The search for alien life outside our solar system has been made a little less daunting thanks to a new list drawn up by astronomer Margaret Turnbull that includes the known stars most likely to support habitable stellar systems.

Turnbull, an astronomer at the Carnegie Institute of Washington, listed 10 of what she believes are likely to be habitable stellar systems, or "habstars," capable of supporting Earth-like planets and life. The list was presented at the recent annual meeting of the American Association for the Advancement of Science in St. Louis.

Five of the stars on the list are thought of as good candidates for SETI astronomers seeking only to listen for radio signals from intelligent alien civilizations. They will be included in a list of targets for the Allen Telescope Array, a network of 42 linked radio dishes that is expected to go online this spring in California.

The other five are for NASA's Terrestrial Planet Finder (TPF), a planned space telescope that would attempt to directly image Earth-like planets around nearby stars. The TPF mission was scheduled for launch around 2016, but is currently on hold indefinitely, according to NASA's 2007 budget plan.

Selection criteria

Turnbull's list of top-10 habstars were winnowed down from a pool of about 19,000 potentially habitable star systems that she and SETI scientist Jill Tarter created in 2003. A star is considered habitable if it is thought able to support a rocky planet with liquid water on its surface.

To narrow the selection, Turnbull chose only those stars that were at least 3 billion years old, which had masses that were no more than 1.5 times that of our Sun and metal contents that were at least 40 percent that of our Sun.

Stars that were less than 3 billion years old are thought to be too young for planets and life to evolve. Stars that are more than 1.5 solar masses tend to burn out before life can emerge and stars with low-metal contents probably formed from clouds that didn't have enough heavy metals to make rocky planets in the first place.

"Our first and favorite targets are stars that are very similar to the Sun, stars that look like they're taking care of a planet that has Earth-like life on it," Turnbull said. "These are places I'd want to live if God were to put our planet around another star."

For now, there are no known Earth-sized planets in favorable orbits around Sun-like stars other than our own. But astronomer are confident such worlds will be found when technology allows.

SETI stars

Turnbull's top habstar candidates for the Allen Telescope Survey:

  • Beta Canum Venaticorum: a Sun-like star about 26 light years away in the constellation Canes Venatici. This was Turnbull's top choice for SETI.
  • HD 10307: a near replica of the Sun but with a companion star. Located about 42 light-years away, this star has almost the same mass, temperature and metal-content as the Sun.
  • HD 211415: has about half the metal content of the Sun and is a little cooler; just slightly farther away than HD 10307.
  • 18 Sco: a near-identical twin of the Sun, located in the constellation Scorpio.
  • 51 Pegasus: The first planet beyond our solar system was detected around this star in 1995. Although that planet was a gas giant, Turnbull thinks 51 Pegasus could harbor rocky planets as well.

All of these stars have already been surveyed by SETI's Project Phoenix in the past without success, but the Allen Telescope Array will have up to five times Project Phoenix's frequency range, making it more likely that a habitable planet around these stars will be detected if one exists.

TPF stars

For the TPF mission, Turnbull chose the following five stars:

  • Epsilon Indi A: Turnbull's top TPF mission choice; this star is only about one-tenth as bright as the Sun and about 11.8 light-years away in the constellation Indus.
  • Epsilon Eridani: This star is a bit smaller and cooler than our Sun; it is located about 10.5 light-years away in the constellation Eridanus.
  • Omicron2 Eridani: A yellow-orange star about 16 light-years away that is roughly the same age as our Sun.
  • Alpha Centauri B: This triple star system is located just 4.35 light-years away and one of the Sun's closest stellar neighbors.
  • Tau Ceti: This star is a G-class star and is in the same brightness category as the Sun. Despite being relatively metal-poor, it is long-lived enough for complex life forms to evolve.

If NASA's TPF mission is cancelled, Turnbull said there is a chance her list could be adjusted for the European Space Agency's Darwin mission, which is expected to launch in 2015 with similar goals.