Astronomers aided by the Hubble Space Telescope have spied two more rings encircling Uranus, the first additions to the planet's ring system in nearly two decades.
The faint, dusty rings orbit outside of Uranus' previously known rings, but within the orbits of its large moons, said Mark Showalter, an astronomer at the SETI Institute in Mountain View, Calif., who made the discovery.
Details will appear online Friday in the journal Science. The discovery, announced Thursday, means scientists now believe the seventh planet from the sun possesses 13 rings.
In 1986, Voyager 2 became the first spacecraft to zip past Uranus and beam back thousands of images of its dazzling rings and numerous moons. It found two new rings in addition to the nine previously discovered from Earth.
Scientists peering through the Hubble Space Telescope made the latest ring discoveries in 2004. Then they went back to process hundreds of images taken by Voyager and found the rings in the pictures. Scientists speculate that the rings may not have been discovered during the spacecraft flyby because of their faintness.
The newly discovered rings are made up of short-lived, faint bands of dust grains that are constantly being replenished by erosion of larger space bodies. Scientist think the dust in the outermost ring is being supplied by the moon Mab, discovered in 2003.
Scientists also measured changes in the orbits of Uranus' inner moons since 1994. The new measurements suggest the moons are in a "random and chaotic" fashion, said Jack Lissauer of the NASA Ames Research Center.
Because of the moons' instability, scientists think the satellites will collide with one another in the next few million years.
Uranus, four times the size of Earth, is one of the solar system's giant, gaseous planets that also include Jupiter, Saturn and Neptune.