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Reconsidering Conventional Wisdom
For decades the conventional wisdom on M dwarfs and habitable planets was "forget it." The stars are so cool, that in order for a planet to have liquid water, the planet would have to be so close to the star that it would become tidally locked. Just as the Moon is tidally locked to the Earth, the planet would have one side constantly in daylight and the other in perpetual night. It was thought that any atmosphere would freeze out on the night side, leaving the day side completely exposed to radiation from the star. We cannot imagine life existing under those conditions. So, with few exceptions, M dwarf stars were excluded from SETI target lists.
Then in the mid 90's people began to question the conventional wisdom. Atmospheric models showed that a tidally locked planet could not only retain its atmosphere, but distribute heat uniformly around the surface with a surprisingly modest amount of carbon dioxide. Other studies showed that ozone, a shield against harmful ultraviolet radiation, could be produced without biology on such a planet, making the surface more accommodating to life. Our conception of habitable conditions also expanded as we discovered "extreme life" (extremophiles) in amazing environments here on Earth. From boiling hot springs and deep ocean volcanic vents to frozen Antarctic lakes to the cooling water of nuclear reactors, life thrives in diverse environments. The environment on planets orbiting M dwarf stars may not be as hostile to life as we thought.
With those discoveries in mind, it seemed appropriate to reconsider the habitability of planets orbiting M dwarfs. NASA's Astrobiology Institute (NAI) is ideally suited to deal with this problem. It is a "virtual institute" composed of 16 teams of scientists from institutions around the US, with expertise in all areas of astrobiology. The SETI Institute NAI team was awarded funding for a series of two workshops to consider the habitability of M dwarf stars' planets. The purpose of the first workshop was to identify the research projects needed to resolve the issue. The second workshop is to be held eighteen months later, allowing the participants and their colleagues time to conduct the research. The second workshop will produce a scientific paper describing which, if any, M dwarfs might host habitable planets
More than thirty scientists, including members of seven NAI teams and twelve outside institutions, attended the first workshop (18-20 July, 2005) at the SETI Institute. After two and a half days of discussion, the consensus was that we could not rule out habitable planets orbiting M dwarfs but that a number of issues needed to be addressed.
