Binary planetary system

Twin or Sister planets were planets whose features were more or less identical, or at the very least similar, and were usually found nearby each other. This included the lifeforms which inhabited them.

List of twin planet systems

 * Earth and Mondas. These planets were so similar that even the shapes of their landmasses were identical, albeit vertically flipped. Similar lifeforms included humans and Mondasians. (DW: The Tenth Planet) (In one account, Mondas had lifeforms similar to the Silurians. DWM: The Cybermen)
 * Androzani Minor and Androzani Major. (DW: The Caves of Androzani)
 * Thoros Alpha and Thoros Beta. (DW: Mindwarp)
 * Raxacoricofallapatorius and Clom - similar lifeforms include Raxacoricofallapatorians and the Abzorbaloff. (DW: Aliens of London / World War Three, Love & Monsters)

There were also other planets which were very similar. New Earth was a planet which had the same orbit, size, atmosphere and terrain as Earth. (DW: New Earth)