Scientist earmarks planets most likely to hold alien life

Astronomers have identified a star in our Milky Way galaxy that is the most likely candidate for possessing a companion planet that harbours intelligent extra-terrestrial life.

It is a sun-like star called beta CVn in the constellation Canes Venatici and it appears to possess all the necessary preconditions that would allow an advanced civilisation to flourish on a nearby planet.

The star is 26 light years away - 153 trillion miles - and it heads a shortlist of five stars that astronomer Margaret Turnbull of the Carnegie Institution in Washington believes could be the focus of fresh attempts to make contact with other intelligent beings.

Dr Turnbull selected her top five from an initial catalogue of 17,129 stars that could be "habitable stellar systems" where the physical conditions would not be too extreme to limit the evolution and development of intelligent life and its technology.

She said she made her choice purely on the characteristics of the stars themselves. "Stars are not all the same, and not all of them are like the Sun," Dr Turnbull told the American Association for the Advancement of Science in St Louis.

The first criterion is that the star had to be at least 3 billion years old, which is about the time it has taken life on Earth to evolve to its present stage. That would be long enough for companion planets to form and for complex life to develop on them. Dr Turnbull said.

Stars on the shortlist also had to be no bigger than about 1.5 times the mass of the Sun - bigger stars tend not to live long enough to produce habitable zones, she explained. Each shortlisted star also had to have enough metallic iron in its atmosphere - at least 50 per cent of the iron content of the Sun - otherwise it is unlikey that rocky planets similar to Earth would form around it.

The stars also had to be at the right stage of stellar evolution, which eliminated red giant stars or dwarf stars, which would not be suitable for complex life to survive for very long on a nearby planet.

"We are intentionally biased towards stars that are like the Sun. These are places I'd want to live if God were to put our planet around another star," Dr Turnbull said.

Jill Tarter of the Search for Extraterrestrial Intelligence (Seti) Foundation, a privately-funded attempt to detect the non-natural radio signals from advanced civilisations in space, said her organisation will now train its radio telescopes on the five shortlisted stars.

Dr Turnbull has also identified the star she believes is most likely to have a companion planet similar to the Earth where simple life could evolve because of the presence of liquid water - thought to be necessary for life.

Her top choice is epsilon Indi A, a star that is only one tenth as bright as the Sun about 11.8 light years away in the constellation Indus. It has enough intrinsic luminosity to suggest good prospects for a habitable zone but not so bright as to overwhelm attemps to take images of the planet with telescopes.

Dr Turnbull said that the shortlist of habitable zone stars with either advanced civilisations or Earth-like planets is by no means definitive but a reasonably accurate guide for other astronomers to follow.

"There are inevitable uncertainties in how we understand these stars. If I took 100 stars, it would be very difficult for me to tell which one is the best," she said.

However, there are certain conditions that would preclude the development of life and by concentrating Seti's efforts on the best candidates, scientists are more likely to get results even though no one is quite sure what will be done if astronomers ever detect a radio signal from ET.

"There is no formal policy of what to do if we discover extraterrestrial life," Dr Turnbull said.

Top five stars for planets with advanced life

* beta CVn, a sun-like star about 26 light years away in the constellation Canes Venatici

* HD 10307, another solar analogue about 42 light years away. It has almost the same mass, temperature and metallicity of the Sun. It also has a benign companion star.

* HD 211415, about half the metal content of Sun and a bit cooler, this star is just a little farther away than HD 10307.

* 18 Sco, a popular target for proposed planet searches. The star, in the constellation Scorpio, is almost an identical twin to the Sun.

* *51 Pegasus. Already famous. In 1995, Swiss astronomers reported they had detected the first planet beyond our solar system in orbit around 51 Pegasus. An American team soon verified the finding of the Jupiter-like object

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