Why do some planets have rings?
What are they made of?
The rings of Jupiter, Uranus, and Neptune are made of dust, very tiny pieces of rock and ice. The rings of Saturn are made of large pieces of ice.
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.
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.)
image source: wikimedia commons
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.
Saturn’s rings are the most magnificent; they are bright, wide and colourful. Uranus has thirteen dark rings around it, and Neptune’s rings are also dark, but contain a few bright arcs.