All of the giant gas planets -- Jupiter, Saturn, Uranus and Neptune -- have rings. But some of Neptune's rings do not completely encircle the planet; rather, they consist of a series of arcs -- stable clumps of dust separated by space.

Such clustering poses a puzzle, because matter orbiting a planet normally gets spread out over time. Until now, the standard explanation was that the arcs are "confined" by the gravity of Galatea, one of Neptune's moons.

But "this theory is not valid anymore," says Christophe Dumas, a planetary astronomer at NASA's Jet Propulsion Laboratory and member of one of two teams that have conducted new observations of Neptune's rings. Both teams' findings are published in the August 19 issue of Nature.

Images of Neptune's rings, taken respectively with the Hubble Space Telescope and the Canada-France-Hawaii Telescope, show that the arcs are not where they would be if previous models were correct. The moon Galatea, says Dumas, "is responsible for only part of the confinement -- not all of it."

Astronomers "need to get back to work" to fully explain the confinement, says Dumas. One possibility, he notes, is that tiny moonlets are helping prevent the clusters from spreading out into full-fledged rings.