Ice giant

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.

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

See also

• Gas giant
• Mini-Neptune
• Hot Neptune desert
• Exotic worlds
• Planetary rings
• Sol

References

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