Pluto may have 3 moons


Monday, October 31, 2005 Posted: 2108 GMT (0508 HKT)

Pluto, its moon Charon, and the planet's two new candidate satellites designated as P1 and P2.

WASHINGTON (Reuters) -- Pluto, that cosmic oddball at the far reaches of our solar system, may have three moons instead of one, scientists announced on Monday.

Astronomers using the Hubble Space Telescope glimpsed the two new satellites back in May, and were intrigued when the pair of possible moons appeared to move around Pluto over three days in what looked like a nearly circular orbit.

If confirmed by the International Astronomical Union, they will get official names based on classical mythology, joining Pluto's moon Charon, which is named for the ferryman of the dead. Pluto is named for the lord of the underworld.

For now, the new satellites are called simply P1 and P2. One of the scientists who discovered the satellites couldn't resist making some spooky allusions with the announcement.

"It's ... strictly coincidental that Pluto of course was named for the god of the underworld and we're describing these Halloween moons," said Alan Stern of the Southwest Research Institute in a telephone news conference.

Pluto's first known moon, Charon, was discovered in 1978. Charon is about half Pluto's size, making it less like a satellite and more like a sibling, and many scientists consider Pluto and Charon to be a binary system, with the moon orbiting about 12,000 miles from the planet.

The newfound putative satellites are likely much smaller than Charon, ranging in size from perhaps 30 miles to 100 miles in diameter. Scientists are still trying to figure this out.

Charon is about 745 miles across, and Pluto is about 1,430 miles across.

The discovery of the two additional satellites means Pluto is the first known object of the Kuiper Belt -- a ring of rocky debris circling outside Neptune's orbit -- with more than one moon, said Hal Weaver, of the Johns Hopkins University Applied Physics Laboratory.

However, the new finding does little to clear up Pluto's planetary status. While it was discovered in 1930, Pluto has such an eccentric orbit around the sun that some have questioned whether it deserves to be called a planet.

The International Astronomical Union, which considers such matters, calls it a planet, but the specific definition of what constitutes a planet is under review.

Mere multiple moons do not change Pluto's status, according to Stern, who serves on an astronomical panel that is working on the new definition.

"Whether or not an object has a moon is not part of the criteria that we've considered, because so many small objects in the solar system have moons," Stern said.

"But I think, just on a visceral level, the fact that Pluto has a whole suite of companions will make some people in the public feel better about its status of planethood."