Big Possibilities for Small Stars
Our galaxy is well-placed for viewing in mid summer and mid winter. In July, the star clouds of the Milky Way feature the bright blue-white stars of the Summer Triangle: Deneb, Vega, and Altair, and the red giant Antares in the heart of the scorpion. Six months later, from the other side of the Earth's orbit, we see the blue-white stars of Orion and the Dog Stars, Sirius and Procyon. The bright red giants Betelgeuse and Aldebaran are rubies in the January night. The splendor of these jewels of the night, when viewed far from city lights, inspires a sense of awe and wonder. It's a beautiful but misleading view of the stars in our galaxy; we're literally missing most of the picture.
Most of the stars in our galaxy, and presumably all galaxies, are small red stars called M dwarfs. If you haven't looked through a telescope, I can guarantee that you've never seen an M dwarf star. They are intrinsically very faint. The largest and brightest have about half the mass of the Sun but emit only a few percent as much energy as the Sun. The smallest are more than four thousand times fainter. They are difficult to study and few astronomers devote themselves to the task. Yet, these small stars may turn out to be the most important stars for Astrobiology.
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 dayside 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.
Among the topics that need further study:
- Better data on the spectrum of solar flares. New studies of our local star will help us predict the effects of M dwarf flares on the atmosphere of a very close planet.
- Better measurements of stellar wind for M dwarfs. Mass loss due to stellar wind could be significant for these stars because they live so long.
- Better models for the evolution of a terrestrial planet over time, especially plate tectonics and the magnetic field.
- Better understanding of how the spectrum of the star, more energy in the red and infrared, will impact life.
There are many more topics needing research. Some were already planned and some require only slight extensions to current research. Other topics will need new funding and we are pursuing possible sources. We are confident that in early 2007, we should know whether M dwarfs make good target stars for SETI. That will be just in time for a new targeted SETI program on the Allen Telescope Array.
One other aspect of M dwarfs makes them intriguing for SETI. They may be ideal hosts for advanced technological civilizations because they live an extraordinarily long time. Stars like the Sun live, i.e., they fuse hydrogen into helium, for only about 10 billion years. No M dwarf that ever formed has yet to die; no M dwarf will die for more than another 100 billion years. With such long lifetimes, there are big possibilities for these small stars.
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