Skip to content
Pax Abyssi
DiscussEditWhat links here (28)Source

Redirected from Ice giants

Planet class · IGC, IGH · IG1-C, IG2-H

Ice giant

ObservedMeasured or catalogued in the real sky, with its source cited.ModelPublished physics or a published model, applied as written.SimWhat the simulation generates or renders; it may depart from reality and says where.This page mixes measured in the real sky, published physics applied as written and how Pax Abyssi models it, built from the physics.How we decide
ContentsShow

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.

How Pax Abyssi classifies ice giants

Ice giants are their own series in Pax Abyssi, with their own physics engine: a planet of roughly 6 to 50 Earth masses, mostly water, methane and ammonia under a hydrogen-helium envelope. Temperature makes the first split, into cold and hot families, and after that each family is sorted by the quantity that most changes its face.

Cold ice giants (IGC) are sorted by internal heat. Uranus and Neptune are near twins in size and mass, yet they look different, and the reason is the heat escaping from inside. Neptune emits about 2.6 times the energy it absorbs from the Sun 2; that heat drives convection, the fastest winds in the Solar System and dark storms the size of Earth. Uranus emits little more than it absorbs 3 4, and it wears a thicker haze and shows far fewer storms. We tie the two together: a weakly convecting ice giant in Pax Abyssi is a hazy one. So our cold ice giants are Active (IGC-CA, emitting two to three times what they absorb, a fast retrograde equatorial jet, thin haze and a chance of a dark spot) or Quiescent (IGC-CQ, emitting little more than they absorb, gentler winds and thick haze). The type then follows the haze, from thin to thick: in Active giants Deep Azure (DP), Royal Blue (RY) and Bright Blue (BR); in Quiescent ones Teal (TL), Pale Cyan (CY) and Palest Aqua (AQ). Thin haze lets light reach deep enough for methane to strip out the red; thick haze reflects it first, which is why Uranus is paler than Neptune 5.

Our Active ice giants wear the deep blue of Voyager's famous Neptune portraits, a choice we made because it is the Neptune most people know and it shows the storms well. The true Neptune, as the section below explains, is a paler greenish blue, only a shade bluer than Uranus; the Quiescent giants show that paler end of the range.

A deep blue planet with faint bands, a dark oval storm and a bright white cloud beside it., open full size
IGC-CA-DPDeep AzureDeep azure with a dark oval and a bright companion cloud: a thin haze lets light reach deep, where methane strips the red and hydrogen scatters the blue, and strong internal heat drives dark storms like the one Voyager 2 saw on Neptune.
A royal blue planet with a few thin white cloud streaks., open full size
IGC-CA-RYRoyal BlueRoyal blue with thin white streaks of methane-ice cloud riding above the main deck, and no dark spot.
A bright blue planet with soft visible bands and faint white clouds., open full size
IGC-CA-BRBright BlueA brighter blue with soft but visible bands under a slightly thicker haze.
A teal-cyan planet, almost featureless., open full size
IGC-CQ-TLTealCool teal with only the ghost of structure: with little internal heat to drive convection, haze builds up and hides the weather.
A smooth pale cyan planet with no visible features., open full size
IGC-CQ-CYPale CyanA smooth, pale cyan ball, the Uranus look: light reflects off the thick haze before methane can absorb much of the red.
A very pale, near-white aqua planet., open full size
IGC-CQ-AQPalest AquaNear-white aqua under the thickest haze of all, so bright that methane's colour barely shows.
Figure 1In Pax Abyssi: the six cold ice giant types, Active above and Quiescent below, drawn by our gas giant generator near full phase. The Deep Azure giant shown is variant 2, with a dark spot on the visible face; every Active and Quiescent type has three variants.
Pax Abyssi

Hot ice giants (IGH) are sorted by temperature, for Neptune-mass planets much closer to their stars. Above about 350 K methane no longer freezes, so the blue methane-ice world gives way to three regimes. Water-Veiled giants (IGH-WV, 350 to 700 K) are the coolest, with water as the top cloud at the cool end and a thinning, greying veil as they warm. Rayleigh-Blue giants (IGH-RB, 700 to 1,200 K) have lost their clouds and hazes: the clear hydrogen atmosphere scatters blue light back to space, a sky with nothing in it. Dark-Stripped giants (IGH-DS, above 1,200 K) are hot enough for sodium and potassium vapour to absorb across the visible, as in the dark hot Jupiters, and a tidally locked form (IGH-DSTL) carries a gradient from its day side to its night side.

A pale blue-white planet swirled with white cloud patches., open full size
IGH-WV-CLWater-Veiled, CoolPale blue-white with soft swirls of cloud: too warm for methane ice, cool enough for water to form the top cloud.
A grey-blue planet with faint pale cloud patches., open full size
IGH-WV-MDWater-Veiled, MidGrey-blue with fading patches, the cloud veil thinner and greyer as the planet warms.
A warm grey planet with very faint broad zones., open full size
IGH-WV-WMWater-Veiled, WarmWarm grey with only the last broad, soft traces of cloud structure.
A vivid bright blue planet with no features., open full size
IGH-RB-VVRayleigh-Blue, VividVivid, featureless blue: with no cloud or haze left, the clear hydrogen scatters blue light back, a sky with nothing in it.
A deep cobalt blue planet with no features., open full size
IGH-RB-DPRayleigh-Blue, DeepDeep cobalt, the clear atmosphere darkening as absorption grows with temperature.
A dark blue planet with a slightly darker band across its middle., open full size
IGH-RB-DKRayleigh-Blue, DarkDark blue with a faint, darker band at the equator, where sodium and potassium vapour begin to absorb.
A dark grey-brown planet with no features., open full size
IGH-DS-MODark-Stripped, ModerateDark grey-brown: sodium and potassium absorb across the visible and remove the blue.
A dark copper-brown planet with no features., open full size
IGH-DS-STDark-Stripped, StrongDark copper-brown and nearly uniform, reflecting only a few per cent of its light.
A nearly black planet with a faint charcoal-brown tint., open full size
IGH-DS-EXDark-Stripped, ExtremeNear-black with a charcoal-brown tint, one of the darkest bodies in the galaxy.
A dark copper-brown planet, slightly brighter on one side than the other., open full size
IGH-DSTL-STDark-Stripped, Tidally LockedDark copper-brown with a smooth gradient from the warmer day hemisphere to the night side, the mark of a planet that always turns one face to its star.
Figure 2In Pax Abyssi: the ten hot ice giant types, cooler to hotter, drawn by our gas giant generator near full phase.
Pax Abyssi

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 6. Their densities, 1.27 g/cm³ for Uranus and 1.64 for Neptune 7 8, 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 9.

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 10; Neptune's is tilted about 47 degrees and offset by more than half its radius 11. 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 12.

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 2. Voyager found Uranus's output almost indistinguishable from the sunlight it absorbs 3; 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 4. 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 13. 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 5.

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, open full size
Figure 3Observation: 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.
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 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

PlanetMassRadiusOrbitNotes
Uranus14.5 Earth masses25,559 km19.2 AU, 84 yearsTilted 97.8 degrees; little internal heat 7 4
Neptune17.1 Earth masses24,764 km30.2 AU, 165 yearsFastest winds; emits 2.6 times what it absorbs 8 2
GJ 436 babout 22 Earth massesabout 4 Earth radii2.6 daysWarm Neptune losing hydrogen 20
LTT 9779 babout 29 Earth massesabout 4.7 Earth radii0.8 daysUltra-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, open full size
Figure 4In Pax Abyssi: a Neptune-class Active ice giant generated for Xi Bootis, a real Sun-like star 22 light years away, seen from four planetary radii with a bright streak of high methane-ice cloud near its limb.

See also

References

  1. 1Helled, 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. doi:10.1098/rsta.2019.0474
  2. 2Pearl, 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. doi:10.1029/91JA01087
  3. 3Pearl, J. C. et al. (1990). The albedo, effective temperature, and energy balance of Uranus, as determined from Voyager IRIS data. Icarus 84, 12-28. doi:10.1016/0019-1035(90)90155-3
  4. 4Wang, X. et al. (2025). Internal Heat Flux and Energy Imbalance of Uranus. Geophysical Research Letters 52, e2025GL115660. doi:10.1029/2025GL115660
  5. 5Irwin, 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. doi:10.1093/mnras/stad3761
  6. 6NASA Science. Neptune Facts. science.nasa.gov/neptune/facts/
  7. 7NASA Space Science Data Coordinated Archive. Uranus Fact Sheet. nssdc.gsfc.nasa.gov/planetary/factsheet/uranusfact.html
  8. 8NASA Space Science Data Coordinated Archive. Neptune Fact Sheet. nssdc.gsfc.nasa.gov/planetary/factsheet/neptunefact.html
  9. 9Millot, M. et al. (2019). Nanosecond X-ray diffraction of shock-compressed superionic water ice. Nature 569, 251-255. doi:10.1038/s41586-019-1114-6
  10. 10Ness, N. F. et al. (1986). Magnetic Fields at Uranus. Science 233, 85-89. doi:10.1126/science.233.4759.85
  11. 11Ness, N. F. et al. (1989). Magnetic Fields at Neptune. Science 246, 1473-1478. doi:10.1126/science.246.4936.1473
  12. 12Stanley, S. and Bloxham, J. (2004). Convective-region geometry as the cause of Uranus' and Neptune's unusual magnetic fields. Nature 428, 151-153. doi:10.1038/nature02376
  13. 13Irwin, 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. doi:10.1029/2022JE007189
  14. 14Limaye, S. S. and Sromovsky, L. A. (1991). Winds of Neptune: Voyager observations of cloud motions. Journal of Geophysical Research: Space Physics 96, 18941-18960. doi:10.1029/91JA01701
  15. 15Sromovsky, 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. doi:10.1016/j.icarus.2015.05.029
  16. 16Hammel, H. B. et al. (1995). Hubble Space Telescope Imaging of Neptune's Cloud Structure in 1994. Science 268, 1740-1742. doi:10.1126/science.268.5218.1740
  17. 17Tsiganis, K. et al. (2005). Origin of the orbital architecture of the giant planets of the Solar System. Nature 435, 459-461. doi:10.1038/nature03539
  18. 18Suzuki, 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. doi:10.3847/1538-4357/833/2/145
  19. 19Mazeh, 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. doi:10.1051/0004-6361/201528065
  20. 20Ehrenreich, D. et al. (2015). A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b. Nature 522, 459-461. doi:10.1038/nature14501
  21. 21Hoyer, S. et al. (2023). The extremely high albedo of LTT 9779 b revealed by CHEOPS. Astronomy & Astrophysics 675, A81. doi:10.1051/0004-6361/202346117
Last edited 28 Sept 2026, 10:18 UTC by Pax Abyssi Science Deskbotstaff(+5,901 bytes)HistoryText CC BY-SA 4.0Report an errorObservedMeasured or catalogued in the real sky, with its source cited.ModelPublished physics or a published model, applied as written.SimWhat the simulation generates or renders; it may depart from reality and says where.