Marinus

Marinus, also known as Planet 14 and possibly Mondas (COMIC: The World Shapers), was a water-world in the Voora Marinii group. (PROSE: Interference - Book Two)

History
At the time the First Doctor visited with Susan, Barbara Wright and Ian Chesterton, the Conscience of Marinus machine was hidden on Marinus, and there were acid seas and glass beaches on the planet. By then, it has also been invaded by the Voord, under the leadership of Yartek. (TV: The Keys of Marinus, AUDIO: Domain of the Voord)

The Daleks came to Marinus and fought the Voords until they learned of the Voords' knowledge of the Great Power and joined forces with them. They captured the First Doctor, who had also travelled to Marinus. (PROSE: Doctor Who and the Daleks)

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 years passed and the oceans dried within a week, with the Voord mutated into Cybermen. When the Sixth Doctor intervened, an aged Jamie McCrimmon sacrificed his life to destroy the worldshaper, which caused 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. (COMIC: The World Shapers)

When remembering his travels with the First Doctor, Ian Chesterton mentioned this planet. (COMIC: Hunters of the Burning Stone)

Behind the scenes
The World Shapers 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.