Under Schulze-Makuch’s criteria, a faraway world racks up points if it has a solid surface and an atmosphere, which act together to support chemical reactions and deflect damaging radiation. Liquid water is not a prerequisite for a high score: A planet with liquids on the surface receives more points than a dry world, but the presence of water confers no additional advantage. “If you didn’t know that water worked on Earth,” Schulze-Makuch says, “you might think methanol would work much better for life.”
The PHI scores of bodies within the solar system reflect Schulze-Makuch’s hypothesis that the most Earth-like places are not necessarily the friendliest for life. Earth gets a near-perfect score of 0.96 on the 0 to 1 scale (it just has less available energy now than it did when life originated 4 billion years ago). But second place goes to Saturn’s moon Titan (0.64), which hosts vast lakes of liquid hydrocarbons but has surface temperatures of –300 degrees Fahrenheit. Mars, the target of more than a dozen robotic missions to hunt for signs of microbial life, comes in third at 0.59.
None of the planets yet found outside our solar system score particularly well. Gliese 581d, a rocky world nestling a cool, dim star, nets a rating of 0.43. Kepler-22b, the most Earth-like planet NASA’s Kepler space telescope has found so far, gets a similar score. However, Schulze-Makuch emphasizes that the numbers are subject to change. Astronomers have been able to determine the surface and atmospheric composition of only a few exoplanets, so for most planets the data are incomplete. Future telescopes that are powerful enough to probe these worlds, such as NASA’s proposed Terrestrial Planet Finder, should make the PHI much more useful.