Planetary Ring Systems

Galileo almost discovered Saturn's rings in 1610. His new telescope revealed something on either side of the planet. Galileo's drawings almost look as if Saturn had grown a pair of giant ears. Galileo was seeing, but not quite resolving, Saturn's rings. In 1655 Christian Huygens correctly described Galileo's appendages as a flat system of coplanar rings that were not attached to Saturn. In 1675, Giovanni Cassini first noticed structure in the ring system, a gap now called Cassini's division. He also first suggested that the rings are composed not of a solid body but of individual particles orbiting Saturn. In the nineteenth century, James Clerk Maxwell proved mathematically that Cassini's suggestion must be correct. In 1895 James Keeler observed the orbital speed of different parts of Saturn's rings, finally proving they are a large number of individual particles. In 1980 the Voyager spacecraft sent back amazing pictures of the rings, showing a wealth of detailed structure.

The rings around Uranus were discovered next. On March 10, 1977, four groups of astronomers observed Uranus pass in front of a star and occult it, in hopes of learning something about Uranus as the starlight dimmed. To their surprise, the star winked, several times, both before and after the occultation. Winking once would suggest a moon, but several symmetric winks before and after suggested rings. The group of astronomers from Cornell University, led by James Elliot, obtained the most complete data and discovered five rings. In 1978, four additional rings were found during another occultation. The Voyager flyby in 1986 confirmed the previously discovered rings and found two more for a total of 11. The rings of Uranus were later observed from the earth, with infrared telescopes, which reveal the long wavelength emission from the icy ring particles. On August 14, 1994 the repaired Hubble Space Telescope photographed Uranus showing, but not fully resolving, the rings.

In 1979, the Voyager 1 and 2 flybys discovered a very thin ring around the planet Jupiter that is not observable from Earth. By 1979 Saturn, Uranus, and Jupiter were known to have rings. What about Neptune? Voyager 2 did not fly past Neptune until 1989. To avoid waiting 10 years to see if Neptune had rings, astronomers observed occultations of Neptune. Perhaps rings could be discovered indirectly from the Earth as for Uranus. Some observations seemed to show rings; others did not. The mixed results suggested partial rings. In 1989, the Voyager photographs finally revealed that Neptune does indeed have a ring system. However the rings vary in width. Narrower parts of the rings would be harder to detect from the Earth, so the occultations gave mixed results. It is not know why these rings vary in width.