In
celestial mechanics, an
orbital resonance occurs when two
orbiting bodies exert a regular, periodic
gravitational influence on each other, usually due to their
orbital periods being related by a ratio of two small
integers. The physics principle behind orbital resonance is similar in concept to pushing a child on a swing, where the orbit and the swing both have a
natural frequency, and the other body doing the "pushing" will act in periodic repetition to have a cumulative effect on the motion. Orbital resonances greatly enhance the mutual gravitational influence of the bodies, i.e., their ability to alter or constrain each other's orbits. In most cases, this results in an
unstable interaction, in which the bodies exchange
momentum and shift orbits until the resonance no longer exists. Under some circumstances, a resonant system can be stable and self-correcting, so that the bodies remain in resonance. Examples are the 1:2:4 resonance of
Jupiter's moons
Ganymede,
Europa and
Io, and the 2:3 resonance between
Pluto and
Neptune. Unstable resonances with
Saturn's inner moons give rise to gaps in the
rings of Saturn. The special case of 1:1 resonance (between bodies with similar orbital radii) causes large
Solar System bodies to eject most other bodies sharing their orbits; this is part of the much more extensive process of
clearing the neighbourhood, an effect that is used in the current
definition of a planet.