In this artist's conception, a Neptune-sized gas giant orbits M-dwarf star Gliese 436 Credit: NASA/G. Bacon. This star is similar in size to Gliese 876 where a newly discovered rocky planet was found in a close orbit.

Last February Phil wrote about the discovery of a brown dwarf solar system. The article Phil referenced added this tidbit:

What is more, these would-be planets could be habitable. The surface temperature of the mini brown dwarf is about 2000°C, which means that any planet 1.5 to 7 million kilometres away could maintain liquid water. The disc probably straddles this range.

Mention of a habitable zone around brown dwarves led to a discussion within the comments on tidal lock. Briefly, tidal lock is the tendency of two close celestial objects to always present the same face to one another. The Moon, for example, presents only one face to the Earth. The thought has been that any body close enough to a small star to have liquid water would tend to be tidally locked to that small star - one side burnt, the other rare. Obviously, that would not be conducive to life.

But now the SETI folks are reevaluating the possibility of habitable planets around small stars.

With the recent announcement of a planet seven to eight times the Earth's mass circling an M dwarf star, the chances for habitable worlds seem greater than ever. "It may well be that there are far more habitable planets orbiting M dwarfs than orbiting all other types of stars combined," explained Frank Drake, the Director of the SETI Institute's Center for the Study of Life in the Universe.

And the problem with tidal lock?

M stars have historically been considered unsuitable for SETI observations. The "Goldilocks" analogy suggested that planets whose temperature was "just right" for life would orbit so close to their star that they would be tidally locked... These stars also flare, producing X and UV radiation that will challenge any life on the surface...

"Simple theory said that terrestrial planets in orbit around M stars will be uninhabitable, and uninhabited," explained Jill Tarter, Director of the Center for SETI Research at the SETI Institute. "But we are not confined to simple theory any more. Recent models predict sustainable atmospheres and substellar liquid water regimes," she said. "A growing appreciation for natural sun screens that developed early in the evolution of life on Earth, and a speculation that niche sterilizing events might actually speed evolution towards complex life, make M stars worth considering once again..."

Hmmm. I'm wondering whether this will move the Foresight Exchange.

Gliese 876