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Anice planet oricy planet is a type ofplanet with an icy surface ofvolatiles such aswater,ammonia, andmethane. Ice planets consist of a globalcryosphere.
Under ageophysical definition of planet, the small icy worlds of theSolar System qualify as icy planets. These include most of theplanetary-mass moons, such asGanymede,Titan,Europa,Enceladus, andTriton;dwarf planetsPluto,Orcus,Haumea,Makemake,Quaoar,Sedna,Gonggong, andEris; and the largestcomets. In June 2020,NASA scientists reported that it is likely thatexoplanets with oceans, including some with oceans that may lie beneath a layer of surface ice, may be common in theMilky Way galaxy, based onmathematical modeling studies.[1][2]OGLE-2005-BLG-390Lb, first observed in 2005, is a possible ice planet.
An ice planet's surface can be composed ofwater,methane,ammonia,carbon dioxide (known as "dry ice"),carbon monoxide,nitrogen, and other volatiles, depending on its surface temperature. Ice planets would have surface temperatures below 260K (−13 °C) if composed primarily ofwater, below 180 K (−93 °C) if primarily composed of CO2 andammonia, and below 80 K (−193 °C) if composed primarily of methane.
On the surface, ice planets are hostile to life forms like those living on Earth because they are extremely cold. Many ice worlds likely havesubsurface oceans, warmed byinternal heat ortidal forces from another nearby body. Liquid subsurface water would provide habitable conditions forlife, includingfish,plankton, andmicroorganisms. Subsurface plants as we know them could not exist because there is no sunlight to use forphotosynthesis. 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 significant atmospheres and surface liquids like Saturn's moonTitan, which could be habitable forexotic forms of life.
Although there are many icy objects in theSolar System, none of them qualify as planets under theIAU definition of planet. However, mostplanetary-mass moons are ice-rock (e.g.Ganymede,Callisto,Enceladus,Titan, andTriton) or even primarily ice (e.g.Mimas,Tethys,Rhea, andIapetus) and so qualify as ice planets undergeophysical definitions of the term. The largestKuiper belt objects, such asPluto,Haumea,Makemake,Charon,Quaoar, andOrcus[3] also qualify as such under geophysical definitions.Europa is also often considered an ice planet due to its surface ice, though its high density indicates that its interior is mostly rocky. The same is true for the scattered-disc objectEris.[4]
Dozens of known exoplanets are very probably ice planets, given their orbits, surfaces, densities, and host stars. Examples of ice planets includeGliese 667 C d,Gliese 667 C g,Kepler-441b,OGLE-2005-BLG-390Lb,OGLE-2013-BLG-0341LBb,OGLE-2016-BLG-1195Lb andMOA-2007-BLG-192Lb.[citation needed]
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