In the search for habitable worlds beyond our solar system, the biggest, hottest opportunities may be found around the smallest, coolest stars. Called M dwarfs, these stars have a mere fraction of the sun's mass and luminosity but are more than 10 times as numerous. Planets circling an M dwarf must be in a close orbit to the star to be warm enough for life, like campers huddling around a small fire. This proximity makes them relatively easy for planet hunters to find, and the prevalence of M dwarfs means there are plenty nearby to investigate.

Astronomers are now gearing up for such an exploration. Multiple independent projects are already monitoring nearby M dwarfs, and a host of new telescopes and satellites are in the works to spot planets orbiting them, including NASA's Transiting Exoplanet Survey Satellite (set to launch in 2017). These efforts make the imminent discovery of potentially habitable M dwarf planets a near certainty. Whether all those bodies will actually prove to be habitable, however, is much less clear: the same sunny properties that make promising M dwarf planets so easy to find may also preclude the possibility for life on those worlds.

3 Reasons M dwarf Planets are Easy to Find but Difficult for Life

Wobbles and Tides Astronomers find some planets by looking for wobbling stars, a periodic back-and-forth motion caused by the gravitational tugging of unseen worlds. An Earth-size planet in a habitable orbit around an M dwarf would shift its motion through space at a detectable meter per second, and the wobble would recur every few weeks or months—much more frequent than the sun's nearly indiscernible wobble. But at such close proximity, tidal forces could sap energy from the planet's spin, causing it to rotate only once per orbit, always presenting the same hemisphere to the star, just as the moon does to Earth. In worst-case scenarios, such a planet's water and air could freeze out and accumulate as a giant ice cap on its cold nightside, rendering the surface uninhabitable.

Shadows and Flares Another way to locate planets is to look for the shadows of transiting worlds, planets that partially eclipse their stars as seen from Earth. Because of their close-in orbits, transiting M dwarf planets would block a larger fraction of starlight than they would if they orbited larger stars, making their shadows easier to see. There is a downside, however, for potential life. M dwarfs are much less luminous than sunlike stars and far more variable, dramatically dimming and brightening because of star spots and stellar flares, bathing planets in x-rays and ultraviolet radiation. Such unpredictable radiation could wreak havoc on climates and biospheres. And cheek by jowl with a tempestuous M dwarf, an otherwise habitable planet could have its atmosphere eroded by powerful flares.

Long Lives and Troubled Youths One reason M dwarfs are so numerous is that they simply live longer than other stars—their numbers growing over time because their small size allows them to slowly, efficiently burn their nuclear fuel. But this longevity comes only after a troubled youth. Somewhat counterintuitively, because of their small size and weaker gravity, M dwarfs can take longer to form than much larger stars. They can spend hundreds of millions of years as protostars, slowly forming from collapsing clouds of gas. Planets, however, may form around such a protostar in only tens of millions of years. In all the intervening time, those worlds would broil in the protostar's light and heat, possibly cooking off most of their life-giving water before the M dwarf was fully formed.