Transcript: All ring systems in the solar system have outer edges that are somewhere between 1.8 and 2.5 times the planet radius from the center of the planet. What is particular about this ratio and how does it arise? The answer was derived in the mid-nineteenth century by the French mathematician Edward Roche. He calculated that the edge of planetary rings is defined by tidal forces. If we consider the gravity force between particles composing a planetary ring, the Roche limit, as it's called, is defined by the distance in which the tidal force caused by the planet on the particles equals the gravity force between them. Therefore beyond the Roche limit rings cannot exist because the tidal force is too large.