The Earth, also known as Sol 3, plays a large part of the Doctor Who fictional universe for a majority of the stories in the series including it's spin-offs ever since the first serial in 1963, An Unearthly Child. The planet is one of the Doctor's most favourite planets as well where most of his companions and friends hail from. Apart from that, it's the planet where most aliens and villains choose to visit or invade which prompts the Doctor and his friends to help out in some way. The planet is also the twin to Mondas from which the Cybermen originate.