A CATALOGUE of habitable worlds is being compiled by scientists as soaring numbers of ''exoplanets'' are detected among the stars.
So far only two marginally Earth-like planets have been confirmed as fully matching the criteria for habitability.
But the Habitable Exoplanets Catalog (HEC) identifies more than 15 planets and 30 ''exomoons'' as potentially habitable.
Future studies with powerful new telescopes will be needed to confirm whether any of these candidates really could harbour life.
Astronomers have already established the existence of more than 700 exoplanets and thousands more are awaiting confirmation.
Most are hot gas giants such as Jupiter and Neptune orbiting close to their stars.
Only a few of those detected so far have the right size and orbit to be considered suitable for life.
The HEC is being compiled by the Planetary Habitability Laboratory (PHL) at the University of Puerto Rico in Arecibo, home of the world's largest single-dish radio telescope.
As well as identifying potentially life-bearing worlds, including ''exomoons'' like Pandora in the movie Avatar, it ranks them according to various indices of habitability.
''One important outcome of these rankings is the ability to compare exoplanets from best to worst candidates for life,'' said project leader Dr Abel Mendez, director of the PHL.
''New observations with ground and orbital observatories will discover thousands of exoplanets in the coming years. We expect that the analyses contained in our catalogue will help to identify, organise and compare the life potential of these discoveries.''
The two confirmed ''habitable'' exoplanets are Gliese 581d and HD 85512b.
Gliese 581d orbits a red dwarf star around 20 light years away and is one of a family of at least six planets.
The planet has about six times the mass of Earth and is just inside the parent star's habitable or ''Goldilocks'' zone, theorbital band where the temperature is ''just right'' to allow liquid surface water.
French researchers have run computer simulations which suggest the planet could have oceans as well as clouds and rainfall.
HD 85512b is about 36 light years away in the constellation of Vela and classed as a ''super-earth,'' having about 3.6 times the Earth's mass. It also lies within its star's habitable zone.
The catalogue uses habitability assessments such as the Earth Similarity Index (ESI), the Habitable Zones Distance (HZD) and Global Primary Habitability (GPH).
Planets are also compared with the Earth both in its current state and in the past.
Dr Jim Kasting, an expert on planetary habitability science from Pennsylvania State University in the US, said: ''I hope this database will help increase interest in building a big space-based telescope to observe exoplanets directly and look for possible signatures of life.''