The Roche limit refers to the distance within which the gravitational tides of a primary celestial body begin to cause the disintegration of an orbiting satellite. In order for this principal to apply, the satellite must be held together by its own gravity.
Within the Roche limit, debris are likely to form rings, while debris outside the limit usually coalesce. Theoretically, the limit is about 2 1/2 times the radius of the more massive primary body. This assumption requires that the two bodies have similar compositions and densities. Saturn's rings are a notable exception, as they lie outside their Roche limit. This phenomenon suggests that the rings are remnants of a moon that passed within its Roche limit.