A binary planetary system, also known as 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. Mondas had lifeforms identical to humans called Mondasians (TV: The Tenth Planet) and lifeforms identical to Silurians called Lizard Kings. (COMIC: The Dead Heart)
- Atrios and Zeos (TV: The Armageddon Factor)
- Androzani Minor and Androzani Major (TV: The Caves of Androzani)
- Thoros Alpha and Thoros Beta (TV: Mindwarp)
- Raxacoricofallapatorius and Clom - similar lifeforms included Raxacoricofallapatorians and the Abzorbaloff. (TV: Love & Monsters)
- Gemino and Gemina (COMIC: The Secret of Gemino)
- The Tornado Nebula contained a binary planetary system which had extremely different lifeforms. One planet was inhabited by carbon-based lifeforms, similar to those on Earth, called Fleshkind, while the other was inhabited by boron-based life called Metalkind. (TV: Sky)
- Antari Two and Antari Three. The Tremas Master tried to start a war these two planets (PROSE: First Frontier) before his visit to the Cheetah World. (TV: Survival)
- Caludaar and Endarra. The inhabitants of the former had a law to not disturb the latter planet, which they believed to be uninhabited. (AUDIO: Scaredy Cat)
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. (TV: New Earth)