Orbital resonance occurs when orbiting bodies exert regular, periodic gravitational tugs on one another because their orbital periods form a simple ratio. Resonances can stabilize orbits, as among Jupiter's moons, or destabilize them, clearing gaps in asteroid belts and rings. A recent chaotic resonance is thought to have melted the interior of Saturn's moon Mimas, giving it a young hidden ocean, and resonant clustering of distant objects is the main clue for a possible Planet Nine.