Scientists Rank Livable Worlds Saturn moon, exoplanet top lists By Matt Cantor, Newser User Posted Nov 24, 2011 8:57 AM CST 15 comments Comments This undated NASA handout shows Saturn's moon, Titan, in ultraviolet and infrared wavelengths. (Getty Images) (Newser) – Looking for extraterrestrials? You might want to start with Saturn’s moon Titan. Scientists have ranked it the alien world most capable of supporting life, according to a “Planetary Habitability Index,” the BBC reports. The index takes into account the planet or moon’s surface, the presence of an atmosphere, and the planet’s usable energy; both Mars and Jupiter’s moon Europa were runners-up. Researchers also ranked planets’ life-sustaining abilities according to an “Earth Similarity Index.” That scale measured planets based on “Earth-like conditions,” says a scientist, including size and density. Topping that list was Gliese 581g, an exoplanet believed to be in the Libra constellation. Within our own solar system, Mars was rated most Earth-like, with Mercury coming in second. This weekend, scientists will launch NASA's biggest-ever Mars rover to get a closer look at the planet's habitability.