Mondas

Mondas is the twin planet to Earth and is the home planet of the Cybermen, more specifically the Mondasian Cybermen. It drifted away from Earth's Solar System many years ago, Before the reformation of the League. The Cybermen had to adapt to deal with the conditions brought about by the lack of a star, and thus Cyberised their bodies and sealed themselves in tombs.

Mondas eventually found its own Star and was slingshotted back, sending it back towards Earth and reversing its thousand year journey. The Cybermen planned to return to our Solar System but eventually they looked to the stars instead, building massive starships and leaving the planet. Mondas now drifts through the Galaxy, lifeless, the Cybermen having abandoned it long ago. The remnants of the old cities built by the humanoids that lived on Monads can still be seen, as well as, if one looks hard enough, the ruined tombs of the Cyberised monsters that they became.