Saturn’s moon Titan and the exoplanet Gliese 581g - thought to reside some 20.5 light-years away in the constellation Libra – are among the most habitable alien worlds, according to scientists.
The researchers propose two different indices: an Earth Similarity Index (ESI) and a Planetary Habitability Index (PHI), to assess the probability of hosting alien life.
“The first question is whether Earth-like conditions can be found on other worlds, since we know empirically that those conditions could harbour life,” the BBC quoted co-author Dr Dirk Schulze-Makuch from Washington State University, US, as saying.
“The second question is whether conditions exist on exoplanets that suggest the possibility of other forms of life, whether known to us or not,” he added.
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.
The Gliese 581 system has been well studied by astronomers and comprises four - possibly five - planets orbiting a red dwarf star.
HD 69830 d, a Neptune-sized exoplanet orbiting a different star in the constellation Puppis, also scored highly (0.60).
It is thought to lie in the so-called Goldilocks Zone - the region around its parent star where surface temperatures are neither too hot nor too cold for life.
The highly rated worlds from our own solar system were Mars, with a value of 0.70, and Mercury, with 0.60.
The Planet Habitability Index produced different results. The top finisher here was Saturn’s moon Titan, which scored 0.64, followed by Mars (0.59) and Jupiter’s moon Europa (0.47), which is thought to host a subsurface water ocean heated by tidal flexing.
The highest scoring exoplanets were, again Gliese 581g (0.49) and Gliese 581d (0.43).
The study has been published in the journal Astrobiology.