Orbital resonance

In celestial mechanics, two bodies are in orbital resonance if their orbital periods can be expressed as a ratio of two integers. For example, two planets, both orbiting a parent star, are said to be in a 2:1 resonance when one of the planets takes approximately twice as long to orbit the star as the other planet. In our own Solar System, Neptune and Pluto are in resonance (in this case 3:2), as are many moons of Saturn and Jupiter. Astronomers have observed several planetary systems around other stars that are also believed to harbour planets in resonance.