Combined observations form two observatories have yielded strong evidence of a ring around the planet neptune, the only one of the solar system’s four giant worlds for which no ring has heretofore been confirmed. “But if it’s a real ring,” says James L. Elliot of Massachusetts Institute of Technology, one of the discoverers of the rings of Uranus, “it’s not like any other one we’ve seen so far.” Last July 22, Neptune passed near the star SAO 186001 (as seen from earth), and a number of astronomers arranged to observe the event.
At such times in the past, blockages, or occultations, of a star’s light have revealed rings (such as Uranus’s) and previously unsuspected satellites, as well as providing precise measurements of a known planet’s size. SAO 186001 did not pass behind Neptune itself, but Patrice Bouchet and colleagues at the European Southern Observatory in Chile, observing for Andre Brahic of the University of Paris, did note one brief occultation, lasting barely a second and reducing the star’s light by only about 35 percent. About 100 kilometers away at the Chilean Cerro Tololo observatory, Faith Vilas from the University of Arizona in Tucson got an almost identical result. Two such findings for the same location in space, both supported by precise recordings of their data, would normally be taken as evidence of a satellite. But the two Chilean observations, says William Hubbard of the University of Arizona, were of locations about 90 km apart, while the duration of the occultations indicates that the starlight was being blocked in each case by an object only 10 to 20 km wide–hardly the likely dimensions of a moon. The likeliest inference, according to Hubbard and Brahic, is that the star passed behind a Neptunian ring, which kept its light from reaching either observatory.
A mystery, however, is that neither group detected any occultation where the other side of the ring hould also have blocked out the star. Could the ring be somehow broken or incomplete? (There are signs of a “partial” ring in Voyager photos of the Saturn ring system’s Encke division, Brahic notes.) Could part of it be “kinked” out of the way or otherwise distorted by the presence of satellites or other factors? Or might part of it be so wide that its density gets too low to cause an occultation–or so narrow that it simply did not show? “There is more diversity in the solar system,” says Brahic, “than in the brains of bright theorists.”