Hubble images show more rings, moons around Uranus

Pictures also show instabilities that could lead to lunar collisions


Astronomers using the Hubble Space Telescope say they have discovered two tiny new moons and two faint new rings spinning around the planet Uranus. The finds bring the current tallies for the remote blue world to 27 moons and 13 rings.

The scientists said Hubble images have also revealed instabilities in the Uranian system since the Voyager II spacecraft flew by in 1986 - changes that could eventually lead to lunar collisions.

"The destruction and accumulation of moons and rings is very exciting," said Jack J. Lissauer, of the NASA Ames Research Center in California, a principal investigator on the project. "It's showing how this system is very rapidly evolving ... and showing that the processes which led to the formation of the planets is still ongoing today."

Following scientific tradition, the discoverers named the two new moons after Shakespearean characters. One is Mab, the fairy queen in A Midsummer Night's Dream. The other is Cupid, the Roman god of love, who appears in one of the bard's lesser-known plays, Timon of Athens.

Since some of Uranus' other moons were named for the likes of Juliet, Portia, Desdemona and Bianca, "we liked the idea of tiny Cupid orbiting among Shakespeare's greatest lovers," said the new moons' co-discoverer, Mark Showalter of the SETI Institute, in Mountain View, Calif.

Uranus is the third-largest planet and the seventh from the sun. About 32,000 miles in diameter, it orbits 1.78 billion miles from the solar system's center, between the orbits of Saturn and Neptune.

It was discovered in 1781 by William Herschel, who named it Georgium Sidus (Georgian Planet) after Britain's King George III. The name Uranus (pronounced YOOR-a-nus), after an early Greek god of the heavens, didn't come into routine use until 1850.

The planet's five largest moons were discovered by Herschel and others between 1787 and 1948.

The next 11 were spotted during the Voyager II spacecraft's flyby in 1985 and 1986. Five more were found by astronomers in the 1990s, and another five were added to the list in 2003. Those included Mab - estimated at 15 miles wide - and Cupid, thought to be just 11 miles across. Hubble also rediscovered Perdita, a moon first spotted by Voyager II and later lost.

The planet's ring system includes 11 inner rings discovered by Voyager II, and the two new outer rings found by Hubble.

The new Hubble images, made in 2003, revealed that one of those new rings is associated with Mab, one of the new moons. "We don't think that is at all coincidental," Lissauer said. Scientists believe the dust that forms the ring is continuously blasted off Mab and into orbit by meteor collisions.

"Some orbits a little faster, some a little bit slower. After a few hundred orbits, the dust spreads out to form a ring," he said. From then on, the dust is continuously "recycled" - falling back onto the little moon and being knocked off again.

The other new ring spotted by Hubble doesn't appear to have a "parent" moon.

"That's sort of mysterious," said Showalter. "A dust ring lasts a year or two, or a decade at most, unless it's continuously replenished." The scientists suspect the ring may hold a belt of very small objects too small for Hubble to see, but big enough to generate a cloud of dust.

"If you see a dust ring," he said, "it's guaranteed there has to be some kind of embedded bodies."

The Hubble pictures also show changes in the orbits of some of Uranus's larger moons since the Voyager flyby. The changes suggest the planet's lunar companions are part of a "chaotic" system that will lead to catastrophe.

A computer simulation by Lissauer, published in 1997, showed that the motions of the Uranian moons Juliet, Cressida and Desdemona would lead to collisions roughly a half-million years from now.

"Everything we find now is consistent with that conclusion," Showalter said.

What scientists are learning about the dynamism of the rings and moons circling Uranus can also be applied on a larger scale, they say, to the planets and asteroid belts circling the sun.

It was once assumed that planets accreted and grew only during the first 100 million years or so after the solar system formed and then settled into an eternal quiet.

"Now we're realizing that planetary systems are continually evolving," Lissauer said. "They change at a slower rate as they get older, but they continue to change."

The team's discoveries were published today by the online journal Science Express. They will appear in the journal Science in January.

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