planet

Orbital resonance

When orbiting bodies fall into a steady lockstep of simple whole-number ratios, tugging on one another gravitationally at regular intervals. In the Jovian system, Io, Europa, and Ganymede sit in a 4:2:1 (Laplace) resonance: the periodic tugs keep their orbits slightly eccentric, feeding the tidal-heating engine. Pandora most likely needs a similar arrangement to stay hot for billions of years.