Etra Prime was the oldest planet in the known universe.
The Daleks removed the planet from time and space so that they could mine the Apocalypse Element from the planet. Twenty years later the Daleks sent Etra Prime on a collision course with the planet Archetryx, which was hosting a temporal treaty attended by twenty of the Temporal Powers. (AUDIO: The Apocalypse Element)
Behind the scenes
Planet One (TV: The Pandorica Opens) and Chrone (COMIC: The Vortex Code) have also been referred to as the oldest planet in the universe.