, and energy balance of Neptune, as determined from Voyager data. Journal of Geophysical Research: Space Physics 96, 18921-18930. 10. Pearl, J. C. et al. (1990). The albedo, effective temperature, and energy balance of Uranus, as determined from Voyager IRIS data. Icarus 84, 12-28. 11. Wang, X. et al. (2025). Internal Heat Flux and Energy Imbalance of Uranus. Geophysical Research Letters 52, e2025GL115660. 12. Irwin, P. G. J. et al. (2022). Hazy Blue Worlds: A Holistic Aerosol Model for Uranus and Neptune, Including Dark Spots. Journal of Geophysical Research: Planets 127, e2022JE007189. 13. Irwin, P. G. J. et al. (2024). Modelling the seasonal cycle of Uranus's colour and magnitude, and comparison with Neptune. Monthly Notices of the Royal Astronomical Society 527, 11521-11538. 14. Limaye, S. S. and Sromovsky, L. A. (1991). Winds of Neptune: Voyager observations of cloud motions. Journal of Geophysical Research: Space Physics 96, 18941-18960. 15. Sromovsky, L. A. et al. (2015). High S/N Keck and Gemini AO imaging of Uranus during 2012-2014: New cloud patterns, increasing activity, and improved wind measurements. Icarus 258, 192-223. 16. Hammel, H. B. et al. (1995). Hubble Space Telescope Imaging of Neptune's Cloud Structure in 1994. Science 268, 1740-1742. 17. Tsiganis, K. et al. (2005). Origin of the orbital architecture of the giant planets of the Solar System. Nature 435, 459-461. 18. Suzuki, D. et al. (2016). The Exoplanet Mass-Ratio Function from the MOA-II Survey: Discovery of a Break and Likely Peak at a Neptune Mass. The Astrophysical Journal 833, 145. 19. Mazeh, T., Holczer, T. and Faigler, S. (2016). Dearth of short-period Neptunian exoplanets: A desert in period-mass and period-radius planes. Astronomy & Astrophysics 589, A75. 20. Ehrenreich, D. et al. (2015). A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b. Nature 522, 459-461. 21. Hoyer, S. et al. (2023). The extremely high albedo of LTT 9779 b revealed by CHEOPS. Astronomy & Astrophysics 675, A81. ## Infobox (planet class) | Field | Value | | --- | --- | | Code | IGC, IGH | | Mass | Uranus 14.5, Neptune 17.1; about 10 to 50 for the class M_Earth | | Name | Ice giant | | Image | File:Ice_giant_IGC-CQ_sim.png | | Level | Series (two sim types) | | Radius | Uranus 25,559; Neptune 24,764 (about 4 Earth radii) km | | Series | IG (ice giant) | | Caption | Sim render: the sim's quiescent, Uranus-like cold ice giant (IGC-CQ), a pale blue-green disc with gentle limb darkening | | 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 | | Bond albedo | Uranus 0.30, Neptune 0.29 dimensionless | | Legacy code | IG1-C, IG2-H | | Rings moons | Both have narrow dark rings and families of moons; Neptune's Triton is a captured body | | Tidal state | Cold ice giants not locked (Uranus 17.2 h, Neptune 16.1 h); hot ones locked | | Wind speeds | Neptune's equatorial winds about 400 westward; Uranus's jets up to about 250 eastward at mid-latitudes m/s | | Bulk density | Uranus 1.27, Neptune 1.64 g/cm³ | | Last verified | 2026-09-27, writer B | | Real examples | Uranus, Neptune; GJ 436 b (warm Neptune); LTT 9779 b (ultra-hot Neptune) | | Typical orbit | Uranus 19.2 AU, Neptune 30.2 AU; Neptune-mass exoplanets from days to many AU | | Dominant gases | H2 about 80%, He 15 to 19%, CH4 1.5 to 2.3% by volume in the upper atmosphere | | Magnetic field | Multipolar and offset: dipoles tilted about 59 (Uranus) and 47 (Neptune) degrees from the spin axis | | Science status | observed (Uranus, Neptune); model (interiors); sim (subtypes) | | Escape velocity | Uranus 21.3, Neptune 23.5 km/s | | Frequency in sim | 752 of the 8,742 planets in the 5,159 generated systems (709 cold, 46 hot, including 3 legacy-coded bodies) | | Gravity at 1 bar | Uranus 8.7, Neptune 11.0 (equatorial) m/s² | | Rendered example | IGC-CQ-AQ and IGC-CA-DP-1 (sim renders); 17 ice giant looks in the procedural generator | | 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 | | 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 | | Temperature at 1 bar | Uranus 76, Neptune 72 K | | Literature equivalent | Ice giant; Neptune-mass planet ('Neptune-like') | | Cloud and haze species | Methane ice near 1 to 2 bar; hydrogen sulfide and haze below; NH4SH and water far deeper; photochemical haze above | ## Related pages - [Gas giant](https://paxabyssi.com/wiki/Gas_giant.md): A planet made mostly of hydrogen and helium, with no solid surface, from about a tenth of Jupiter's mass up to the brown dwarf boundary near 13 Jupiter masses. Jupiter and Saturn are the Solar System's two; thousands more are known around other stars. - [Cold gas giant](https://paxabyssi.com/wiki/Cold_gas_giant.md): A gas giant far enough from its star that ammonia freezes into clouds high in its atmosphere, like Jupiter and Saturn. Its own internal heat rivals the sunlight it absorbs, and it can keep large families of moons and rings. - [Hot Jupiter](https://paxabyssi.com/wiki/Hot_Jupiter.md): A gas giant orbiting so close to its star that its year lasts days and its dayside is hotter than lava. Hot Jupiters were the first planets found around Sun-like stars and remain the best-studied exoplanet atmospheres, though fewer than one Sun-like star in a hundred has one. - [Temperate gas giant](https://paxabyssi.com/wiki/Temperate_gas_giant.md): A gas giant warmer than Jupiter but cooler than a hot Jupiter, roughly 150 to 800 K, where ammonia clouds give way to water clouds and then to clear air. It spans the widest range of looks of any giant, from brilliant white to dark blue. - [Exotic worlds](https://paxabyssi.com/wiki/Exotic_worlds.md): Four kinds of planet that physics allows but the Solar System lacks. Iron-rich super-Mercuries are observed; carbon planets, helium-atmosphere planets and chthonian planets (the stripped cores of giants) remain hypotheses with candidates but no confirmed member. - [Mini-Neptune](https://paxabyssi.com/wiki/Mini-Neptune.md): A planet between about 1.7 and 4 times Earth's radius with a rocky or icy core wrapped in a thin envelope of hydrogen and helium. The most common kind of planet Kepler found, and one the Solar System lacks. Categories: [Ice giants](https://paxabyssi.com/wiki/Category:Ice_giants.md), [Planet classes](https://paxabyssi.com/wiki/Category:Planet_classes.md), [Giant planets](https://paxabyssi.com/wiki/Category:Giant_planets.md)