Orbital resonance occurs when two orbiting bodies have periods of revolution that are in a simple integer ratio so that they exert a regular gravitational influence on each other. This may serve to destabilize one of the orbits. For instance, there is a series of lanes in the asteroid belt called Kirkwood gaps where the asteroids would be in resonance with Jupiter. Alternatively, it may serve to stabilize the orbits and protect them from gravitational perturbations. For instance Pluto and some smaller bodies called Plutinos were saved from ejection by a 3:2 resonance with Neptune and the Trojan asteroids may be regarded as being protected by a 1:1 resonance with Jupiter.
A Laplace resonance occurs when more than two orbiting bodies have a simple integer ratio between their orbital periods. For example, Jupiter's moons Ganymede, Europa, and Io are in a 1:2:4 orbital resonance.
See also Tidal locking.