The controversy over the definition of the word “planet” could have a similar counterpart in the definition of “moon”, at least at Saturn where the line between moons and ring particles is a bit fuzzy. While I’m sure schoolkids are not going to be as interested in whether Saturn’s moons Pan and Daphnis, for example, are called moons or relegated to ring particle (or dwarf moon?) status, the same general underlying principles hold for this situation as for the planet definition. In fact, Saturn’s rings are frequently used as a local small-scale model of the disks in which planets formed. There are some very important differences, of course, but there are also a lot of similar processes at work in Saturn’s rings to what we expect took place in our forming solar system 4.5 billion years ago.
This cool picture from Cassini illustrates all of these points. This is the outer edge of the A ring, near the outer edge of the main ring system of Saturn. That gap, about 30 km across, is kept clear by that little moon, Daphnis. Since it is capable of clearing the neighborhood of its orbit (one of the IAU planet criteria), it makes sense to distinguish this as a moon rather than a ring particle. Unofficial Cassini project nomenclature calls Daphnis and many of the other small moons of Saturn “rocks” to distinguish them from the main satellites that are hundreds of km across (Daphnis is less than 10 km). Moonlet is a more distinguished term for the same idea: a moon that is too small to be round (another criterion for planethood, remember, is that it is big enough to be round). Daphnis is doing some interesting things to the ring material around it. Notice the wavy edge of the gap near the moonlet, and the diagonal wakes trailing up and to the right away from Daphnis (Daphnis is moving down relative to those particles, and up relative to the ring material on the left of the image, so the wake it leaves, unlike that from a boat, goes in opposite directions on either side of the moon - er, moonlet). The similarity to planet-forming disks can be seen in the many bright features in the ring. These are density waves produced from the gravity of nearby moonlets Pandora and Prometheus. Density waves in protoplanetary disks are believed to be created by forming giant planets. These waves in a protoplanetary disk can play an important role in the formation of the planets themselves. Cassini is allowing us to study the physics of density waves right here in our own backyard so we can have a better idea about not only Saturn and its rings, but planetary systems in general.