Hubble spots two moons orbiting smallest planet

Discovery of additional satellites might help Pluto maintain its planetary status


Scientists using NASA's Hubble Space Telescope have found that Pluto has not one, but three moons.

The discovery is likely to prompt searches for undiscovered moons elsewhere and shed light on the evolution of the Kuiper Belt, the vast icy region of space beyond Neptune where Pluto resides.

"Pluto just became even more interesting," said Hal Weaver, a planetary scientist at the Johns Hopkins Applied Physics Laboratory in Laurel and co-leader of the team that made the discovery.

The newly discovered moons are constantly being pelted by objects in space, sending out debris that gives Pluto an unusual appearance, researchers say.

"They should generate rings around Pluto," said the team's other co-leader, Alan Stern of the Southwest Research Institute in Boulder, Colo. But no one has seen such rings.

The most distant planet from the sun, Pluto is about 1,475 miles in diameter, two-thirds the size of Earth's moon and so tiny that some scientists question whether it qualifies as a planet.

"It tells us that Pluto is a much more complex world than we previously imagined," Richard Binzel, a planetary scientist at the Massachusetts Institute of Technology, said of the discovery. "How do you get so many moons around such a small planet?"

Moons have been discovered orbiting every planet in the solar system except Venus and Mercury. Jupiter, by far the largest planet, has 63 known moons.

Like Earth's moon, most are thought to have formed when meteors or other space objects collided with the nearby planet, shooting debris into space that is gradually shaped by gravitational forces.

"It's the easiest way for a planet to get such a huge satellite," Binzel said.

Training Hubble's Advanced Camera for Surveys on Pluto on May 15 and again three days later, scientists found two very faint satellites moving in a counterclockwise orbit. They were about 5,000 times fainter than light from the planet itself.

The new moons are 30 to 100 miles in diameter and orbit about 30,000 miles and 40,000 miles, respectively, away from Pluto. The closer moon takes about 25 days to orbit; the more distant takes about 38 days.

Charon, the moon discovered in 1978, is 12,000 miles from Pluto and orbits the planet every 6.4 days.

The observations were confirmed by Hubble images taken June 14, but they must be reconfirmed by the space telescope in February before the International Astronomical Union will consider names for the moons. For now, they're known as P1 and P2.

Weaver and Stern said they have discussed naming the moons after their wives but will talk with the nine-member team that worked on the project before making any decisions. "We're going to be pursing that with the whole team and consulting with the IAU," Stern said.

Pluto was discovered in 1930 and named for the Roman god of the underworld. Its orbit ranges from 2.7 billion miles to 4.6 billion miles from the sun. Earth is 93 million miles from the sun.

Some astronomers say Pluto's small size, its highly elliptical orbit and the methane ice surface make it more like a Kuiper Belt object and less like the outer gas-giant planets such as Jupiter and Neptune.

Scientists at New York's Hayden Planetarium caused an uproar when they put eight planets in a display that opened in 2000 and classified Pluto as a Kuiper Belt object. The debate intensified this summer with the discovery of a Kuiper Belt object, UB313, that is larger than Pluto.

But experts say discovery of the moons might help Pluto keep its planetary status.

"It's going to be harder and harder not to call it a planet," said Stephen Maran, a retired astronomer from the Goddard Space Flight Center in Greenbelt and author of Astronomy for Dummies.

The astronomical union has assigned a group of astronomers to develop criteria for planets.

Stern, a member of that astronomical union group, said the discovery of the moons will probably add support to maintaining Pluto's designation as a planet. Stern is also principal investigator of New Horizons, an unmanned NASA mission designed to probe Pluto and the Kuiper Belt over the next 15 years. Its launch is scheduled for January.

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

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