Either approach is tricky and relies on choosing the right targets for scrutiny out of the many thousands of nearby stars in our galactic neighborhood.
Margaret Turnbull, an astronomer at the Carnegie Institution of Washington, has devoted herself to the painstaking search for candidate stars that may harbor zones of habitability where life--primitive or otherwise-- might thrive. Turnbull announced her shortlist of so-called "habstars" at the 2006 Annual Meeting of the American Association for the Advancement of Science in St. Louis.
Out of an initial catalogue of 17,129 "habitable stellar systems" that Turnbull and her colleagues published in 2003, she selected a handful of stars that she considers her best bets, based on a variety of screening criteria.
Turnbull offered five top candidate stars for those seeking only to listen for radio signals from intelligent civilizations--the Search for Extraterrestrial Intelligence or SETI--and five candidates for those who undertake the demanding job of trying to detect Earth-like planets in orbit around nearby stars.
Astronomers have found evidence during the past decade for dozens of planets around nearby stars by studying how an object's gravity affects the orbit of the parent star. Virtually all of the discovered planets are gas giants like Jupiter and are presumed to be inhospitable to life. There have been hints of smaller, rocky planets like Earth, but definitive detection of such terrestrial planets likely awaits the deployment of more capable space-based observatories in about a decade. "It's impossible to know the true nature of those planets until we can directly image them," Turnbull said.
NASA had a mission on the dra