A fluid planet is a very rare type of planet that is entirely made of a single fluid compound, usually water. Fluid planets are formed when terrestrial planets made entirely out of ice in the outer reach of a system somehow move closer to a system's star, heating it up. The surface of the planet is a vast liquid and may have a vapor atmosphere. The core of the planet is solid, made up of stable solid states of the fluid such as hot ice and superfluid water. The solid hot ice core of the planet may also be magnetically conductive, allowing fluid planets to have a magnetic field.
Gliese 1214 b is an example of a partial fluid planet, a planet with a rocky core but with over 75% of its mass being composed of water.
Fluid planets are more commonly found in hyperspace, where they often have life within them.