A Hubble Telescope picture of Saturn's thin rings seen edge-on.

Galileo Galilei was accustomed to extraordinary discoveries. Using his primitive telescope he had found new worlds orbiting Jupiter, watched planet-sized spots crossing the Sun, and explored craters on the Moon. But when Galileo turned his telescope toward Saturn in 1610, even he was amazed.

The planet looked nothing like others in the solar system. Through 17th Century optics, Saturn appeared to be one bright star closely flanked by two dimmer ones -- a blurry suggestion of the planet's magnificent rings.

What Galileo did next was nearly as unusual as Saturn itself.

He wanted to tell everyone what he had seen, but he also wanted to keep his work secret while he studied the puzzling planet. So, he published his discovery in code: smais mr milmep oet ale umibunen ugttauir as. Unscrambled, the anagram means "I have observed the highest planet tri-form."

Nowadays anyone with a department store telescope can get a better view of Saturn's rings than Galileo did. Otherwise, matters stand much as they did four hundred years ago. First-time observers of the planet still step back from their telescopes speechless. And scientists are still puzzled.

Where do the rings come from?

"After all this time we're still not sure about the origin of Saturn's rings," says Jeff Cuzzi, a planetary scientist at the NASA Ames Research Center.

Astronomers once thought that Saturn's rings formed when Saturn did: 4.8 billion years ago as the Sun and planets coalesced from a swirling cloud of interstellar gas. "But lately," Cuzzi says, "there's a growing awareness that Saturn's rings can't be so old."

Cuzzi speculates that some hundreds of millions of years ago -- a time when the earliest dinosaurs roamed our planet -- Saturn had no bright rings. Then, he says, something unlikely happened: "A moon-sized object from the outer solar system might have flown nearby Saturn where tidal forces ripped it apart. Or maybe an asteroid smashed one of Saturn's existing moons."

The debris encircled the planet and formed the rings we see today.

Saturn's ring particles range in size from microscopic dust to barn-sized boulders. If you assembled them all in one place, notes Cuzzi, you would have enough material to make an icy satellite 100 kilomoeters or 200 kilometers wide -- much like Saturn's present-day moon Mimas.

The debris layer is extraordinarily thin, he marvels. "Saturn's rings are 250,000 kilometers wide, but only a few tens of meters thick. A sheet of paper the size of San Francisco would have about the same ratio of width to depth." Indeed, if you made a 1-meter-wide scale model of Saturn, the rings would be 10,000 times thinner than a razor blade.

See Saturn, and its rings, disappear behind the Moon tonight.

Next Page: How old are the rings?