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'Exciting' find: Possible planets without orbits

Staff reports Updated: May 18, 2011 at 12:00 am

NEW YORK — Are these planets without orbits? Astronomers have found 10 potential planets as massive as Jupiter wandering through a slice of the Milky Way galaxy, following either very wide orbits or no orbit at all. And scientists think they are more common than the stars.

These mysterious bodies, apparently gaseous balls like the largest planets in our solar system, may help scientists understand how planets form.

"They're finding evidence for a lot of pretty big planets," said Alan Boss of the Carnegie Institution of Washington, who wasn't involved in the research.

If they orbit stars, their sheer number suggests every star in the galaxy has one or two of them, "which is astounding" because that's five or 10 times the number of stars scientists had thought harbored such gas-giant planets, he said.

And if instead they are wandering free, that "would be really stunning" because it's hard to explain how they formed, he said.

If that's the case, it would give a boost to some theories that say planets can be thrown out of orbit during formation, said Lisa Kaltenegger of the Harvard-Smithsonian Center for Astrophysics, another outside expert.

Other scientists have reported free-wandering objects in star-forming regions of the cosmos, but the newfound objects appear to be different, said one author of the new study, physicist David Bennett of the University of Notre Dame.

Bennett and colleagues from Japan, New Zealand and elsewhere report the finding in Thursday's issue of the journal Nature. They didn't observe the objects directly. Instead, they used the fact that massive objects bend the light of distant stars with their gravity, just as a lens does. So they looked extensively for such "microlensing" events.

They found 10, each caused by one of the newfound objects. They calculated each object has about the mass of Jupiter, and estimated how common such objects are. They also found no sign of a star near these bodies, at least not within 10 times the distance from Earth to the sun. (For comparison, within our solar system that would basically rule out an orbit closer than Saturn's.)

So the newfound objects either orbit a star more distant than that, or they don't orbit a star at all, the researchers concluded. They drew on other data to determine most of the objects don't orbit a star.

Scientists believe planets are formed when disks of dust that orbit stars form clumps, so that these clumps — the planets — remain in orbit. Maybe the newfound objects started out that way, but then got tossed out of orbit or into distant orbits by the gravitational tugs of larger planets, the researchers suggest.

The work suggests that such a tossing-out process is quite common, Bennett said.

Boss said maybe the bodies formed around a pair of stars instead, one of which supplied the gravitational tug. But even that would take some explaining to produce an object without an orbit, he said. Or maybe they somehow formed outside of any orbit. So the theoretical challenge in explaining the existence of such bodies is "exciting," he said.

Boss said he suspects most of these are in a distant orbit, and that maybe they even formed at that great distance rather than being tossed outward from a closer orbit.

Kaltenegger also said the new results can't rule out the possibility that these possible planets are in orbit, and that they may only have the mass of Saturn, about a third of Jupiter's.

But if they aren't orbiting a star, she noted, they don't fit the official definition of a planet — at least not the definition applied to objects in our own solar system.

All in all, Boss said, the new work is "pretty exciting in telling what is out there in the night sky... Lots of theories will grow in this environment."

___

Nature: www.nature.com

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