From this catalog she further narrowed the search.
The list was prepared for a the SETI (Search for Extraterrestrial Intelligence) Institute. Scientists at SETI are listening for radio signals from other intelligent civilizations in our galaxy.
Stars that SETI might want to aim radio scanners at, Turnbull said, would likely be similar to our sun and should therefore meet several key criteria.
A star's age is a primary factor: It should be old enough for planets to form and complex life to arise, an age Turnbull estimates to be at least three billion years.
A star's size and brilliance can indicate its age, so Turnbull chose small, stable, dim stars with no other stars orbiting them.
Activity is also important: "Some stars do a lot of flaring. Ours has big flares, but nothing like flares of other stars in our neighborhood,'' she said. Huge flares could wipe out life on nearby planets.
She also looked for stars that, like our sun, have significant amounts of metals. Stars and their orbiting planets form from the same cloud of materials, she explained, so Earthlike planets rich in metals should have metal-laden stars.
Based on these and other criteria, Turnbull's top SETI stars are Beta Canum Venaticorum, HD 10307, HD 211415, 18 Scropii, and 51 Pegasus.
But if any of Turnbull's picks turn out to have planets that are worth a closer look, it may be a long while before anyone on Earth could make the voyage.
"The closest one is about five light-years away,'' she said.
When we do begin the actual search for life, scientists will have to consider what they have learned about extreme climate zones on Earth and elsewhere, said Nathalie Cabrol, principal investigator at NASA's Ames Research Center.
In recent years scientists have found life on Earth in places thought inhospitableat very high altitudes, in extremely cold regions, in places of high ultraviolet exposure, and in areas that receive little sunlight, Cabrol says.
"Wherever you look on Earth, we have yet to find a limit on life,'' she said. Planets that mimic Earth's extreme environments might not host life as we know it, but they are not out of the running.
A Second List
Turnbull created a second list for a NASA observatory program called Terrestrial Planet Finder.
Since this project aims to image Earthlike planets, Turnbull focused on stars that could have planets that would be visible to a telescope.
For Terrestrial Planet Finder, Turnbull based her search solely on the stars' brightness: bright enough to suggest they could support planets with liquid waterbelieved to be a key ingredient for lifebut dim enough to not interfere with taking images.
Her top candidates for this program are Epsilon Indi A; Epsilon Eridani; Omicron2 Eridani; Alpha Centauri B; and Tau Ceti.
Meanwhile, another NASA team plans to shoot a giant telescope dubbed Kepler into space in 2008 to detect suns with planets.
"It will be the first time that this will be done,'' said Dimitar Sasselov, an astronomy professor at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
The Kepler telescope will help answer the question: "How many stars like the sun have planets orbiting them like the Earth?'' said Sasselov, who is assisting with the project.
Named after the medieval astronomer Johannes Kepler, the telescope will be aimed at stars in and around the constellation Cygnus, which contains many stars like our sun.
Kepler will take photographs continuously for five years, documenting when orbiting planets pass in front of their stars.
"What we are trying to do is observe [planets] eclipsing their stars,'' Sasselov said. "Kepler will be put into space to avoid our unstable atmosphere,'' which often hampers ground-based observations.
The Kepler research will dovetail nicely with Turnbull's work, Sasselov said.
"Out of the hundred thousand stars Kepler will observe, we expect a thousand to two thousand will have transiting planets, and about 30 of these will be like Earth,'' Sasselov said.
Since stars in Cygnus are about a thousand light-years away, trips to potentially habitable planets found there are not in our near future. But that doesn't worry Sasselov.
"I'm not intimidated by this," he said. "We can do a lot with remote sensing, and one day we will probably figure out how we can visit them.''
Free Email News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES