Lars A. Buchhave / University of Copenhagen
An artist's conception of a newly formed star surrounded by a swirling protoplanetary disk of dust and gas, where debris coalesces to create rocky 'planetesimals' that collide and grow to eventually form planets. A new study suggests small rocky planet may actually be widespread in our Milky Way galaxy.
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updated 6/13/2012 2:09:17 PM ET 2012-06-13T18:09:17

Small, rocky planets can coalesce around a wide variety of stars, suggesting that Earth-like alien worlds may have formed early and often throughout our Milky Way galaxy's history, a new study reveals.

Astronomers had previously noticed that huge, Jupiter-like exoplanets tend to be found around stars with high concentrations of so-called "metals" — elements heavier than hydrogen and helium. But smaller, terrestrial alien planets show no such loyalty to metal-rich stars, the new study found.

" Small planets could be widespread in our galaxy, because they do not require a high content of heavy elements to form," said study lead author Lars Buchhave of the University of Copenhagen in Denmark.

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A diversity of stars
Buchhave and his colleagues analyzed data from NASA's planet-hunting Kepler space telescope, which has been continuously observing more than 150,000 stars since its launch in March 2009.

Kepler watches those stars for tiny brightness dips, some of which are caused by alien planets that cross the stars' faces from the telescope's perspective. To date, Kepler has flagged more than 2,300 exoplanet candidates. While just a small fraction have been confirmed, Kepler scientists estimate that at least 80 percent will end up being the real deal. [ Gallery: A World of Kepler Planets ]

In the new study, the researchers looked at Kepler observations of 226 planet candidates circling 152 different stars. More than three-quarters of these potential planets are smaller than Neptune — i.e., their diameters are less than four times that of Earth — and some of them are as diminutive as our own planet, researchers said.

The astronomers studied the stars' spectra and found that small, rocky worlds circle stars with a much broader range of metal content than do giant planets.

"Naively, one might think that the more material you have in the (protoplanetary) disk, the more likely you are to form (small) planets," Buchhave told Space.com via email. "What we see, though, is that small planets form around stars with a wide range in heavy element content, while the close-in Jupiter -type planets seem to predominantly form around stars with a higher metal content."

In fact, small, terrestrial planets can form around stars nearly four times more metal-poor than our own sun, researchers said. The results suggest that Earth-size worlds may be common throughout the Milky Way, perhaps providing many places for life to gain a foothold.

"Since small planets could be widespread in our galaxy, the chances of life developing could be higher, simply because there could be more terrestrial-sized planets where life could evolve," Buchhave said.

The team's results are published Wednesday in the journal Nature.

NASA / JPL-Caltech
This chart compares the smallest known alien planets to Mars and Earth. Astronomers using data from NASA's Kepler space telescope announced the discovery of KOI-961.01, KOI-961.02 and KOI-961.03 on Jan. 11,; the Kepler team announced Kepler-20e and Kepler-20f in December 2011.

Early planets, early life?
Metals weren't present at the universe's birth. Rather, they're created inside stellar cores, implying that gas giants had a hard time forming until multiple generations of stars had been born, died and spread their innards throughout the cosmos in massive supernova explosions.

But the fact that rocky planets can take shape around metal-poor stars means the first roughly Earth-like worlds may have formed long, long ago.

"This work suggests that terrestrial worlds could form at almost any time in our galaxy’s history," co-author David Latham, of the Harvard-Smithsonian Center for Astrophysics, said in a statement. "You don’t need many earlier generations of stars."

Our solar system has been around for 4.6 billion years, and the earliest signs of life on Earth date from about 3.8 billion years ago. But the new study suggests that we may be relative latecomers as far as Milky Way biology goes.

"Knowing that the formation of rocky planets can occur in envi­ronments of lower metallicity than those of gas giants implies that there could be some places in the universe where rocky planets and life got an earlier start than did Earthlings," Debra Fischer, an astronomer at Yale University, wrote in a commentary accompanying the study in the same issue of Nature.

Follow Space.com senior writer Mike Wall on Twitter @michaeldwall  or Space.com @Spacedotcom.  We're also on  Facebook  and Google+.

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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
  4. Special delivery

    The International Space Station's Canadian-built robotic arm moves toward Orbital Sciences Corp.'s Cygnus autonomous cargo craft as it approaches the station for a Jan. 12 delivery. The mountains below are the southwestern Alps. (NASA) Back to slideshow navigation
  5. Accidental art

    A piece of art? A time-lapse photo? A flickering light show? At first glance, this image looks nothing like the images we're used to seeing from the Hubble Space Telescope. But it's a genuine Hubble frame that was released on Jan. 27. Hubble's team suspects that the telescope's Fine Guidance System locked onto a bad guide star, potentially a double star or binary. This caused an error in the tracking system, resulting in a remarkable picture of brightly colored stellar streaks. The prominent red streaks are from stars in the globular cluster NGC 288. (NASA / ESA) Back to slideshow navigation
  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
  8. Fire on the mountain

    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
  9. Where stars are born

    An image captured by NASA's Spitzer Space Telescope shows the Orion Nebula, an immense stellar nursery some 1,500 light-years away. This false-color infrared view, released on Jan. 15, spans about 40 light-years across the region. The brightest portion of the nebula is centered on Orion's young, massive, hot stars, known as the Trapezium Cluster. But Spitzer also can detect stars still in the process of formation, seen here in red hues. (NASA / JPL-Caltech) Back to slideshow navigation
  10. Cygnus takes flight

    Orbital Sciences Corp.'s Antares rocket rises from NASA's Wallops Flight Facility on Wallops Island, Va, on Jan. 9. The rocket sent Orbital's Cygnus cargo capsule on its first official resupply mission to the International Space Station. (Chris Perry / NASA) Back to slideshow navigation
  11. A long, long time ago...

    This long-exposure picture from the Hubble Space Telescope, released Jan. 8, is the deepest image ever made of any cluster of galaxies. The cluster known as Abell 2744 appears in the foreground. It contains several hundred galaxies as they looked 3.5 billion years ago. Abell 2744 acts as a gravitational lens to warp space, brightening and magnifying images of nearly 3,000 distant background galaxies. The more distant galaxies appear as they did more than 12 billion years ago, not long after the Big Bang. (NASA / NASA via AFP - Getty Images) Back to slideshow navigation
  12. Frosty halo

    Sun dogs are bright spots that appear in the sky around the sun when light is refracted through ice crystals in the atmosphere. These sun dogs appeared on Jan. 5 amid brutally cold temperatures along Highway 83, north of Bismarck, N.D. The temperature was about 22 degrees below zero Fahrenheit, with a 50-below-zero wind chill.

    Slideshow: The Year in Space (Brian Peterson / The Bismarck Tribune via AP) Back to slideshow navigation
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