NASA / JPL-Caltech / T. Pyle An artist's rendering shows a typical close-in Earth-size planet, Kepler-20e, which is about 0.87 times as wide as our planet but orbits its parent star more closely than Mercury orbits our sun.





A simulation based on data from NASA's planet-hunting Kepler mission has determined that about one out of every six stars has an Earth-sized planet, which would translate to at least 17 billion such worlds in our Milky Way galaxy. And that's not even counting the alien Earths we'd want to live on.

These 17 billion planets would be circling their parent stars more closely than Mercury orbits our own sun — which means that, in many cases, the planets would be too hot for liquid water to exist. A few such worlds already have been found, including a "lava planet" known as Alpha Centauri Bb that's just 4.3 light-years away from us.

Someday, the type of simulation that astronomers used to estimate the number of hot Earths can be used to estimate how many habitable Earths could provide a home for life as we know it in the Milky Way. But not just yet.

"For an estimate of Earth-sized planets in the habitable zone, it's simply too early to call," said Francois Fressin of the Harvard-Smithsonian Center for Astrophysics, or CfA.

Fressin and his colleagues lay out their estimates for Earth-sized planets, as well as bigger worlds, in a paper that's been accepted for publication in the Astrophysical Journal. Their research is being discussed today at the American Astronomical Society's winter meeting in Long Beach, Calif.

The estimates are based on a list of 2,400 planet candidates that have been detected by the Kepler probe since its launch in 2009. Kepler looks for planets in a patch of sky overlapping the constellations Cygnus and Lyra, by checking for the faint dimming of a star as an alien world passes across its disk. One of the challenges is to make sure the dimming is really caused by a planet, rather than some other phenomenon such as an eclipsing binary star. Another challenge is that Kepler is sure to miss some planets, because those planets are not in a position to block the light of its parent star, as seen from Earth.

Now that the Kepler mission has been churning out detections for more than three years, there's enough of a database to arrive at some statistical conclusions about the total number of planets in the Milky Way — at least 100 billion. There's also enough data to determine what the breakdown of detections should be, and even how many of those detections will be wrong.

"We have a knowledge of false positives that's good enough that we can do a study from scratch," Fressin said.

The simulation suggests that the false-positive rate should vary depending on the size of the planet candidates, from a low of 6.7 percent for small Neptune-scale planets to a high of 17.7 percent for Jupiter-type giants. The false-positive rate for close-in planets between 0.8 and 1.25 times as wide as Earth is 12.3 percent. When all these factors were added to the calculations, the astronomers arrived at a breakdown for five types of planets currently detectable by Kepler:

17 percent for Earths with orbital periods up to 85 days.

26 percent for super-Earths (1.25 to 2 times as wide as Earth) with orbits up to 145 days.

26 percent for small Neptunes (2 to 4 times Earth's width) with orbits up to 245 days.

3 percent for large Neptunes (4 to 6 times Earth's width) with orbits up to 418 days.

5 percent for giants (6 to 22 times Earth's width) with orbits up to 418 days.

The results indicate that for every size of planet except for gas giants, the type of star doesn't matter. Earth-sized planets should be just as likely to form around red dwarfs as around sunlike stars. That runs counter to what was previously thought.

"Earths and super-Earths aren't picky. We're finding them in all kinds of neighborhoods," the CfA's Guillermo Torres, a co-author of the study, said in a news release.

The researchers emphasized that these are just minimum estimates — and that as Kepler provides more planet candidates at smaller scales and wider orbits, the numbers could increase. Eventually, such simulations could spit out a long-sought number: the tally of Earth-sized planets in the Milky Way expected to have conditions capable of supporting life.

"This result is a significant step towards the determination of eta-earth, the occurrence of Earthlike planets in the habitable zone of their parent stars," they wrote in their research paper.

More about the planet search:

In addition to Fressin and Torres, the authors of "The False Positive Rate of Kepler nd the Occurrence of Planets" include David Charbonneau, Stephen Bryson, Jessie Christiansen, Courtney Dressing, Jon Jenkins, Lucianne Walkowicz and Natalie Batalha.

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.