Zanak

Zanak was a hollow planet which, using powerful engines, could materialise around other planets and steal their mineral wealth. It would finally crush the planets into compressed cores small enough to carry. The surface was pleasant, with green fields, lush vegetation and friendly Near-Human natives.

Not a native of that world, cyborg space pirate known as the Captain ruled from a mountain fortress known as the Bridge. He, in turn, answered to Queen Xanxia.

The Fourth Doctor visited Zanak and helped the Mentiads overthrow both the Captain and Xanxia. The Bridge was then destroyed. (DW: The Pirate Planet)