Rotation also creates an obvious equatorial bulge on Jupiter and Saturn, which spin in about 10 hours. The spinning motion, when combined with convection in their metallic hydrogen cores, generates powerful magnetic fields that in turn contribute to the formation of auroras and energetic, charged particle belts like the Van Allen belts of our Earth. Each of the four giant planets has a ring system composed of small orbiting particles. The rings are probably debris from small objects that have broken up either as the result of collisions or of tidal forces exerted by the planet. Collisions continue to break up small particles, replacing ring material that falls into the planets' atmospheres.
Many satellites orbit each of the four giant planets.
Some of these moons are comparable in diameter to Mercury. They are thought to be made mostly of water, ice, and rock, although Io, an exceptionally active moon of Jupiter, may be mostly sulfur and rock. Pluto, despite being an outer planet, is more like a moon in size and mass. Many astronomers now think Pluto is a surviving icy planetesimal. The McGraw-Hill Companies.