An ice planet is a theoretical type of exoplanet where everything is frozen. None has been found so far, but astronomers think they do exist.
They might have an icy surface of substances which are volatile on Earth, such as water, ammonia, and methane. Ice planets consist of a global cryosphere. They are bigger versions of the small icy worlds of the Solar System, like the moons Europa, Enceladus, and Triton, the dwarf planets Pluto and Eris.
Some ice worlds might have subsurface oceans, warmed by internal heat or tidal forces from another nearby body. Liquid subsurface water might provide habitable conditions for life. Microorganisms can produce nutrients using specific chemicals (chemosynthesis) that may provide food and energy for other organisms. Some planets, if conditions are right, may have strong atmospheres and surface liquids like Saturn's moon Titan, which could be habitable for exotic forms of life.
- Stern, Alan; Mitton, Jacqueline (2005). "Pluto and Charon : ice worlds on the ragged edge of the solar system". Weinheim: Wiley-VCH.