Milky Way has more planets than stars

The more astronomers look for other worlds, the more they find that it is a crowded and crazy cosmos. They think planets easily outnumber stars in our galaxy and they are finding them in the strangest of places.

And they have 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 study 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 Luke Skywalker's home planet in Star Wars, with twin suns and a mini-star system with a dwarf sun and shrunken planets.

"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 different techniques and peering through telescopes in space and on the ground.Confirmed planets outside our solar system, called exoplanets, now number well over 700, but 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 is moving the field of looking for some kind of life outside Earth from science fiction toward 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 further 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."

Kepler also found three rocky planets smaller than Earth that are circling a dwarf star that itself is only a bit bigger than Jupiter. They are so close to their small star that they are too hot for life.

"It's like you took your shrink ray gun and you set it to seven times smaller and zap the planetary system," said California Institute of Technology astronomer John Johnson, co-author of the study presented Wednesday at the astronomy conference. Because it is so hard to see these size planets, they must be pretty plentiful, Johnson said. "It's kind of like cockroaches. If you see one, then there are dozens hiding."

It's not just the number or size 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.

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.

"It just feels like it's inevitable that Kepler is going to come up with a habitable Earth-sized planet in the next couple of years," Caltech's Johnson said.