Gravitational centrifuge

A gravitational centrifuge was a method of generating energy by harnessing the gravitational potential energy of planets.

The Ninth Doctor hypothesised that the near-perfect orbits of the planets in the Justicia system would make them ideal for a gravitational centrifuge, only to discover that the Blathereen had been gradually tweaking the planets' orbits to make this possible. (PROSE: The Monsters Inside)