An artist's conception shows Gliese 581g, an extrasolar planet that is thought to have three times the mass of Earth and to orbit within its parent star's habitable zone.
By Space.com contributor
updated 10/28/2010 3:09:43 PM ET 2010-10-28T19:09:43

There may be a bonanza of Earth-size alien worlds in the universe, scientists now suggest — about one out of every four sunlike stars might have a planet roughly the size of Earth orbiting close around it.

The new study found that there may be no shortage of planets with masses ranging from five to 30 times that of Earth, conflicting with previous planet models, researchers said. The findings also suggest that solar systems with Earth-size planets like our own may be common, they added.

The scientists focused on 33 known alien planets orbiting around 22 of the stars, some of which had been first discovered by the researchers themselves. Another 12 exoplanets were detected, but have not yet been confirmed. [Gallery: The Strangest Alien Planets]

Astronomers studied sunlike class G and K stars within 80 light-years of Earth with the powerful Keck telescopes in Hawaii for five years. Our sun is the best known of the yellow G stars, while K-type dwarfs are slightly smaller, orange-red stars. In all, they analyzed 166 of these stars, split roughly equally between G and K.

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Alien planet-palooza
The researchers looked for any minute wobbles in each star potentially caused by planets between three and 1,000 times the size of Earth orbiting closely around them — just a quarter of the distance between Earth and the sun.

The scientists estimate that about 1.6 percent of the sunlike stars in the sample had Jupiter-size planets, while 12 percent had super-Earths three to 10 times Earth's mass, the smallest currently detectable. This revealed a trend of increasing numbers of smaller planets, suggesting that planets the size of Neptune and smaller are probably much more common than giants such as Jupiter.

To extrapolate further, "of about 100 typical sunlike stars, one or two have planets the size of Jupiter, roughly six have a planet the size of Neptune, and about 12 have super-Earths between three and 10 Earth masses," researcher Andrew Howard, an astronomer at the University of California at Berkeley, said in a statement. "If we extrapolate down to Earth-size planets between one-half and two times the mass of Earth, we predict that you'd find about 23 for every 100 stars."

While the researchers spotted an additional 12 possible planets in the new study, they have not been confirmed, said researcher Geoffrey Marcy, an astronomer at the University of California at Berkeley.

If those were included in the count, the team detected a total of 45 planets around 32 stars.

"As NASA develops new techniques over the next decade to find truly Earth-size planets, it won't have to look too far," Howard said.

Howard, Marcy and their colleagues detailed their findings in Friday's issue of the journal Science.

Bucking the planet formation trend
The new findings conflict with current models of planet formation and migration.

"These results will transform astronomers' views of how planets form," Marcy said.

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After planets form a protoplanetary disk, researchers had thought only giant planets spiraled inward. Instead, where current models predict no small planets, the researchers found a surplus.

"I don't know for sure what's missing from the models, but I have a few guesses," Howard told Space.com. "One guess is that the disks of gas that planets are thought to migrate within during the birth of solar systems are more complicated than the models allow for. Another guess is that many small planets in a solar system may undergo a phase of scattering off of each other after the gas clears, a sort-of planetary billiard balls."

Based on these statistics, the researchers suggest NASA's Kepler mission to survey 156,000 faint stars for planets will detect 120 to 260 "plausibly terrestrial worlds" orbiting near some 10,000 nearby G and K dwarf stars.

"This is a first estimate, and the real number could be 1-in-8 instead of 1-in-4," Howard said. "But it's not 1-in-100, which is glorious news."

What are these planets made of?
The researchers hope to learn much more about extrasolar planets by combining the results of their study with forthcoming data from NASA's planet-hunting Kepler spacecraft. While their study can detect planetary masses, Kepler can measure planet size with exquisite sensitivity.

Image: Planet distribution
UC-Berkeley
The sizes of planets around 166 sunlike stars suggest that small planets outnumber larger ones. Each bar on the chart represents the percentage of planets within a range of masses. Based on these data, astronomers estimate that 23 percent host close-in, Earth-size planets.

Since the astronomers only detected planets very near their stars, there could be even more Earth-size worlds at greater distances, including within the habitable zone located at about the same distance as our planet is from our sun.

The habitable or "Goldilocks" zone is the distance from a star neither too hot nor too cold to allow liquid water to be present on the surface.

"By combining the planet masses with planet sizes, we're going to get a sense of the typical planetary densities and we'll be able to figure out whether these small planets we're finding are made mostly of iron, rock, water or gas," Howard said in an interview.

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Video: New frontiers in planetary science

Photos: Month in Space: January 2014

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  1. Southern stargazing

    Stars, galaxies and nebulas dot the skies over the European Southern Observatory's La Silla Paranal Observatory in Chile, in a picture released on Jan. 7. This image also shows three of the four movable units that feed light into the Very Large Telescope Interferometer, the world's most advanced optical instrument. Combining to form one larger telescope, they are greater than the sum of their parts: They reveal details that would otherwise be visible only through a telescope as large as the distance between them. (Y. Beletsky / ESO) Back to slideshow navigation
  2. A balloon's view

    Cameras captured the Grandville High School RoboDawgs' balloon floating through Earth's upper atmosphere during its ascent on Dec. 28, 2013. The Grandville RoboDawgs’ first winter balloon launch reached an estimated altitude of 130,000 feet, or about 25 miles, according to coaches Mike Evele and Doug Hepfer. It skyrocketed past the team’s previous 100,000-feet record set in June. The RoboDawgs started with just one robotics team in 1998, but they've grown to support more than 30 teams at public schools in Grandville, Mich. (Kyle Moroney / AP) Back to slideshow navigation
  3. Spacemen at work

    Russian cosmonauts Oleg Kotov, right, and Sergey Ryazanskiy perform maintenance on the International Space Station on Jan. 27. During the six-hour, eight-minute spacewalk, Kotov and Ryazanskiy completed the installation of a pair of high-fidelity cameras that experienced connectivity issues during a Dec. 27 spacewalk. The cosmonauts also retrieved scientific gear outside the station's Russian segment. (NASA) Back to slideshow navigation
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  6. Supersonic test flight

    A camera looking back over Virgin Galactic's SpaceShipTwo's fuselage shows the rocket burn with a Mojave Desert vista in the background during a test flight of the rocket plane on Jan. 10. Cameras were mounted on the exterior of SpaceShipTwo as well as its carrier airplane, WhiteKnightTwo, to monitor the rocket engine's performance. The test was aimed at setting the stage for honest-to-goodness flights into outer space later this year, and eventual commercial space tours.

    More about SpaceShipTwo on PhotoBlog (Virgin Galactic) Back to slideshow navigation
  7. Red lagoon

    The VLT Survey Telescope at the European Southern Observatory's Paranal Observatory in Chile captured this richly detailed new image of the Lagoon Nebula, released on Jan. 22. This giant cloud of gas and dust is creating intensely bright young stars, and is home to young stellar clusters. This image is a tiny part of just one of 11 public surveys of the sky now in progress using ESO telescopes. (ESO/VPHAS team) Back to slideshow navigation
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    This image provided by NASA shows a satellite view of smoke from the Colby Fire, taken by the Multi-angle Imaging SpectroRadiometer aboard NASA's Terra spacecraft as it passed over Southern California on Jan. 16. The fire burned more than 1,863 acres and forced the evacuation of 3,700 people. (NASA via AP) Back to slideshow navigation
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    Slideshow: The Year in Space (Brian Peterson / The Bismarck Tribune via AP) Back to slideshow navigation
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