A mirror Earth, right around the corner

The discovery of an Earth-like world is only the first of many steps to finding Earth-like life. For one thing, most of the candidate planets Kepler has seen to date are so close to their stars that they’re much too hot for life. Only three, in fact, appear to be in orbits that make them plausibly habitable. What’s more, Charbonneau cautions that M-dwarfs tend to be much more active than the Sun, with lots of sunspots, flares and potentially lethal ultraviolet radiation.

Still, the more such Earth-like exoplanets there are, the greater the odds of finding just the right one. And that’s where the new study gets exciting. Since a planet has to orbit edge-on from Earth’s perspective for it to pass in front of its parent star in a way that would allow Kepler to notice it at all — and since only a small fraction of planetary systems have that orientation — Dressing, Charbonneau and their collaborators were able to calculate that 6% of red dwarfs are likely to host an Earthlike world, which is actually quite a lot. Spreading the 6% evenly out over the entire galactic population of M-dwarfs is what led to the statistical the conclusion that the nearest one is 13 light years away. “Astronomically speaking,” says Dressing, “this is like a stroll across the park.”