Observatoire de Paris
<-   The Roche limit   ->
The Shoemaker Levy 9 comet fragmented by Jupiter in 1994.
Copyright : NASA/HST

The Roche limit is the minimal distance, with respect to the center of a planet, at which a satellite is able to orbit without being destroyed by tidal forces. If the planet and the satellite have the same density, the Roche limit is 2.5 times the radius of the planet. Within this limit, the satellite is destroyed by tidal forces.

Each of the ring systems in the Solar System are within the Roche zone of their planets.

Solid satellites can exist inside the Roche zone if they are sufficiently small, since the tension of the rocks prevent them from breaking up.

In a disk of remnants around a newly formed planet, the matter outside the Roche limit can form satellites, whereas nearer to the planet, the tidal forces prevent the formation of any satellite.

This mechanism still holds in the neighbourhood of a star : no planets can exist nearer than 2.5 times the radius of its star.