Want to know which moons and planets have the highest probability of hosting extra-terrestrial life?

An international team has outlined a list of the most habitable alien words, using two rating systems: the Earth Similarity Index and the Planetary Habitability Index, the BBC reports.

For the ESI, the team rated the planets and moons on how similar they are to Earth, taking into account such factors as size, density and distance from the parent star.

The PHI, on the other hand, looks at whether the world has a rocky or frozen surface, whether it has an atmosphere or a magnetic field. In addition, scientists look at the energy available to any organisms, either through light from a parent star or via a process called tidal flexing, in which gravitational interactions with another object can heat a planet or moon internally.

Lastly, the PHI takes into account chemistry - such as whether organic compounds are present - and whether liquid solvents might be available for vital chemical reactions.

"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. "The second question is whether conditions exist on exoplanets that suggest the possibility of other forms of life, whether known to us or not."

Using the ESI, the team gave the following ranking of most livable moons and planets:
• Earth - 1.00
• Gliese 581g - 0.89
• Gliese 581d - 0.74
• Gliese 581c - 0.70
• Mars - 0.70
• Mercury - 0.60
• HD 69830 d - 0.60
• 55 Cnc c - 0.56
• Moon - 0.56
• Gliese 581e - 0.53

As expected, Earth got the maximum value of 1.00. 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 PHI list of most habitable planets include:
• Earth - 0.96
• Titan - 0.64
• Mars - 0.59
• Europa - 0.49
• Gliese 581g - 0.45
• Gliese 581d - 0.43
• Gliese 581c - 0.41
• Jupiter - 0.37
• Saturn - 0.37
• Venus - 0.37

The PHI results showed that 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.
For exoplanets, the highest scoring were Gliese 581g (0.49) and Gliese 581d (0.43).

More moons and planets are expected to be discovered with the search for potentially habitable planets outside our solar system has stepped up several gears.