Scientists rank 'most liveable' alien worlds
Scientists have outlined which moons and planets are most likely to harbour extra-terrestrial life.
Among the most habitable alien worlds were Saturn's moon Titan and the exoplanet Gliese 581g - thought to reside some 20.5 light-years away in the constellation Libra.
The international team devised two rating systems to assess the probability of hosting alien life.
They have published their results in the journal Astrobiology.
In their paper, the authors propose two different indices: an Earth Similarity Index (ESI) and a Planetary Habitability Index (PHI).
"The first question is whether Earth-like conditions can be found on other worlds, since we know empirically that those conditions could harbour life," said co-author Dr Dirk Schulze-Makuch from Washington State University, US.
"The second question is whether conditions exist on exoplanets that suggest the possibility of other forms of life, whether known to us or not."
The maximum value for the Earth Similarity Index was 1.00 - for Earth, unsurprisingly. The highest scores beyond our solar system were for Gliese 581g (whose existence is doubted by some astronomers), with 0.89, and another exoplanet orbiting the same star - Gliese 581d, with an ESI value of 0.74.