The hunt for habitable planets beyond the Solar System just became more difficult. A study posted today on the arXiv server suggests that the same factors that make planets near M-dwarf stars easy to probe for potential life also diminish the chances that life could actually exist on those planets.
Researchers have often cited the environs of M-dwarfs, a type of red dwarf star, as a relatively easy place to look for planets that might be habitable. The stars are the most common type in the Galaxy, and their small size and mass makes it easier to detect planets orbiting them and use starlight to probe the planets' atmospheres. M-dwarfs are cooler than the Sun, so their habitable zones — the region surrounding a star where water could exist as a liquid on a solid surface — are closer in than the Sun’s. Planets in that region therefore complete an orbit in less time than Earth takes to orbit the Sun, providing astronomers with more opportunities to study them.
But the habitable zones around M-dwarfs may be too close to the stars to sustain life, says astronomer Ofer Cohen of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, who announced the findings during a press briefing today at a meeting of the American Astronomical Society in Boston, Massachusetts. Just as the Sun blows a steady stream of charged particles — the solar wind — M-dwarfs generate their own wind. That wind can strip the protective atmosphere of a planet in the habitable zone, making it harder for life to gain a foothold, Cohen says. Only if the planet had a magnetic field stronger than that of Earth — powerful enough to deflect the stellar wind — could it hold on to its atmosphere, Cohen notes.
Earlier findings had led astronomers to question the viability of life on these planets. M-dwarf flares, for example, have been shown to erode the atmosphere of surrounding planets. “This is one more knock against habitable planets orbiting M-dwarf stars,” says geoscientist James Kasting of Pennsylvania State University in State College, who was not part of the study.
Cohen and his team examined the influence of M-dwarfs on three planets that had been identified by NASA’s Kepler spacecraft and shown to reside in the habitable zones of their stars. Because key properties of the actual parent stars were not known, the team chose a dwarf star, Lacertae (EV Lac), which is a relatively young 300 million years old, as a stand-in. EV Lac’s luminosity and magnetic activity, which drives the stellar wind, is well characterized. The three candidate planets are much closer to their stars than Mercury is to the Sun.
The researchers found that the pressure from the stellar wind encountered by the planets would be 10–1,000 times stronger than that exerted on Earth.
However, M-dwarfs older than EV Lac are likely to have weaker winds, notes astrophysicist Edward Guinan of Villanova University in Pennsylvania. If so, a habitable-zone planet that survived for the first billion years with most of its atmosphere intact might still support life, he says.
Even if astronomers might be more likely to find life around stars resembling the Sun in size and mass, there is still a rationale, says Cohen, for missions like NASA’s Transiting Exoplanet Survey Satellite (TESS), which will study, among other things, planets in the habitable zone of M-dwarfs. Such observations will offer insight about the potential for life throughout the Galaxy, Cohen says. Astronomers have always been wary of how M-star activity might affect potentially habitable planets, says TESS scientist Sara Seager of the Massachusetts Institute of Technology in Cambridge. “Observers will always search for habitability without limits from theory,” says Seager. “What do we have to lose?”