"We thought we would have to search vast distances to find an Earth-like planet. Now we realize another Earth is probably in our own backyard, waiting to be spotted," said Harvard astronomer and lead author Courtney Dressing (CfA).
Dressing presented her findings today in a press conference at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
Red dwarf stars are smaller, cooler, and fainter than our Sun. An average red dwarf is only one-third as large and one-thousandth as bright as the Sun. From Earth, no red dwarf is visible to the naked eye.
Despite their dimness, these stars are good places to look for Earth-like planets. Red dwarfs make up three out of every four stars in our galaxy for a total of at least 75 billion. The signal of a transiting planet is larger since the star itself is smaller, so an Earth-sized world blocks more of the star's disk. And since a planet has to orbit a cool star closer in order to be in the habitable zone, it's more likely to transit from our point of view.
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Dressing identified 95 planetary candidates orbiting red dwarf stars. This implied that at least 60 percent of such stars have planets smaller than Neptune. However, most weren't quite the right size or temperature to be considered truly Earth-like. Three planetary candidates were both warm and approximately Earth-sized. Statistically, this means that six percent of all red dwarf stars should have an Earth-like planet.
"We now know the rate of occurrence of habitable planets around the most common stars in our galaxy," said co-author David Charbonneau (CfA). "That rate implies that it will be significantly easier to search for life beyond the solar system than we previously thought."
Our Sun is surrounded by a swarm of red dwarf stars. About 75 percent of the closest stars are red dwarfs. Since 6 percent of those should host habitable planets, the closest Earth-like world is likely to be just 13 light-years away.
It should be noted that 13 light-years is only an average--there are numerous red dwarf stars closer than 13 light-years, some further. Nevertheless.