Scott Holmes, of Kings High School, asks :-

Why is there a ring around Saturn?

Pam Kilmartin, an astronomer at the University of Canterbury's Mt John Observatory, responded.

A ring probably forms around a planet when a comet or asteroid passes so close that the planet's gravity can pull it apart. The collision of Comet Shoemaker-Levy 9 (SL9) with Jupiter in 1994 showed how a ring might be made. SL9 was originally a small comet or asteroid that was captured into orbit around Jupiter. On one orbit it passed very close to the giant planet and was pulled to pieces. On the next orbit the pieces fell into Jupiter's atmosphere.

Had the orbit of SL9 been slightly different then the fragments would not have hit Jupiter but would have continued to orbit as before. At each close approach they would have been broken into smaller bits. These bits would begin colliding with each other, breaking into still finer stuff. Eventually the collisions would lead to a thin ring being formed.

Rings always orbit around the equator of their planet. This is because a planet's spin stretches it out at the equator. Put simply, the extra diameter at the equator causes more gravity there. This extra gravity eventually pulls the ring particles into orbits around the equator.

Continuing collisions of the material in the ring cause it to gradually spiral onto the planet. So rings fade in time. The ring around Saturn is probably quite young; only a few hundred million years old.