Jupiter or Mars-like planets beyond our Solar System may be serious contenders for harbouring life, says a British astrophysicist.
According to Professor Tim Naylor, of Exeter University, planets that do not resemble home should not be ruled out in the search for primitive lifeforms.
He is calling on biologists to draw up new parameters for extra-terrestrial life using their knowledge of the toughest organisms on Earth.
Microbes which thrive in boiling hot springs or in volcanic vents are stretching the limits of conditions that can support life.
According to Professor Naylor, it boosts the chances of finding life on non Earth-like planets circling stars other than our Sun.
International experts are meeting this week at Exeter University in the south-west of England to discuss just what type of conditions really are necessary for life on other worlds.
It could lead to a search for life on the growing list of so-called extrasolar planets that have been discovered.
Many of these 100 or so planets are huge gaseous objects close to their stars which, to a large extent, have been ignored as serious contenders favourable to living organisms.
"Planets that have been ruled out completely in the search for life could in fact be candidates for harbouring life," says Professor Naylor.
"If we can find life in the extremes of Earth where thermophiles (heat-loving microbes) are, then it could be that life could get a foothold on the giant exoplanets that we've discovered."
Various space telescope missions are being prepared by the US and European space agencies to search for planets around other stars.
The most ambitious of these is possibly Darwin, a plan by the European Space Agency to station a telescope 1.5 million kilometres from Earth.
The flotilla of six telescopes would seek to pinpoint other worlds capable of supporting life.
With so many stars to choose from, the obvious candidates are the nearest and most Earth-like. But not all scientists agree.
British planet hunter Dr Hugh Jones of Liverpool John Moores University says we need to get a much better idea of the conditions for life in different environments ahead of costly projects such as Darwin.
"Thermophiles in a whole range of environments should certainly be part of the search for life," he says.
"But when we launch these missions, it would be a shame not to configure them to look for planets like the Earth."