Image: Exoplanet lineup
PHL @ UPR Arecibo, ESA/Hubble, NASA
Planets are being added to the Habitable Exoplanets Catalog regularly. So far none of them is small enough to be considered "Earth's twin," but that's likely to change.
updated 1/2/2013 8:22:38 PM ET 2013-01-03T01:22:38

Our Milky Way galaxy is home to at least 100 billion alien planets, and possibly many more, a new study suggests.

"It's a staggering number, if you think about it," lead author Jonathan Swift, of Caltech in Pasadena, said in a statement. "Basically there's one of these planets per star."

Swift and his colleagues arrived at their estimate after studying a five-planet system called Kepler-32, which lies about 915 light-years from Earth. The five worlds were detected by NASA's Kepler Space Telescope, which flags the tiny brightness dips caused when exoplanets cross their star's face from the instrument's perspective.

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The Kepler-32 planets orbit an M dwarf, a type of star that is smaller and cooler than our sun. M dwarfs are the most common star in the Milky Way, accouting for about 75 percent of the galaxy's 100 billion or so stars, researchers said.

Further, the five Kepler-32 worlds are similar in size to Earth and orbit quite close to their parent star, making them typical of the planets Kepler has spotted around other M dwarfs. So the Kepler-32 system should be representative of many of the galaxy's planets, scientists said. [The Strangest Alien Planets (Gallery)]

"I usually try not to call things 'Rosetta stones,' but this is as close to a Rosetta stone as anything I've seen," said co-author John Johnson, also of Caltech. "It's like unlocking a language that we're trying to understand — the language of planet formation."

Kepler can detect planetary systems only if they're oriented edge-on to the telescope; otherwise, the instrument won't observe any star-dimming planetary transits. So the researchers calculated the odds that an M-dwarf system in the Milky Way would have this orientation, then combined that with the number of such systems Kepler is able to detect to come up with their estimate of 100 billion exoplanets.

The team considered only planets orbiting close to M dwarfs; their analysis didn't include outer planets in M-dwarf systems, or any worlds circling other types of stars. So the galaxy may actually harbor many more planets than the conservative estimate implies — perhaps 200 billion, or about two per star, Swift said.

The new analysis confirms three of the five Kepler-32 planets (the other two had been confirmed previously). The Kepler-32 worlds have diameters ranging from 0.8 to 2.7 times that of Earth, and all of them orbit within 10 million miles of their star. For comparison, Earth circles the sun at an average distance of 93 million miles.

Because the Kepler-32 star is smaller and less luminous than our sun, the five planets are likely not as heat-blasted as their tight orbits might imply. In fact, the outermost planet in the system appears to lie in the habitable zone, a range of distances that could support the existence of liquid water on a world's system.

The new analysis also suggests that the Kepler-32 planets originally formed farther away from the star and then migrated closer in over time, researchers said.

Several pieces of evidence point toward this conclusion. For example, the team estimated that the five Kepler-32 worlds coalesced from material containing about as much mass as three Jupiters. But models suggest that this much gas and dust cannot be squeezed into the small area circumscribed by the planets' current orbits, researchers said.

"You look in detail at the architecture of this very special planetary system, and you're forced into saying these planets formed farther out and moved in," Johnson said.

The new study was published Jan. 2 in The Astrophysical Journal.

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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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