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Curiosities: Why do some planets have rings?

October 18, 2010

Saturn imaged by the Cassini Orbiter.

Image: courtesy Jet Propulsion Lab

Planetary ring systems are complicated, notes UW Space Place Director Jim Lattis, and they are more common than once believed.

For ages, Saturn was thought to be the only planet in our solar system with a ring system. But in recent years ring systems have been discovered around Jupiter, Uranus and Neptune as well.

“There are various theories about planetary rings, like the fantastic rings around Saturn, but we cannot say for sure how they are formed,” explains Lattis.

One theory is that the rings formed at the same time as the planet and its major moons. In this case, if material is close to the planet, the planet’s gravitational pull is too strong to coalesce into a moon and the particles that would otherwise form a moon spread out in orbit around the planet as a ring.

Another idea is that a close call by a moon or comet results in the planet’s gravitational tidal force breaking up those bodies, the debris of which then becomes a ring system.

Although astronomers can’t say for certain what causes planetary rings, Lattis says that the Cassini spacecraft now in orbit around Saturn is beginning to provide tantalizing new clues to the forces that govern the physics of planetary rings.