Dozens of candidate worlds reside within the "habitable zones" of their parent stars.
- NASA's Kepler telescope has found more than 1,200 extrasolar planet candidates.
- Smaller worlds, like Earth, appear to be more common than gas giants, like Jupiter.
- One six-planet system is unique in that the planets orbit very close to their sun.
A NASA telescope taking a nose count of planets in one small neighborhood of the Milky Way registered more than 1,200 candidates, including 54 residing in life-friendly orbits around their parent stars.
Scientists have no way of knowing yet if any of the newly discovered planets are solid-body worlds like Earth. But the census, collected by NASA's Kepler Space Telescope after just four months of work, shows that small planets like Earth are much more prevalent than Jupiter-sized worlds and that multiple-planet systems are common.
"We think we're seeing about 200 multi-planet systems," astronomer Daniel Fabrycky, with the University of California, Santa Cruz, told Discovery News. "That really blew us away. We didn't expect that this would be one of Kepler's discoveries."
The findings increase the number of planet candidates identified so far by Kepler to 1,235. Of these, 68 are approximately Earth-size; 288 are two to three times the size of Earth; 662 are Neptune-size; 165 are the size of Jupiter and 19 are larger than Jupiter.
Of particular note: a brood of six -- the largest extrasolar planet family found so far -- circling a sun-like star named Kepler 11. Five are puffy planets formation-flying closer than Mercury orbits the sun.
Astronomers suspect Kepler 11, located about 2,000 light years from Earth, has more offspring farther away or beyond Kepler's viewing angle.
The telescope sees a planet's footprints as it passes across the face of its parent star, just like a gnat flying past your computer screen will block a bit of light.
When a planet orbits behind the star, relative to Kepler's view, there is a slight increase in brightness. From the frequency and duration of the dimming and brightening scientists can figure out how far away a planet orbits and how much mass it contains.
Kepler, which monitors stars in the constellation Cygnus, was launched to find out how many Earth-like worlds are orbiting a sampling of 156,000 stars like the sun.
Actually finding an Earth-sized world circling as far from its star as Earth orbits the sun will take 365 days of observations to detect one pass, plus another year or two of data to verify the orbital period.
If Kepler's latest head count is confirmed, the list of extrasolar planets will more than triple.
"One of the things that is still a work in progress is to figure out how many of the candidate planets are real planets," Jonathan Fortney, assistant professor of astronomy and astrophysics at University of California, Santa Cruz, told Discovery News.
"This is some kind of planetary tsunami," said astronomer Geoff Marcy, with the University of California, Berkeley. "The implication is that there are lots and lots of Earth-sized planets in our Milky Way."
An independent analysis by astrophysicists at the California Institute of Technology (Caltech) shows Kepler's track record of finding extrasolar planets is better than 80 percent.
The Kepler 11 study appears in this week's Nature. The Caltech analysis appears on arXiv.org. The Kepler science mission data, collected between May 2 and Sept. 16, 2009, was released by NASA Wednesday and will be published in an upcoming issue of The Astrophysical Journal.