Zanak

Zanak was a pleasant planet, with green fields, lush vegetation and friendly, humanoid natives. It was quite prosperous until the reign of Queen Xanxia. The queen launched galactic wars which ultimately ended up ruining Zanak, leaving only a few nomadic tribes alive.

Later, the Captain crash-landed on Zanak in his ship, the Vantarialis. He hollowed out the planet, which, using powerful engines, could materialise around other planets and steal their mineral wealth. It would then crush the planets into small, compressed cores. The Captain ruled from a mountain fortress known as the Bridge. He, in turn, answered to Xanxia, who had recreated him in his current cyborg form. Xanxia had gutted her entire planet to provide the power needed to control the time dams that kept her body frozen in its last few seconds of life. Her consciousness was then projected into a new, virtually-generated body in the form of the Captain's nurse.

The Fourth Doctor visited Zanak and helped the Mentiads overthrow both the Captain and Xanxia. The Bridge was destroyed, allowing Zanak to settle in the position that had once been occupied by the planet Calufrax. Its interior mass was replaced by the remnants of the planets (with the exception of Calufrax) that Zanak had destroyed, and which the Captain had preserved as part of a plan to destroy Xanxia. (TV: The Pirate Planet)

In 2009 K9 Mark IV told Clyde Langer about "a hollow world called Zanak", which was a pirate planet. Clyde learned that the planet jumped through space around other planets and drained them dry. (PROSE: The Wedding of Sarah Jane Smith)

Behind the scenes
No explicit narrative connection has ever been made between Zanak and Zanak IV, a planet featured in the audio story The Wreck of the Titan.