Astronomers seeing more planets than stars in galaxy

WASHINGTON -- The more astronomers look for other worlds, the more they find that it's a crowded cosmos. They think planets easily outnumber stars in our galaxy and they're even finding them in the strangest of places.

And they've only begun to count.

Three studies released Wednesday in the journal Nature and at the American Astronomical Society's conference in Austin, Texas, demonstrate an extrasolar real estate boom. One of those studies shows that in our Milky Way, most stars have planets. And since there are a lot of stars in our galaxy -- about 100 billion -- that means a lot of planets.

"We're finding an exciting potpourri of things we didn't even think could exist," said Harvard University astronomer Lisa Kaltenegger, including planets that mirror "Star Wars" Luke Skywalker's home planet with twin suns.

"We're awash in planets where 17 years ago we weren't even sure there were planets" outside our solar system, said Kaltenegger, who wasn't involved in the new research.

Astronomers are finding other worlds using three techniques and peering through telescopes in space and on the ground.

Confirmed planets outside our solar system -- called exoplanets -- now number well over 700, still-to-be-confirmed ones are in the thousands.

NASA's new Kepler planet-hunting telescope in space is discovering exoplanets that are in a zone friendly to life and detecting planets as small as Earth or even tinier. That's moving the field of looking for some kind of life outside Earth from science fiction toward just plain science.

One study in Nature this week figures that the Milky Way averages at least 1.6 large planets per star. And that is likely a dramatic underestimate.

That study is based on only one intricate and time-consuming method of planet hunting that uses several South American, African and Australian telescopes. Astronomers look for increases in brightness of distant stars that indicate planets between Earth and that pulsating star. That technique usually finds only bigger planets and is good at finding those farther away from their stars, sort of like our Saturn or Uranus.

Kepler and a different ground-based telescope technique are finding planets closer to their stars. Putting those methods together, the number of worlds in our galaxy is probably much closer to two or more planets per star, said the Nature study author Arnaud Cassan of the Astrophysical Institute in Paris.

Dan Werthimer, chief scientist at the University of California Berkeley's search for extraterrestrial intelligence program and who wasn't part of the studies, was thrilled: "It's great to know that there are planets out there that we can point our telescopes at."

It's not just the number of planets, but where they are found. Scientists once thought systems with two stars were just too chaotic to have planets nearby. But so far, astronomers have found three different systems where planets have two suns, something that a few years ago seemed like purely "Star Wars" movie magic.

"Nature must like to form planets because it's forming them in places that are kind of difficult to do," said San Diego State University astronomy professor William Welsh, who wrote a study about planets with two stars that's also published in the journal Nature.

The gravity of two stars makes the area near them unstable, Welsh said. So astronomers thought that if a planet formed in that area, it would be torn apart. He said these are planets "so close to the edge where it would teeter over and fall" if they moved a bit closer in, Welsh said.

Late last year, Kepler telescope found one system with two stars. It was considered a freak. Then Welsh used Kepler to find two more. Now Welsh figures such planetary systems, while not common, are not rare either.

The two systems that Welsh found have another trait that excites him. They are near -- but not in -- the all-important habitable zone. That's the area that's not too hot and not too cold, so that liquid water could exist and thus so could life. With two stars, the planet goes through a strange and rapid heating and cooling in a few months, something most planets don't do. Overall they average about 100 degrees, he said.

"Planets are extremely frequent, extremely common," Welsh said. "More common than we ever imagined. That's a really good sign if you are searching for life beyond Earth."

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Online:

Nature: www.nature.com/nature

American Astronomical Society: http://aas.org/