An astronomer ranks her top 10 places to search for extraterrestrials.

The truth may be out there, but without a good guidebook, you may never find it.

Researchers probing the universe for signs of extraterrestrial intelligence got their treasure map this week. It came in the form of a list of 10 stars likely to be at the center of planetary systems that could support life.

Margaret Turnbull, an astronomer at the Carnegie Institution, picked her top 10 from a catalogue of 17,129 potentially life-supporting solar systems that she assisted in compiling in 2003.

“What I needed to do was think about the characteristics of the sun that make it such a great host for us,” Turnbull said. “We’ve had this planet with liquid water on it for billions of years, long enough to have life evolved to the point where it’s got civilization and radio telescopes and is capable of communicating between the stars.”

Turnbull evaluated the candidate stars based primarily on their ages, sizes and chemical make-ups: A star had to be old enough to have developed complex, at least three billion years old. The stars chosen are all relatively small in mass—less than one and a half times the mass of our sun—so that they may survive long enough to allow life to evolve; larger stars tend to burn out quicker. The stars also need to have high levels of iron in their atmospheres, which indicates that the nebulae in which they were formed contains sufficient amounts of heavy metals necessary to foster planets.

While Turnbull has no way of knowing what the planets in her top solar systems actually look like, she is confident that they could, under the right circumstances, sustain life. She expects, however, that extraterrestrial life may first be found in our own solar system. The planet Mars and Jupiter’s moon, Europa, are frequently mentioned as potential hosts of organisms, the latter due to its large oceans of water.

“It’s probable that [Europa] has hydrothermal activity at the bottom of the oceans, just like ours do,” said Turnbull. “Those hydrothermal vents could very well support all kinds of life forms, maybe even big animal life forms like what we see around the vents at the bottom of our oceans.”

Dimitar Sasselov, director of the Harvard-Smithsonian Center for Planetary Astrophysics, called Turnbull’s work “timely.”

“It is important to revisit our lists of best nearby neighborhoods every few years and see what has changed in our knowledge about them,” he said via e-mail. “Great habitable places are hard to find in the universe.”

At the moment though, astrobiologists are finding NASA to be an uninhabitable place, Turnbull said. On January 14th, President Bush promised to send a man to Mars by 2020, but he did not allocate sufficient funds in his 2007 budget to make this financially possible. NASA has scrambled to divert money from, among other places, its Astrobiology Institute. As part of the shake-up at the space agency, G. Scott Hubbard, the director of the NASA Ames Research Center, where the Astrobiology Institute is based, left at the end of January to join the SETI Institute.

While the 2007 budget is still pending approval in Congress, scientists who study life in outer space may come up short.

Originally published February 27, 2006


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