QUESTION #188

# How are the rings of planets formed and what are they made out of?

Asked by: Claas Behrens

The rings around planets like Jupiter and Saturn (yes Jupiter has rings!) are made up of bits of ice and rock.

They form when asteroids,comets, or any other large objects pass too close to the planet and are torn apart by the planet's gravity. There is a point around the planets called the Roche Limit. This is the point where gravity will tear apart an incoming object, and prevent the particles from re-accreting back into a larger object.

Essentially, rings are just thousands of tiny moonlets that orbit a planet and don't clump back into larger objects.
Answered by: Mike Perkins, Physics/Astronomy Major, Penn State

A planet is believed to form when the mutual gravitational attraction of debris orbiting a star accumulates into a growing mass. Some of the coalescing material orbiting a protoplanet can do the same thing, forming the planet's satellites, or 'moons'. (Some smaller satellites, though, are believed to be just 'captured', already formed, asteroids.)

There is a problem, however, if the debris orbiting a planet orbits too closely. Because gravitational force varies with distance, the different accelerations between two points at different distances from a massive object create a tidal force that has a stretching effect on nearby objects. This force from the Sun and Moon creates tides on opposite sides of the Earth, for example. When the tidal force is large enough, it overcomes the gravitational forces trying to bring individual particles together. Within a given distance from a planet, called its 'Roche Limit', tidal forces prevent debris from aggregating into larger bodies.

Planetary rings, then, consist of millions of separate small rock and ice particles, each maintaining their own orbit around the host planet. From a distance, these small orbiting particles only APPEAR to be a continuous, solid ring.
Answered by: Paul Walorski, B.A., Part-time Physics Instructor