George Dvorsky writes:

When we look up into the starry sky at night, we only see a small portion of what's really up there. Red dwarfs comprise nearly 75% of all the stars in the Milky Way, yet they remain invisible to the naked eye. Given their predominance, astronomers and astrobiologists are keen to learn more about these small, cool stars, and their potential to host habitable planets. Now, new research from the Harvard-Smithsonian Center for Astrophysics has offered some clues, suggesting that as many as 6% of all red dwarfs host Earth-sized planets within their habitable zones — a calculation that brings the total number of red dwarf alien Earths across the galaxy to 4.5 billion.

Top image: Artist's impression of a sunset from the super-Earth Gliese 667Cc courtesy ESO/L.Calçada. The large sun is the red dwarf, 667C.

To complete their study, "Small Planets Around Small Stars," astronomers Courtney Dressing and David Charbonneau took data from NASA's Kepler space telescope and parsed through the catalog of 158,000 target stars (hey, no one said astronomy wasn't hard work). The researchers were looking to isolate as many red dwarfs as possible, an analysis that ultimately led to the short-listing of 3,897 candidates, all of which featured temperatures below 4,000K (3,726 degrees Celsius, 6,740 degrees Fahrenheit). By comparison, our sun is nearly 5,800K (5,526 degrees Celsius, 9,980 degrees Fahrenheit).

Interestingly, the analysis showed that most red dwarfs were smaller and cooler than previously assumed.

From these 3,897, the researchers identified 64 red dwarfs that hosted 95 planets, a sample that included three Earth-sized planets situated within a habitable zone.

Now, it's important to remember that a red dwarf's ‘habitable zone' is far removed from what we see in our solar system. Red dwarfs are smaller (about one-third the size of our sun) and fainter (about one-thousandth the brightness of our sun), so they have a Goldilocks Zone that's much closer in. Consequently, any planet — Earth-like or not — would be subject to increased rates of solar flares, along with an increased vulnerability to celestial impacts.

Dressing and Charbonneau discovered that 60% of red dwarfs have planets smaller than Neptune, the majority of which are super-Earths (planets bigger than Earth, so not considered "Earth-like"). As a recent study noted, these super-Earths are more accurately described as mini-Neptunes — terrestrial planets surrounded by a suffocatingly thick and deep layer of hydrogen.

But of interest to the researchers was the discovery that 6% of all red dwarfs feature an Earth-like planet within its habitable zone. And by "Earth-like," they mean a rocky planet roughly the same size as earth and with a potentially temperate climate (a trait that would allow for liquid water, among other things).

As it so happens, our solar system is surrounded by red dwarfs. In fact, the astronomers suspect that an Earth-like planet with a moderate temperature may be as close as 13 light-years away (which is mere speculation at this point).

The full journal article:

http://www.cfa.harvard.edu/news/2013/dressing+charbonneau2013.pdf