The most common stars in the Milky Way go by a less-than-ennobling name: M-dwarfs. You may or may not have heard of them, but you’ve definitely never seen one — at least not with the naked eye. Even the very nearest M-dwarf is too dim to be spotted without a telescope, so until astronomers began scanning the sky in real detail, nobody had guessed these small, reddish stars even existed. “If the Sun is a hundred-watt bulb,” says Harvard astronomer David Charbonneau, “M-dwarfs are Christmas-tree lights.”
But M-dwarfs make up for their low wattage with sheer numbers. A whopping 248 M-dwarfs (and counting) are sprinkled right in the Sun’s immediate neighborhood, compared with a mere 20 or so Sun-like stars. Now, a new analysis says that at least one of those M-dwarfs is likely to host a habitable, Earthlike planet. Where there is one such familiar world, there are probably more. “The punchline,” says Courtney Dressing, the Harvard graduate student who led the analysis, “is that the nearest Earthlike planet should be no more than 13 light-years from the Sun.”
Finding a twin of Earth has been the ultimate goal of planet-hunters ever since the very first extrasolar worlds were discovered in the mid-1990’s. Until recently, such planets, about the size of Earth, and with a reasonably balmy, life-friendly surface temperature, have been too small to spot. But that changed dramatically in 2009, with the launch of the space-based Kepler Mission, an orbiting telescope that has been staring ever since at a single patch of nearly 160,000 stars, waiting patiently for one of them to wink in a regular rhythm that would indicate that a planet was moving across its face, blocking just a bit of starlight.
So far, Kepler has seen not just one, but nearly 3,000 sets of winks, all almost certainly caused by orbiting planets. In more than 300 cases, the amount of light blocked and the speed of the wink suggested Earth-size planets. Significantly, while only a small fraction of Kepler’s target stars are M-dwarfs, this is where all of these Earths have been seen. It’s no surprise, since an M-dwarf’s relatively small size allows even a modest-sized planet to block a relatively large percentage of starlight.
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.”
That’s crucial, since the closer an exoplanet is, the easier it is to study in detail — allowing astronomers to analyze its atmosphere, for example, for telltale molecules such as methane and oxygen that might betray the presence of life. Such analysis of even close-up worlds is not possible today, but when the James Webb Space Telescope goes into orbit in 2018, and when a new generation of giant ground-based telescopes goes on line at about the same time, the study of Earth-like exoplanets in our cosmic neighborhood can begin.
Before that happens, of course, those nearby Earths will have to be found for real — instead of merely statistically inferred as they were in the current study. But the study’s estimates of how common the exoplanets are makes the prospect of spotting at least one look better all the time.
Once they are found, there’s one more thing that increases the odds that that they could harbor life: M-dwarfs, Charbonneau points out, “are incredibly long-lived.” Earth has been habitable for less than five billion years, and has only about another billion to go before it ages to the point at which the Sun gets too hot for comfort. An M-dwarf, by contrast, changes little for tens of billions of years. Given the 13-billion-year age of the universe, it’s plausible that we could find a second Earth that has been sitting in its star’s habitable zone — and nurturing life — for as much as 10 billion years. And now we know that at least one of them might be right next door.