Marinus

Marinus, also known as Planet 14 and possibly Mondas, was a water-world and the homeworld of the Voord. The Conscience of Marinus computer was hidden on Marinus. There were acid seas and glass beaches on the planet. (DW: The Keys of Marinus)

Designated as Planet 14 and given a worldshaper at some point, the Voord took control of the worldshaper and rapid-evolved themselves. When activated, thousands upon thousands of gyres passed and the oveans dried within a week, with the Voord mutated into Cybermen. When the Sixth Doctor intervened, Jamie McCrimmon sacrificed himself and destroyed the worldshaper, causing time to accelerate and seasons to change in seconds with massive geological activity. By this time Marinus was barren, yet the worldshaper's last energy pulse completed the evolution of the Voord into Cybermen. (DWM: The World Shapers)


 * This complicates the continuity of Doctor Who, though Cyberman history is complicated enough without this. Marinus, according to this account, becomes Mondas, and would therefore make Marinus the twin planet of Earth.

Cities

 * Morphoton
 * Millennius