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ice planet
An ice planet is a type of hypothetical exoplanet with an icy surface of volatiles such as water, ammonia, and methane. Ice planets consist of a global cryosphere. They are bigger versions of the minor planets of the Solar System, which includes the icy moons Europa, Enceladus, and Triton, the dwarf planets Pluto and Eris, and many other icy small Solar System bodies such as comets.
== Characteristics and habitability ==
Ice planets usually appear nearly white with geometric albedos of more than 0.9. An ice planet's surface can be composed of water, methane, ammonia, carbon dioxide (known as "dry ice"), carbon monoxide, and other volatiles, depending on its surface temperature. Ice planets would have surface temperatures below 260 K if composed primarily of water, below 180 K if primarily composed of CO and ammonia, and below 80 K if composed primarily of methane.
On the surface, ice planets are hostile to life forms similar to those living on Earth because they are very cold. Many ice planets likely have subsurface oceans, warmed by internal heat or tidal forces from another nearby body. Liquid subsurface water would provide habitable conditions for life, including fish, plankton, and microorganisms. Subsurface plants cannot exist because they and microorganisms cannot engage in photosynthesis because sunlight is blocked by the overlying ice. Microorganisms can produce nutrients using specific chemicals (chemosynthesis). Some planets, if conditions are right, may have significant atmospheres and surface liquids like Saturn's moon Titan which could be habitable for exotic life forms.
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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