Three researchers from France, Canada and the United States discovered 12 additional moons orbiting Saturn this past year. Having now studied them in greater depth, they conclude that three or four larger moons collided with asteroids or comets and broke apart to create the new satellites. The findings are published in this week's Nature.

Most larger moons form from an accretion disk of dust and rocks that swirls around a fledgling planet. As a result, they orbit the planet in the direction that it spins. About half of the new moons, however, travel in the opposite, or retrograde, direction, suggesting that they came about in another way. Such retrograde orbits can occur when a planet's gravitational pull captures a passing body. But because the new moons follow irregular orbits and appear in clusters, the scientists suspect that they resulted from collisions. "With asteroids that cluster, the belief is they are pieces of what once was a big asteroid that got hit by something," Carl Hergenrother of the University of Arizona says. "It's possible that we're seeing the same thing with the satellites."

The scientists are still uncertain as to when the collisions occurredeither when the planet formed about four billion years ago or more likely, at a later date. But the search for new moons is far from over. As astronomers discover increasingly smaller moons, they will have to address the question of what exactly qualifies as a moon. "Right now, we see irregular satellites as small as three kilometers around Saturn," Hergenrother says, "but there may be many smaller than that. These may go on a continuum in size all the way down to the size of dust."