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Ice giant
An ice giant is a giant planet composed mainly of elements heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, and sulfur. There are two ice giants in the Solar System, Uranus and Neptune. They consist of only about 20% hydrogen and helium in mass, as opposed to the gas giants (Jupiter and Saturn), which are both more than 90% hydrogen and helium in mass. In the 1990s, it was realized that Uranus and Neptune are a distinct class of giant planet, separate from the other giant planets. They have become known as ''ice giants'' because their constituent compounds were ices when they were primarily incorporated into the planets during their formation, either directly in the form of ices or trapped in water ice. The amount of solid volatiles within the ice giants today is, however, very small.〔
==Terminology==
In 1952, science fiction writer James Blish coined the term ''gas giant''〔Science Fiction Citations, (Citations for gas giant n. )〕 and it was used to refer to the large non-terrestrial planets of the Solar System. However, in the 1990s, the compositions of Uranus and Neptune were discovered to be significantly different from those of Jupiter and Saturn. They are primarily composed of elements heavier than hydrogen and helium, constituting a separate type of giant planet altogether. Because during their formation Uranus and Neptune incorporated their material as either ices or gas trapped in water ice, the term ''ice giant'' came into use.〔
Today, very little of the water in Uranus and Neptune remains in the form of ice. Instead, H2O primarily exists as supercritical fluid at the temperatures and pressures within them.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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