Ice giant

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This article is about the type of planet. For other uses, see Ice giant (disambiguation).

An ice giant is a giant planet composed mainly of 'ices'—volatile substances heavier than hydrogen and helium, such as water, methane, and ammonia—as opposed to 'gas' (hydrogen and helium).[1] It became known in the 1990s that Uranus and Neptune were really a distinct class of giant planet, composed of about 20% hydrogen, compared to the heavier gas giant's 90%.[1] These materials were actually ices during the ice giants' formation, but now they exist in different phases, primarily supercritical fluids.[1] The 'ice' is primarily H2O and is generally treated as such, making the equation of state of water important for modeling ice giants.[2] Ice giants are thought to lack metallic hydrogen at their cores, unlike the gas giants.[1]

Different atmospheric patterns have been observed, including polar vortices, strong zonal winds, and large-scale circulation.[1] There is no satisfying model that describes why these features exist.[1] Because of their large sizes and low thermal conductivities, planetary interior pressures range up to several 100 GPa and temperatures of several 1000 K.[2] In March 2012, it was found that the compressibility of water used in ice-giant models could be off one third.[3] The value is important for modeling ice giants, and has a ripple effect understanding of them.[3] Ice giants include Uranus, Neptune, and exoplanets so categorized.[3]

Formation of ice giants[edit]

The ice giants have gas envelopes that are smaller than those of the gas giants Jupiter and Saturn but that are still substantial (several Earth masses).[4] The existence of these envelopes provides a critical constraint: giant planets must form relatively quickly, before the gas in the protoplanetary disk is dissipated.[4] Observations of protoplanetary disks around stars in young clusters pin the gas disk lifetime in the 3–10 million year range.[4]

Magnetic fields[edit]

The magnetic fields of Uranus and Neptune are both unusually displaced and tilted.[5] Their field strengths are intermediate between those of the gas giants Jupiter and Saturn and those of the terrestrial planets, being 50 and 25 times that of Earth's, respectively.[5] Their magnetic fields are believed to originate in an ionized convecting molten ice mantle.[5]

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