Ice giant
An ice giant is type of giant planet composed largely of materials less volatile than 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] They are primarily composed of 'ices'—volatile elements heavier than hydrogen and helium.[1] These were actually ices during the ice giants' formation, though now they exist primarily in different phases, primarily supercritical fluids.[1] They are thought to lack metallic hydrogen at their cores, unlike the gas giants.[1]. Different 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]
In March 2012, it was found that the compressiblity of water used in ice-giant models could be off one third.[2] The value is important for modeling ice giants, and has a ripple effect understanding of them.[2] Ice giants include Uranus, Neptune, and exoplanets so categorized.[2]
Formation of ice giants
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).[3] The existence of these envelopes provides a critical constraint: giant planets must form relatively quickly, before the gas in the protoplanetary disk is dissipated.[3] Observations of protoplanetary disks around stars in young clusters pin the gas disk lifetime in the 3–10 million year range.[3]
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