With exquisite timing, the SETI Institute has announced the first of a series of workshops to study the habitability of planets around M-class red dwarfs. The issue became highly visible recently with the announcement of the rocky planet discovered around the red dwarf Gliese 876, some 15 light years from Earth. Although thought to be too hot for life as we know it, the new planet is a solid world orbiting a main sequence star, raising the question of genuinely terrestrial worlds around such stars.

‘Main sequence’ refers to stars that burn hydrogen in their cores, those that show up in a well-defined band on the famous Hertzsprung-Russell (HR) diagram, which plots the intrinsic brightness of stars against their surface temperatures (intrinsic brightness is the observed brightness of the star corrected for distance). Moving off the main sequence takes you into the domain of red giants, red and yellow supergiants, and white dwarfs. But way down on the lower right of the HR diagram, and still on the main sequence, are low mass red dwarfs like Gliese 876.

And there are plenty of such stars. According to Jill Tarter, the Director of the Center for SETI Research at the SETI Institute, “Most stars in our galactic neighborhood are M stars; historically we’ve excluded them because planets within their classically defined ‘habitable zone’ would be tidally locked to the star and have to endure periodic flares of hard radiation. This historical wisdom may require revision in light of newer atmospheric models and a new appreciation of extremophiles on Earth. Our list of target stars for SETI may be about to get a lot bigger.”

Image: Here’s another star from the Gliese catalog (not the one around which the new planet was found). Gliese 623b is one of the least massive main sequence stars ever found, some 60,000 times fainter than the Sun. This double star system is 25 light years from Earth in the constellation Hercules. Can tiny red dwarfs like this produce conditions suitable for life? Credit: Cesare Barbieri, University of Padua, and NASA/ESA.

The Tarter quote comes from a news release from the SETI Institute, which will host the workshop from July 18 to 20 at the Institute’s headquarters in Mountain View, CA some 40 miles south of San Francisco and 10 miles north of San Jose (this is virtually next door to NASA’s Ames Research Center). Full information including directions to the conference site can be found here. A second workshop is planned in another 12 to 18 months to examine preliminary work initiated by the first conference.

With 70 percent of all stars in the Milky Way now thought to be red dwarfs, and with the target selections for Terrestrial Planet Finder an obvious priority, this conference could not be more timely. “It may well be that there are far more habitable planets orbiting M dwarfs than orbiting all other types of stars combined,” said Frank Drake, the Director of the SETI Institute’s Center for the Study of Life in the Universe. The Institute will use the results of its workshops to plan future observations using the Allen Telescope Array (ATA), and the work should feed into future studies with SonATA, a system now being designed to examine one million stars for extraterrestrial signals.

Be aware of Heath, M.J., Doyle, L.R., Joshi, M.M. et al., “Habitability of planets around red dwarf stars,” Origins of Life 29, 405-424 (1999). Also note Jill Tarter’s paper with Margaret C. Turnbull, “Target Selection for SETI. I. A Catalog of Nearby Habitable Stellar Systems,” Astrophysical Journal Supplement Series 145, 181-198 (March, 2003), available on the SETI Institute site (PDF warning).