Top stars picked in alien search

An US astronomer has drawn up a shortlist of the stars most likely to harbour intelligent life.

Scientists have been listening out for radio signals from other solar systems in the hope of detecting civilisations other than our own.

Margaret Turnbull at the Carnegie Institution in Washington DC looked at criteria such as the star's age and the amount of iron in its atmosphere.

Her top candidate was beta CVn, a Sun-like star 26 light-years away.

Dr Turnbull had previously identified about 17,000 stellar systems that she thought could be inhabited.

From these, she has selected five stars that look most likely to support intelligent extraterrestrial life forms - if they exist.

"I've chosen five to advertise the very best places to move to if we had to, or to point the telescope at," she told the BBC.

Shortlist

Seti, the Search for Extraterrestrial Intelligence, is an exploratory science that seeks evidence of life in the universe by looking for some signature of its technology.

Astronomers principally do this by using telescopes to look for radio signals from distant stars.

But the enormity of the task means that scientists have been looking for ways to narrow down the search.

"There are bazillions of stars in the sky to look at, but we can't look at every single one with the scrutiny that we'd like to," said Dr Turnbull.

"We have been able to prioritise our search so that we are looking at stars that are most like the ones around which we live. We need to know which ones to spend our telescope time on."

Several of the criteria she used were related to age.

For stars to be considered in the shortlist, they had to be at least three billion years old - long enough for planets to form and for complex life to develop.

"Fully-formed advanced civilisations don't just spring up overnight. On planet Earth, it took billions of years for civilisation to arise."

Looking for Goldilocks

Candidate stars also had to have at least 50% of the iron content of the Sun.

If the atmosphere of a star is low in iron, it is likely there were not enough heavy metals present early in its existence for planets to form from.

Dr Turnbull threw out variable stars prone to lots of solar flares because they tend to be young.

Stars more than 1.5 times the mass of the Sun do not tend to live long enough to produce so-called "habitable zones".

This is the area around the star where a planet within the zone can support copious amounts of liquid water on the surface - a key requirement for life.

Put the planet too close and the heat will evaporate the water, put it too far away and any water freezes.

Budget cuts

The Carnegie Institution researcher also removed from her shortlist any stars with a companion.

These companion stars can interfere with the habitable zone.

Astronomers have put together a set of principles - called the Seti principles - that outline what should be done if a signal from an extraterrestrial civilisation is ever detected.

"The scientific community - and the world - is told right away," said Gill Tarter, from the Seti Institute in California.

"Before a decision is made to send a message back everyone will consult - that's in the ideal world."

The search for life in other stellar systems has been hit by cuts in the 2007 Nasa budget.

Dr Turnbull announced details of her work at the American Association for the Advancement of Science annual meeting in St Louis, US.