The climate of planets in close orbit around low-mass stars may be more like Earth than previously thought, according to a new study, potentially greatly expanding the chance of life existing around some of the sun's nearest neighbors.
When we look into the sky, most of the stars we see are brighter and more massive than the sun. That's just selection bias though. Most of the stars in the galaxy are low-mass red dwarfs. Within 15 light-years, there are nine stars with at least half the mass of the sun and 43 M-class red dwarfs. Over the last few years, we have learned that planets are common around red dwarfs, including at distances which suggest the planet's average temperature should allow for liquid water. This has unleashed debate about whether such planets are suitable hosts for life.
One issue is the length of the planetary day. Most moons in the solar system are tidally locked, like our own. This means they turn at the same rate as they orbit, so that they always keep the same face to their planet. The closer a planet is to its parent star, the more likely it is to have a similar tidal locking, which would leave one side roasting hot and the other permanently frozen. There might be a narrow ribbon in between where temperatures are acceptable, but it is unlikely the atmosphere of such places would support life.
Planets close enough to a dim star to have average temperatures above 0°C risk a similar fate. However, Dr. Jérémy Leconte of the Canadian Institute for Theoretical Astrophysics says examples such as the moon make bad models because they don't have an atmosphere to start with.
"Atmosphere is a key factor affecting a planet's spin, the impact of which can be of enough significance to overcome synchronous rotation and put a planet in a day-night cycle," Leconte says. Winds on Earth are created by the difference between the parts of the planet heated by the sun and those due to night, clouds or high latitude that are cooler. The redistribution of mass on the planet produced by winds is enough to overcome the tidal drag a parent star would exert on a planet in its habitable zone, Leconte concludes, provided the atmosphere was at least as thick as Earth's.
Leconte's modeling, published in Science Express, was on stars with masses 0.5-0.7 times that of the sun, so lower mass stars may still prove inhospitable. He also acknowledges that many of these planets would have long days, equal to weeks or even months on Earth. This could certainly make life uncomfortable, but probably not impossible.
Day length is just one obstacle to life on such planets. Intense outbursts of stellar activity could strip the atmosphere off planets located too close to the star, unless it was protected by a particularly strong magnetic field.