A planetary ring is an annulus of material, in the form of countless numbers of small objects, in orbit around a planet. Ring matter – whether it is dust particles or huge boulders – will orbit a planet without coalescing into a single body if it remains within a certain distance, known as the Roche limit. Any bodies that orbit outside this limit won't be adversely affected by the gravity of the parent planet and may therefore accrete into a larger body. All four giant planets in the Solar System have ring systems, though only Saturn's is bright enough to be seen at visible wavelengths from Earth. The various rings have different proportions of ice and rock, and different distribution of particle sizes.