A new index scores planetary bodies on their suitability for life: Two decades ago astronomers suspected that planets might orbit other stars, but no one had ever seen one. These days hundreds are known. In a (subscription-only) paper published in the journal Astrobiology researchers, led by Dirk Schulze-Makuch of Washington State University, have come up with an index that aims to describe just how friendly to life such exoplanets might be. Tipping its hat to the possibility that aliens could have dramatically different biochemistry from earthlings, the index confines itself to measuring big-picture factors such as the presence of a solid surface, the average surface temperature, the strength of a planet’s magnetosphere (which helps shield it from cosmic radiation) and the like. Unsurprisingly, Earth comes top of the list. Interestingly, though, Titan, a Saturnian moon covered in hydrocarbon lakes, takes the second spot in our solar system, ahead of Mars. There is still some doubt about whether Gliese 581g, the highest-scoring exoplanet, actually exists; but the existence of its companion world Gliese 581d, which scores nearly as highly, is uncontroversial. Sadly we won’t be visiting any time soon—the Gliese-581 system is around 20 light-years from Earth, in the constellation of Libra.