In the British TV series 'Doctor Who', what is Earth's twin planet?

Mondas is a fictional planet in the British science fiction television series Doctor Who. Mondas is the homeworld of the Cybermen, a race of cyborgs.

Mondas is first mentioned in the 1966 Doctor Who serial 'The Tenth Planet'. This episode is also the first appearance of the Cybermen, a race that recurs in numerous Doctor Who stories. In 'The Tenth Planet', a New Zealand astronaut discovers a newly arrived planet near Earth, between the orbits of Mars and Venus. Later, a Cyberman informs the personnel of a nearby space station that this planet is the home of his people, the Mondasians. When the Cyberman mentions the planet's name, a Doctor Barclay inquires whether Mondas is not an ancient name for Earth. The Cyberman confirms, and claims that, billions of years ago, Mondas and Earth were twin planets until Mondas drifted into deep space. (In the historical section of his book Cybermen, David Banks suggests that the Moon's orbit around Earth was the cause of Mondas' departure from orbit.)