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An ice giant is a planet consisting mostly of elements somewhat heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, and sulfur. Uranus and Neptune are examples. (Jupiter and Saturn are primarily hydrogen and helium.) The term ice refers to the astronomy/planetary-science sense, specifically, a volatile material (gaseous at our "everyday" temperatures) with a freezing point of around 100 K or above. Much of the heavier material in Uranus and Neptune is frozen or is trapped within water ice.