Life near cool stars?

A recent paper by Guillen Anglada-Escudé (A planetary system around the nearby M dwarf GJ 667C with at least one super-Earth in its habitable zone) announces the discovery of yet another planet in the habitable zone of its star. The planet, GJ 667Cc, is a super-Earth (with a mass in excess of 4.5 Earth masses). It orbits its star with a period of 28.15 days.

This is a particularly interesting finding because the planet is orbiting an M-class dwarf. Thing is, more than three-quarters of the stars in our neighbourhood are M-class stars (mainly dwarfs, though there are a some red giants too). If rocky planets in the habitable zone are common around M-class dwarfs then there are lots of potentially habitable planets out there!

I doubt that M-class dwarfs are ideal locations for advanced life forms, however.

M-class dwarfs pump out much less energy than our Sun. Thus a planet in orbit around such a star must be close to the star if it is to have a surface temperature that’s similar to Earth’s. That in turn means that the planet is much more likely to be tidally locked. The problem is that tidal locking leads to extremes of climate: the star-facing side of a tidally locked planet would be in permanent light, the other side would be in neverending night. And that in turn means that surface temperatures actually wouldn’t be like Earth’s. One side would be extremely hot, the other side frigid. Furthermore, the temperature on the frigid side would be so low that any atmospheric gases would be frozen out; the day side would be left dry. (If a large planet possessed a moon, however, then conditions on the moon might be more hospitable: a moon that was tidally locked to its planet would have a day-night cycle as it orbited the planet.)

Another problem with M-class dwarfs is that they can be quite variable. Starspots are common, and they reduce the star’s energy output by up to 40% for significant periods; flares are less common, but when they occur they can double the star’s brightness in a matter of minutes.

The discovery of GJ 667Cc suggests that the Galaxy might contain billions of rocky planets where liquid water can exist. But whether those planets can host life … well, that’s a different question. Soon the search for exoplanets needs to become a search for biosignatures.