• Bands of dust and rock orbiting planets. Saturn’s is the most spectacular in the solar system, but others are known to orbit Jupiter, Uranus and Neptune. Ring systems are found within the Roche limits of large planets and have a number of characteristics in common. One is that they are paper-thin by comparison with their diameter: another is that the rings are always divided by gaps containing little or no material, swept clear by resonances with satellite orbits. There are also wave systems in rings caused by gravitational forces concentrating material at particular points along them. Spoke-like structures in the rings of Saturn appear to be due to electromagnetic forces.

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