Rings Around the Planets

Natural History, Oct, 2000 by Richard Panek

Saturn's most prominent feature is no longer a singular phenomenon.

Saturn is the ringed planet, right? Wrong. A little over twenty years ago, it was demoted to a far less privileged position in the solar system: a ringed planet. To be precise, it is merely one of four--or possibly more--such celestial objects.

Saturn's seemingly distinctive look has been an object of astronomical fascination since July 1610, when Galileo first observed the planet's "triple-bodied" appearance through a telescope and tried (although unsuccessfully) to explain it (see "The Sharp-Eyed Lynx Outfoxed by Nature," Natural History, May 1998). It wasn't until 1659 that the Dutch astronomer Christiaan Huygens, working with better equipment and more favorable viewing conditions, finally announced a solution to the puzzle: "a thin, flat ring, nowhere touching."

Thin, flat rings, actually, as French-Italian astronomer Jean-Dominique Cassini determined in 1675. (Thereafter, the space between the inner and outer rings was called the Cassini Division.) Having figured out that Saturn actually had rings, astronomers next asked themselves: Why rings, of all things? Specifically, are the rings of Saturn solid, or are they ... something else?

Definitely something else, said French theoretician Pierre-Simon Laplace, who in 1785 proved mathematically that the rotation rates of solid rings would violate Kepler's third law of planetary motion. As to what that something else might be, Laplace had a suggestion: not two rings but millions of rings, all rotating at varying rates. In 1856 the British astronomer James Clerk Maxwell refined that hypothesis, arriving at an interpretation more in keeping with the satellite pattern elsewhere evident in the solar system: not millions of rings but millions of moonlets. Indeed, American astronomer James E. Keeler validated this interpretation in 1895 through spectroscopic observation.

There the matter of planetary rings more or less rested until March 10, 1977. On that day, two groups of astronomers observed the disappearance (occultation) of a ninth-magnitude star behind Uranus. One group of astronomers, aboard an airborne observatory, noticed several spikes on the data plot just before the star vanished from view, indicating some sort of irregularity in the immediate vicinity of the planet. On a hunch, the researchers radioed the ground-based team and urged them to continue gathering data when the star reemerged from behind Uranus. Sure enough, the second group of astronomers found an identical set of spikes at the same distance from the planet. The only possible explanation was rings.

Suddenly Saturn's centuries-old reign as the ringed planet was over. Then, visits of the Voyager 1 spacecraft to Jupiter in 1979 and of Vobyager 2 to Neptune in 1989 revealed that they too had ring systems. Some astronomers have even hypothesized the presence of very thin rings around Mars as well (they're looking). Apparently rings are a common feature among planets--a realization that has only complicated astronomers' efforts to study what rings are, where they came from, and how they work.

The rings of Saturn and Uranus appear to consist primarily of ice particles, ranging in size from mote to iceberg, while Jupiter's seem to be rock. Neptune's rings, however, remain a mystery. The particles that comprise the rings of these four planets might be relics from the formation of the solar system or remnants of moons that broke apart after venturing too close to the planet, or maybe of moons that suffered from violent collisions. Whatever their source, these particles have settled over the eons into a central plane, apparently stabilized by the host planet's gravitational field.

Still, mathematicians have shown that gravity alone can't explain why millions of particles would maintain a ring shape over time. One further possible explanation has met with great success: bodies called "shepherding moons" may gravitationally corral the particles. Predicted in 1979, they've since been photographed around Saturn and Uranus. But even after more extensive and precise surveys of these shepherds have been conducted, the mechanics of rings will probably remain, as one planetary specialist recently told a meeting of the American Astronomical Society, "hideously complicated."

Despite the discoveries of the past two decades, Saturn does retain its distinction as the only planet whose rings are visible through telescopes available to the backyard astronomer. By the end of October, the planet will be nearer to Earth than at any time since February 1977. Coincidentally, the planet's tilt will leave the rings more "open" to view than they have been in nine years. Check the eastern sky about two hours after nightfall. One look through even a modest telescope and you'll see for yourself that even though this ringed planet may no longer be singular, it remains no less spectacular.