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Pluto offers clues to alien worlds

Model system Scientists wanting to learn more about how to find families of planets are getting some lessons from Pluto and its partner moon Charon.

The frozen duo, which orbit about 40 times as far away from the sun as Earth, balance a brood of at least three smaller moons, Nix, Hydra and a newly discovered body, designated P4, that flies between the two.

The whole system takes up less room than the span between Earth and our moon, providing a delicate orbital ballet that has implications for finding planets around dual-star systems beyond the solar system.

Following last year's discovery of P4 in images taken by the Hubble Space Telescope, astronomers ran computer models to try to pin down the moons' locations relative to each other and to Pluto and Charon, a proportionally large moon that has about 12 per cent of Pluto's mass.

"I was very surprised that they found a new moon in between the other two. It basically meant that it was getting kicked around by these other moon. I thought about what the effects of being kicked around like that and wondered what we could learn about them," says astronomer Andrew Youdin, with the Harvard-Smithsonian Center for Astrophysics.

"This is generally an issue with extrasolar planets," he says. "One tries to study the stability of their orbits over time scales of billions of years."

Lessons for Kepler

The research by Youdin and colleagues not only put some limits on how big Pluto's moons are and where they are located. It also is helping scientists figure out where to look for planets that circle two parent stars.

So far, astronomers using NASA's planet-hunting Kepler space telescope, for example, have found systems with a single planet orbiting two stars, but not multiple planets, though that may be about to change.

"We're learning from Pluto how far away moons have to be for a given mass to last for millions or billions of years," says Youdin, whose research will appear in the Astrophysical Journal and is available on the pre-press website on the pre-press website arXiv.org.

The computer simulation will soon get a field test. NASA's New Horizons probe is en route to the outer corner of the solar system to study Pluto, Charon and the small moons, as well as other objects in the Kuiper Belt region.

"We'll know in a few years if we're right or wrong," says Youdin. "You can study the (moons') motion through images by Hubble, but it is not as precise as what you can find by going there even on a single flyby."

The geometry doesn't leave much space for additional moons close to Pluto, though there could be more orbiting farther away.

Science aside, the prospect of even bits of moons flying near Pluto has the New Horizons team on high alert.

"We're very concerned about the spacecraft being destroyed," says Alan Stern, New Horizons lead scientist. "If we hit something even the size of a grain of rice, it would shred the spacecraft."

Hubble will be tapped to search for potential crash zones until the spacecraft gets close enough to scan with its own cameras. The team also is developing an alternative route to fly by Pluto in case it finds danger looming on the primary path.

"We don't to put something into the black widow's web," says Stern.