By Helen Briggs
BBC News Online science staff
Astronomers estimate about half the planetary systems so far discovered in our galaxy could contain Earth-like worlds.
More than 100 gas giant planets have recently been discovered
And they say that space telescopes will be capable of observing these planets and investigating them to see if they support life in about 15 years' time.
Scientists have recently discovered more than 100 stars other than our Sun with planets circling about them.
But they are all giant planets like Jupiter that cannot sustain life.
Planets more like the Earth should, in theory, exist too. But they are too small to be seen using current technology.
Research work by the UK's Open University suggests there are perhaps 50 or so of these small, rocky bodies on which there is liquid water and possibly life.
"We would certainly expect them to be something like Earth in size and in mass, to have a reasonable atmosphere; they'll have oceans and continents, they'll be potential abodes of life, but the big question is - has there actually been life there?" OU astronomer Professor Barrie Jones told BBC News Online.
His team used computer modelling to calculate the likely number of habitable planets, based on what we know about how planets form and the conditions needed for life.
The planets would exist in what is sometimes referred to as the "Goldilocks" zone, a region set back from the parent star where it is neither too hot for liquid water, nor too cold.
By launching "Earths" into a variety of orbits in this zone and following their progress with the computer model, the small planets have been found to suffer a variety of fates.
In some systems, the proximity of one or more Jupiter-like planets results in gravitational ejection of the "Earth" from anywhere in the habitable zone.
However, in other cases there are safe havens in parts of the Goldilocks zone, and in the remainder the entire zone is a safe haven.
Nine of the known exoplanetary systems have been investigated in detail using this technique, enabling the team to derive the basic rules that determine the habitability of the remaining 90 or so systems.
At the moment, it is just a theory, but by the middle of the next decade, we should have the technology to search directly for Earth-like planets.
The new generation of space telescopes will look for gases given off by living things in the atmospheres of distant worlds.
Finding chemicals such as carbon dioxide, water and ozone would be intriguing evidence of life.
But even if those chemical signs were detected, it would not be possible to send a space mission, Professor Jones said.
"It's likely the nearest Earth-type world in the habitable zone would be a few tens of light-years away, maybe 100 light-years away; that's being optimistic," he told BBC News Online.
"There's not much prospect of travelling to these worlds; all we can do is rely on instruments orbiting somewhere in the solar system and making observations.
"I think we could be in the rather frustrating position of having indirect evidence of life through the nature of the atmosphere but not being absolutely certain and not having any prospect of becoming absolutely certain in the foreseeable future."
Professor Jones gave details of the modelling work at the UK's National Astronomy Meeting in Milton Keynes.
Its results are more optimistic than a similar study led by Dr Kristen Menou, of Princeton University, US, last year which found perhaps less than a quarter of the known extrasolar planetary systems might contain Earth-like worlds.