Binary planetary system

Twin or Sister planets are planets whose features are more or less identical, or at the very least similar, including the lifeforms which inhabit them, and are usually found nearby each other.

List of twin planet systems

 * Earth and Mondas - similar lifeforms include Humans and Mondasians.
 * While not strictly speaking a twin planet system, Earth and Mars both have reptillian species - the Silurians and Sea Devils of Earth compared to the Ice Warriors of Mars - and both are relatively close to each other.
 * Raxacoricofallapatorius and Clom - similar lifeforms include Raxacoricofallapatorians and the Abzorbaloff.