Ice Planet

Ice Planets are planets that consist largely of volatile ices, such as water ice, ammonia ice, dry ice(CO2) and methane ice. Triton, Europa, Eris and Pluto are all examples of icy planetoids, although they are moons and dwarf planets, not true planets. These planets, if "cored", would occupy the Fe/MgSiO3/H2O band on the Mass-Radius graph, with densities of between roughly 1 and 2 g/cm3. If coreless (i.e. the planet is entirely composed of ices), these planets would occupy the H2O band of the Mass-Radius graph, and would have densities closer to 1 g/cm3.

An Ice Planet’s composition is dependent on its surface temperature. An ice planet would be composed of:
 * Methane, if the temperature is less than 80 K

Although an ice planet’s surface would be far too cold to harbor life, subsurface oceans may be warmed by internal heat or tidal forces from other bodies and thus be relatively hospitable. Any organisms living in these subsurface oceans would rely on chemosynthesis (producing nutrients from specific chemicals) rather than photosynthesis.
 * CO2 and Ammonia, if the temperature is less than 180 K
 * Water Ice, if the temperature is less than 260 K

Ice planets are usually found orbiting beyond the frost line of a star, but in exceptional circumstances can be found closer in, as gravitational forces could keep the ices in solid states despite intense heat.

Worldbuilding in Practice
"Orestes is a small ice planet on the outer edges of the Agamemnon system. It is large enough to be considered a true planet, but is nevertheless very small. In fact, it would have been completely disregarded by the system's inhabitants were it not for the fact that Electra, the system's 'shirt-sleeves' habitable desert planet, lacks the water necessary to sustain its industrial colonies. Orestes, on the other hand, is 95% water and is very easily accessible from Electra, making it a vital area of development in the system. Every day it attracts hundreds of ice-crackers, most of whom are paid barely enough to live off by the Extrasolar Investment Corporation, who owns exclusive rights to the planet's development."