QUESTION: Why does Saturn have so many Moons? Why does Saturn have rings and the other planets don't? ANSWER from Linda Spilker on April 6, 2000: Why does Saturn have so many moon? We don't know for sure why Saturn has so many moons. The other planet of comparable size, Jupiter, also has a large number of moons. Both of these planets are very large and when they formed from a disk of gas and dust there was probably a fair amount of material left over in the disk. This material condensed to form most of the moons. Some of the outer moons at both Saturn and Jupiter are probably captured asteroids. We know this because the moons closest to the Saturn all orbit in a prograde direction (the same direction in which Saturn rotates) and their orbits are near the equatorial plane of Saturn. This is consistent with moons that formed from the left-over disk material from which Saturn accreted. The outer moon may have orbits that are inclined (tipped) 20 or more degrees with respect to Jupiter's or Saturn's equatorial plane and some of these moons even have retrograde orbits (they orbit Jupiter or Saturn in the opposite direction from the inner moons). Saturn has one outer moon, Phoebe, with a very inclined orbit which is retrograde. The following moons formed from the disk of gas and dust that formed Saturn. They are all in prograde orbits near the equatorial plane of Saturn. Pan (actually orbits in a gap inside the rings), Atlas, Prometheus, Pandora, Epimetheus, Janus, Mimas, Enceladus, Tethys, Telesto, Calypso, Dione, Helene, Rhea, Titan, Hyperion, and Iapetus. At Saturn, only Phoebe is believed to be a captured asteroid. Jupiter has 8 outer moons that are probably captured asteroids. Four of them have retrograde orbits as well. Why does Saturn have rings and the other planets don't? Actually 4 (and maybe 5) planets in our solar system have ring systems. Saturn's rings are just the biggest and brightest. The other planets with known rings are Jupiter, Uranus and Neptune. Rings are a common feature of the 4 gas giant planets. At Saturn we think that the rings may have formed from the breakup of a moon that got too close to Saturn. We now think that Mars may also have dust rings generated from dust particles that are kicked off the two moons which orbit Mars. Japan is sending a spacecraft to Mars to specifically look for these dust rings. The moons at Mars are much smaller than our own moon. Their small size makes it much easier to get dust into space from the Mars moons than it would be to get dust into space from our moon.