Shortlist drawn up of stars likely to have habitable planets

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Astronomers have drawn up a shortlist of the stars most likely to have habitable planets. They are the prime candidates for detecting signals with the first radio telescope designed specially to find extraterrestrial intelligence elsewhere in the universe.

Margaret Turnbull of the Carnegie Institution of Washington released the catalogue of “habitable zones” at the American Association for the Advancement of Science meeting in St Louis. It will guide the search for extraterrestrial intelligence to be carried out by the SETI Institute with the Allen Telescope Array, a network of 350 radio dishes under construction in California. The array, which will begin operating later this year, will vastly increase astronomers’ ability to detect radiowaves from space that might be transmissions from alien civilisations rather than natural processes.

“We can’t point the array at all of the gazillions of stars in the sky,” said Prof Turnbull. So she and her colleagues narrowed down the candidates first of all to a long-list of 19,000 stars and then produced a shortlist of five “habstars” – the ones most likely to have planets and moons on which intelligent life might exist.

The astronomers used several criteria. Habstars are at least 3bn years old, giving enough time for evolution to lead to complex life forms. They are stable, because variable stars prone to pyrotechnics are unlikely to be hospitable to life. And they are rich in iron and other metals.

“We are intentionally biased toward stars that are like the sun,” said Prof Turnbull. “These are the places I’d want to live if God were to put our planet around another star.”

Her number one candidate is Beta CVn, 26 light-years away and visible to the naked eye in the Hound Dogs constellation. Another top five Habstar is 51 Pegasus, which became famous in 1995 when astronomers discovered the first planet outside the solar system in its orbit. Although this giant Jupiter-like planet is not habitable, Prof Turnbull believes 51 Pegasus could also harbour terrestrial planets.

Current technology does not allow astronomers to image Earth-like planets around other stars. Nasa’s planned Terrestrial Planet Finder space telescope, scheduled for launch in 2015, would be capable of doing so, though it is threatened by the Bush Administration’s plans to cut the agency’s budget next year.

Prof Turnbull has produced a different shortlist of Habstars for direct observation in this way. These are younger and less luminous than the SETI candidates – so that their brightness does not overwhelm the telescope’s imaging system – but could still have planets or moons capable of supporting life.

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