October 31, 2005
Pluto may have three moons, instead of one
By Deborah Zabarenko
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.
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
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
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."
More information and images are available online at