When the base personnel's suspicions are roused, the Doctor informs them that the space capsule is being affected by the gravitational pull of another planet — a tenth planet in the Solar system.

The Tenth Planet is the First Doctor’s farewell adventure ... also survives and this short clip is included as part of the DVD box set, Lost in Time. The full audio soundtrack to all four ...

Doctor Who (1963–1996) — Season 4, The Tenth Planet: Episode 1 Landing at the South Pole the travellers find a new planet is disrupting a space shot. BBC One ...