Really Old Stars Perhaps Ideal for Advanced Civilizations

Planets abound in the galaxy. Over the past decade, scientists have discovered giant planets mostly by radial velocity techniques that detect the spectral shift in a star's light caused by the to and fro tug of an unseen planetary companion.

This method has detected more than 200 planets, dominantly large close-in planets called "hot Jupiters" that are inhospitable to life as we know it.

In the near future, with the launch of NASA's Kepler Mission in 2008, we'll have the tools to seek evidence of Earth-size planets in the habitable zone of distant stars.

The search for life beyond Earth is the search for a good place to live, a habitable planet, in orbit about a long-lived star where life may arise and evolve. The first place we looked was at stars like our own Sun, a middle-sized, middle-aged star. G-Stars like the Sun are stable for about 10 billion years, which is a good long time for planets to form, and life to evolve. We also expected to find solar systems like our own with small terrestrial planets near the star, and larger gaseous planets farther out. This particular pre-conception was discarded with the discovery of hot Jupiters on 4-day orbits about their stars.

The idea that other, less-massive, dimmer stars than the Sun could also host habitable worlds has long been debated. A particular class, M-Stars, are of interest simply because there are so many of them-they are the most common star in the galaxy. They're the cool stars that inhabit our neighborhood.

There's considerable interest in the question of whether M-Stars could host habitable planets. Would the planets be tidally locked with one face always directed toward the M-Star? Would flares wipe out life on the local planet? If M-Stars could host habitable planets, life may be much more widespread that we've previously thought. Thus, M-Stars are of interest to astrobiologists including SETI scientists who are searching for life beyond Earth.

In July 2005, a team of SETI institute scientists, as part of our NASA Astrobiology Institute research program, brought together a diverse group of scientists to consider-frankly reconsider-the possibility of life on planets orbiting M-Stars. The results of this workshop are now published as the current issue of "Astrobiology": Search for Habitable Planets Outside Earth's Solar System.  According to the publisher, "These reports present the preliminary results and conclusions from recent studies on the habitability of M Star Planets, which are planets about the size and mass of Earth that contain sufficient amounts of carbon dioxide (CO2) in their atmosphere to support a stable source of water on the planet's surface. The habitability of terrestrial planets depends in large part on the distance of their orbit from the nearest star. Most of the stars closest to the Earth's Sun are characterized as M Stars, and planets orbiting M Stars are of particular importance in the ongoing Darwin/Terrestrial Planet Finder missions being developed jointly by the U.S. National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA)."

The collected papers present the current understanding of M-stars, explore various aspects of M Stars (including dwarf M Stars and low mass M Stars), describe efforts to simulate Earth-like planets, consider the possible greenhouse effects in the atmosphere of Earth-like planets, and review the spectral signatures of photosynthesis.

"M stars are the most accessible, yet challenging, targets for habitable zone terrestrial planet searches," says journal Editor-in-Chief, Sherry L. Cady, Ph.D., Associate Professor in the Department of Geology at Portland State University. "The potential for M Star habitable zone planets to evolve biospheres and retain them are but two of the many reasons to include M stars in the search for evidence of life beyond the confines of Earth."

Why are SETI scientists interested in M-Stars? As Dr. Peter Backus, Observing Programs Manager for SETI, concluded in a preliminary report on the M-Stars workshop, "One...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."

For those of you who wish to delve deeper into this truly cool subject, the results of the first workshop are available to download, free-of-charge, at the Astrobiology magazine.

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