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How do planets acquire rings?

Astrophysicist George F. Spagna, Jr., of Randolph-Macon College provides this explanation:

The bright rings of Saturn have puzzled scientists ever since Galileo first observed them in about 1610. He initially thought of Saturn as a triple planet because he could discern only two irregular blobs of light, one on either side of the planet itself. Then in 1655 Christian Huygens proposed that these blobs were actually a flattened system of rings circling the planet above its equator.

We now know that rings circle all four of the gas giant planets in our solar system¿Jupiter, Saturn, Uranus and Neptune¿though only Saturn¿s are obvious when viewed from Earth. Contrary to their appearance, they are not solid rings or disks at all. Rather they are composed of myriad bits and pieces of ice, rock and dust. In the case of Saturn¿s brightly visible rings, they consist of more ice, which reflects sunlight effectively. The rings of the other planets contain mostly dust, which is dark and doesn¿t reflect much light. Additionally, whereas Saturn's rings are wide, the other planets sport thin rings.

The rings of Jupiter, Uranus and Neptune probably arose as a result of meteor impacts on their tiny inner satellites. Dust and rocky debris blasted off the satellites' surfaces continues to orbit the planet for many years. Indeed, we have observational evidence from the Galileo space probe, currently orbiting Jupiter, that traces the ring material directly to three of the smallest inner satellites¿the rings are even in similar orbits. Saturn's rings probably represent a moon-shattering collision that left debris from an icy moon too close to the planet to reassemble. Of interest, Earth is acquiring its own "ring system" of space junk and debris from old satellites and launch vehicles.

saturn's rings
Image: JPL/NASA
SATURN'S RINGS, as photographed by Voyager 2, show that the C ring (blue) and the B ring (yellow) contain traces of different elements.

The rings most likely have minimal effect on the planets, except perhaps as a reservoir for "shooting stars" when the ring material eventually falls into the atmosphere as its orbit decays over time. Although the rings are large, they contain virtually no mass as compared with the planets. Saturn¿s rings would certainly be visible from the planet itself, but because it lacks a solid surface, you¿d have to wait for a break in the clouds while you float in your balloon gondola. The darker rings of the other planets might be similarly visible, but only if you had the appropriate observation instruments.

Answer originally posted December 4, 2000.

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