Astrobiologist Margaret Turnbull recently compiled the list for the SETI Institute in Mountain View, Calif., a non-profit institute dedicated to the search for extraterrestrial intelligence.

"As soon as one discovers that our sun, which makes life and our civilization on Earth possible, is one of hundreds of billions of suns in the galaxy, the very next question is: Does that mean there are other living planets and civilizations in the galaxy, too?" Turnbull said.

"Is the galaxy a desert, with a single little oasis of life, or is it a garden, teeming with diverse creatures? To me, this is one of the most important questions that can be asked, because it begins to tell us about our connection to the entire universe."

Answering these questions requires stars bright enough for telescopes to detect. Because the distance from Earth is an indication of a star's brightness, Turnbull initially considered 120,000 stars from the Hipparcos catalog, which contains information on the distance of each star.

She created computer programs to organize information about each star, and she narrowed the list to 17,000 by using criteria characteristic of our sun. Now she's working with other scientists to provide the National Aeronautics and Space Administration with the 30 stars that seem most promising.

Her first criterion was the age of a star, because if an older star has planets, they are likely to be older and easier to spot. Her second criterion, star variability, tells how much a star changes in brightness.

Turnbull said some stars such as our sun shine pretty steadily, whereas others change brightness from day to day and even hour to hour. Turnbull chose stars with low variability because planets orbiting those stars are more likely to have a stable climate, one more conducive to life.

Her third criterion is whether a star contains metal, because a star with a metal content of zero cannot form planets. She also eliminated stars that have other stars orbiting them, called binary star systems, because in such systems the faint light reflected by planets will be hard for NASA's Terrestrial Planet Finder to see.

Lastly, Turnbull considered water. To support life, a planet has to be at the right temperature for liquid water to exist on the surface. Therefore the planet has to be not too close, but not too far, from its sun. If a giant planet like Jupiter is nearby, its presence could pull an Earth-sized planet out of that prime location. So Turnbull has taken care to exclude stars from her list that have giant planets orbiting too close for comfort.

For the 30 stars chosen for NASA's Terrestrial Planet Finder, the ability of a sun to support life in its system is only one factor. Ultimately, engineering decides what the TPF is capable of observing and which 30 stars are on the list.

To create the high-resolution space telescope needed to detect faint planets next to a bright star, astronomers and engineers are collaborating. Dennis Ebbets, an engineer at Ball Aerospace & Technologies Corp. in Boulder, Colo., is providing an engineering point of view for the TPF project. According to Turnbull, Ebbets has also generated a list of stars that will be observable according to the engineering requirements.

The telescope will also provide information on the chemical composition of the candidate planets' atmospheres, a key factor determining whether a planet could support life. Turnbull anticipates that the final telescope design will be chosen in 2006.

Turnbull is looking forward to what Terrestrial Planet Finder discovers when it starts reviewing the stars on her list. "Aside from all the beautiful and bizarre discoveries that come out of studying the skies, ultimately the real questions that drive us are more personal," she said. "We want to know if we ourselves have some kind of purpose here, if there is a deeper meaning to the universe than what we've grasped, and if perhaps others are asking the same things."

Related Links
UA Department of Astronomy and Steward Observatory
NASA Astrobiology Institute
SETI Institute
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