Rocky planets around cool stars may have Earthlike climates

There may be more Earthlike environments in the universe than previously thought. Warm, rocky planets that orbit close to their stars might not end up with one side in perpetual daylight as suspected, allowing such planets to sustain an environment hospitable to life.

For the first time, researchers have shown that the gentle tug of a star’s gravity on a relatively thin atmosphere can keep a planet spinning even when other forces threaten to slow it down. While planetary scientists have long suspected that this process keeps Venus slowly turning, the mechanism could also work on a planet without Venus’ massive atmosphere, Jérémy Leconte, an astrophysicist at the University of Toronto, and colleagues report online January 15 in Science.

Most stars in the galaxy are M dwarfs, red stars that are smaller and dimmer than the sun. Because these stars are relatively cool, their habitable zones — the region around a star where liquid water could, in principle, survive on a planet’s surface — are small. Planets have to huddle up close to the star to stay warm. For planets so close, the star’s gravity slows down their rotation so that one side always faces the star, a circumstance known as tidal locking. The climate on a locked planet may be too harsh for life; a permanent warm spot on the dayside could perpetually blow ferocious winds and all the water might end up as ice on the nightside.

Leconte and colleagues used computer simulations to determine that heat in a planet’s atmosphere may prevent tidal locking. A planet’s equator receives more heat from its star than the poles do. This extra heat drives winds that push air toward the nightside. When the star pulls on the atmosphere, it tugs a little more on the side with more mass, which gradually causes the planet to spin faster over billions of years. If Earth were as close to the sun as Venus, the researchers found, these subtle nudges would make our days shorter and shorter. “It’s quite impressive,” Leconte says, “that the atmosphere — which is something like one-millionth the mass of the Earth — can spin up the whole planet.”

Because Earth is so far from the sun, the effect of these atmospheric forces is negligible, Leconte says. But on an Earth-sized world in the habitable zone of an M dwarf, the relentless tug on the atmosphere — even a relatively thin atmosphere like Earth’s — could be enough to keep the planet spinning, the researchers found.  

Edwin Kite, a planetary scientist at the University of Chicago, says these results should stimulate a lot of research. Roughly three-quarters of the stars in the galaxy are M dwarfs. “It’s reasonable, though not proven,” Kite says, “that most habitable zone planets orbit M dwarfs.” Because M dwarfs outnumber other stars near the sun, that’s where the closest habitable planets are likely to be found. The James Webb Space Telescope, scheduled to launch in 2018, could be able to detect the atmospheres of rocky worlds around M dwarfs and test ideas about their habitability.

Even if atmospheric forces don’t keep planets spinning, that doesn’t rule out M dwarfs as homes to alien life. “People still have this knee-jerk reaction that a tidally locked planet is hazardous to life,” says Nicolas Cowan, an astronomer at Amherst College in Massachusetts. Atmospheres are good at moving heat, he says, which should help a close-in planet maintain its temperature. And on a tidally locked planet, thick dayside clouds created by localized water evaporation might help regulate the planet’s thermostat, even for planets very close to their suns (SN Online: 7/8/13). The habitable zone for a tidally locked planet, he says, may even be wider than for one that spins normally.