Many of the moons in the solar system could have been spawned from giant rings around planets. According to a new model, Saturn, Uranus, Neptune and even the Earth may have once had ring systems that gave rise to satellites.
We used to think that moons form around planets in the same way as planets form around stars: coalescing from a gaseous disc that surrounded the planet as it formed. That model still applies to some moons, like those of Jupiter.
But Saturn's moons follow a peculiar pattern. Their orbits bunch near the edge of the rings, and the moons get more spread out and more massive as they get further away.
The rings mark Saturn's Roche limit: the distance from the planet beyond which its gravitational tidal forces are weak enough to let moons there survive. Inside the Roche limit, however, Saturn's gravity would pull moons apart and add them to its rings. Some astronomers think this is how the planet gained its rings in the first place.
But theory says that such rings do not remain static. The constituent fragments that lie near the inner, planetary side of the ring should constantly exchange angular momentum with fragments further out. This means the inner fragments lose energy and fall towards Saturn while the outer ones gain energy and retreat from the planet.
Aurélien Crida at the Observatory of the Cote d'Azur in Nice, France, and Sébastien Charnoz at the Denis Diderot University in Paris have now run simulations of this effect ( via newscientist.com ).