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). 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%. These materials were actually ices during the ice giants' formation, but now they exist in different phases, primarily supercritical fluids. The 'ice' is primarily H2O and is generally treated as such, making the equation of state of water important for modeling ice giants. Ice giants are thought to lack metallic hydrogen at their cores, unlike the gas giants.
Different atmospheric patterns have been observed, including polar vortices, strong zonal winds, and large-scale circulation. There is no satisfying model that describes why these features exist. Because of their large sizes and low thermal conductivities, planetary interior pressures range up to several 100 GPa and temperatures of several 1000 K. In March 2012, it was found that the compressibility of water used in ice-giant models could be off one third. The value is important for modeling ice giants, and has a ripple effect understanding of them. Ice giants include Uranus, Neptune, and exoplanets so categorized.
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). The existence of these envelopes provides a critical constraint: giant planets must form relatively quickly, before the gas in the protoplanetary disk is dissipated. Observations of protoplanetary disks around stars in young clusters pin the gas disk lifetime in the 3–10 million year range.
The magnetic fields of Uranus and Neptune are both unusually displaced and tilted. 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. Their magnetic fields are believed to originate in an ionized convecting molten ice mantle.