Question: how big is Saturn's ring and how many other planets we know have rings like it?

Susan Cartwright answered on 19 Jun 2015:

Saturn has a whole series of rings, not just one. The main rings – the ones you see on photos of Saturn – start about 7000 km from the surface of Saturn, and extend out to 80000 km. They’re very thin by comparison, varying from only 10 m up to 1 km depending on which bit of the ring system you are looking at. This means that when we view the ring system edge on, which we sometimes do, it almost completely disappears (this puzzled early observers considerably!).

The rings are not solid: they are made of chunks of ice, each orbiting separately. The chunks are basically boulder sized – from about 1 cm across to about 10 m. The total mass of the rings is about the same as a smallish moon – only about 5 millionths of the Earth’s mass.

All the giant planets – Jupiter, Saturn, Uranus and Neptune – have ring systems, but Saturn’s is by far the most prominent and contains the most mass. Also, Saturn’s rings are made of the biggest, best-reflecting chunks. The ring systems of the other giant planets are made of fine dark dust rather than ice boulders, so they reflect very little light – they were first seen clearly when the Pioneer and Voyager spacecraft in the 1970s few past them and viewed them back-lit.