An ice giant is a huge planet made mostly of elements heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, and sulfur. There are two known ice giants in the Solar System, Uranus and Neptune.
In astrophysics and planetary science the term "ice" refers to volatile chemical compounds with freezing points above about 100 K (−280 °F; −173 °C), such as water, ammonia, or methane, with freezing points of 273, 195, and 91 K (31.7, −108.7, and −295.9 °F; −0.1, −78.1, and −182.2 °C), respectively.
In the 1990s, it was realized that Uranus and Neptune are a distinct class of giant planet, separate from the other giant planets, Jupiter and Saturn. They have become known as ice giants. Their constituent compounds were solids when the planets were stuck together during their formation. Today, little of the water in Uranus and Neptune is in the form of ice. Instead, the water is a supercritical fluid at the temperatures and pressures inside them.
Ice giants consist of only about 20% hydrogen and helium in mass, as opposed to the Solar System's gas giants, Jupiter and Saturn, which are both more than 90% hydrogen and helium in mass.
- Hofstadter, Mark 2011. The atmospheres of the ice giants, Uranus and Neptune, White Paper for the Planetary Science Decadal Survey, US National Research Council, pp. 1–2.