Saturn, the second-largest planet in the solar system, is often lauded with the title “Jewel of the Solar System.” This title is well-deserved; the planet’s magnificent rings stretch far off into space, a beautiful circuit adorning Saturn’s yellow lateral stripes. However, Saturn’s spectacular rings are not the only example of the incredible ring structure; there are plenty of other examples. In the solar system, all the gas giants sport rings of varying size and opacity. A few small planetoids, such as the centaur-class Chariklo, also have rings. Looking beyond the solar system, there is evidence that some of the multitude of identified extra-solar planets have rings as well.
What are Planetary Rings?
Planetary rings are composed of many tiny objects orbiting a planet. These rings are extremely thin—1 km thick, in Saturn’s case—yet incredibly wide—480 000 km in radius for Saturn. In many cases, the density of material can vary by huge amounts depending on the radius of the orbit. This is particularly obvious with Saturn, since it has the most prominent rings; there are rings of darkness as well as yellow-gold. From a far-away Earth perspective, with a small telescope, these rings of darkness—regions of space and emptiness—appear to have sharp edges. Surprisingly, work from satellites such as the Voyager probes and Cassini that flew past and orbited Saturn, respectively, confirms this observation; Saturn’s rings have, in some cases, sharp and definite edges.
Without even considering the extra-solar planetary rings, there is a huge diversity in planetary ring composition. Jupiter’s faint, narrow rings are made of dust, probably originating from impacts between the planet’s moons and asteroids. Saturn’s rings consist of ice and some rock, with many pebbles about 2 cm in size. The rings of Uranus are even bigger, including large ice boulders that can be metres across. Neptune’s rings are the least well-studied of these rings, but are thought to consist of small particles of rock and ice.
Planetary rings are mysterious, and their formation seems almost implausible or designed. The structures are incredibly wide and incredibly thin. In some cases, the line between dense rings and empty space is unnaturally stark. It seems incredible to think that they are naturally-occurring.
The first thing that is required for rings is material for the rings, broken up into tiny pieces. There are, it is thought, several possible sources. As previously mentioned, much of Jupiter’s ring material is probably expelled from its moons during asteroid impacts. Volcanism can also play a role: some of the material for Saturn’s rings is thought to originate from its icy moon Enceladus, which throws plumes of water from its surface into space. Other material is thought to come from collisions between moons—for instance between Saturn’s moons Prometheus and Pandora.
A final source of material for planetary rings is through gravitational tearing-apart of planetary bodies. This process is caused by tidal forces between the planet and some smaller body like a moon or comet which become too extreme when the Roche Limit, aka the Roche Radius, is surpassed.
Tidal forces occur because some parts of an orbiting “secondary” body is closer to the larger “primary” than others. The parts of the secondary that are closer to the primary want to orbit more quickly than the parts that are further away, since objects in lower orbits orbit faster. If the force caused by this discrepancy is greater than the gravitational force the body exerts to hold itself together, the body splits into smaller pieces. The Roche Limit is only true for bodies that are only held together by gravity and have no internal strength which, it turns out, is true for a wide variety of small stellar objects such as some comets, asteroids, and moons. The Roche Limit increases with body size, which is why material is not ripped apart indefinitely once a certain orbital radius is reached.
The flatness of planetary rings is notable. Saturn’s rings, for instance, are so flat that they disappear from view twice per Saturnian year when the rings are exactly aligned with the Earth. The fundamental reason for this spectacular organization of countless particles is conservation of angular momentum; over time, the objects that make up the rings collide again and again, exchanging angular momentum. This process continues until all objects have approximately uniform angular momentum.
Any object that doesn’t conform to the average orbit must pass through the rings twice on each orbit around the planet. During each of these passes, there is a high (at least by space standards) risk that the rebel object will collide with material in the ring. The rebel object will lose some if its errant angular momentum, while the collided object will gain some. Both objects will now pass into and out of the rings, colliding into yet more material. Over time, the angular momentum of the initial rebel object is distributed throughout the whole ring by a branching tree of less and less drastic collisions.
In some cases, such as Saturn’s innermost “F Ring”, the ring is very thin and has well-defined edges. Similar patterns are also observed with all of the other gas giant rings and with Chariklo. This is surprising, since it would be expected that collisions within the ring would cause the objects to slowly diffuse away from the defined ring (assuming a way to make a well-defined ring could be contrived).
The cause of these distinct edges, it is thought, is shepherd moons. These are moons that orbit the primary body and, by some mechanism, keep the material in the rings in place. There is still a significant amount of research going on in this area. For instance, it used to be thought that Pandora and Prometheus were excellent examples of shepherd moons, keeping Saturn’s F Ring well-defined from outside and inside, respectively. The mechanism, it was thought, was that each moon would either force diffusing material back into the ring or eject it from the low-density region. However, at least one paper from 2014 suggests that Pandora does not act as a shepherd, and that Prometheus’ shepherding effect comes by it disturbing the ring in one way during one orbit, than disturbing it the other way during the subsequent orbit.
Planetary rings seem like some sort of epic, fantastic planetary feature. They adorn the sky in science fiction movies such as James Cameron’s Avatar. They capture the imagination of generation after generation of astronomers. The limited evidence is that these rings occur around many planets and even small bodies throughout the galaxy. What is known is that, common or not, there is plenty more work to be done in figuring out their intricate details.