LONDON [ PA] - Hundreds of thousands of life-bearing planets could be scattered throughout the galaxy, according to a new estimate.

The nearest inhabited Earth-like world might be a few hundred light years away, experts believe.

In 1961, US scientist Frank Drake suggested a simple formula for calculating the number of technologically advanced civilisations in our galaxy, the Milky Way.

But the Drake equation contains some parameters that are very difficult to estimate, such as the number of Earth-like planets orbiting stars and the percentage of those that are likely to evolve life.

German climate researchers from the Potsdam Institute for Climate Impact Research near Berlin have now set new limits on these parameters, New Scientist magazine reported.

They first calculated how many planets lie in a star's habitable zone - the region where the temperature is right to allow life. The researchers arrived at a figure of 500,000 "Gaias", as they called extra-solar planets with a globally active biosphere.

Another part of their estimate was based on the theoretical assumption that only one of all stars in the Milky Way are accompanied by Earth-like planets.

They also assumed that life will form and evolve on only one per cent of all planets that are habitable.

In some respects their calculations were much more conservative than those made earlier. For instance, some believe that if a planet is habitable, life will inevitably evolve there.

A number of other experts think the conclusions are premature.

"We have no way to know if the estimate is good or not. We have no data," said Donald Brownlee, an astronomer at the University of Washington in Seattle.

Geoffrey Marcy, at the University of California at Berkeley, who has helped discover a number of extra-solar planets, said: "We don't know enough to predict habitability".

Some of these riddles might be solved later this century with the launch of sensitive planet-finding space telescopes by the American space agency Nasa and the European Space Agency.

Both ESA's Darwin telescope and Nasa's Terrestrial Planet Finder will be able to analyse the atmospheres of extra-solar planets for chemical signs of life. They should be operational by about 2015.

•Story originally published by:
The Age / Australia - Oct 25.01

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