For more than 300 years we thought Saturn was the only body in the solar system with rings. But by 1989, after ground-based measurements and flybys from Voyagers 1 and 2, we had discovered rings around the other three gas giants—Jupiter, Uranus, and Neptune. Now the Cassini spacecraft appears to have found a ring system around Saturn’s second-largest moon, Rhea. The discovery took astronomers by surprise. Just what is a ring doing around a moon, especially one that is significantly smaller than our own?

Normally a ring forms when two things are present: space debris, either from a collision with a small body or left over from the solar system’s formation, and sufficient gravity to attract the debris and hold it in place. While the gas giants all have what it takes to hold a ring system in place, “a ring system around a moon comes as a surprise because the parent planet’s gravity should destabilize any ring system that might form,” says Richard P. Binzel, a professor of planetary science at MIT.

That leaves two possible explanations for Rhea’s ring. It may be that the debris was kicked off recently enough that Saturn has not yet had a chance to destabilize it. But it is more likely that Rhea is uniquely suited to hold debris in orbit. Simulations indicate that, given Rhea’s size and distant orbit around Saturn, this moon could potentially hang on to a ring for millions of years or more before the planet’s pull overcomes Rhea’s hold. Whatever the case, the mission scientists are eager to gather more data when Cassini returns to Rhea in 2010.