On July 19, scientists at the University of Puerto Rico at Arecibo’s Planetary Habitability Laboratory released a list of the top five potentially habitable exoplanets. Here's a brief rundown of the PHL's best candidates:
Gliese 581g, which is located just 20 light-years away, is likely two to three times as massive as Earth and zips around its parent star every 30 days or so. This orbit places the planet squarely in its star's "habitable zone" — that just-right range of distances where liquid water, and perhaps life as we know it, could exist.
The alien world is a so-called "super Earth" that's at least 4.5 times as massive as our planet, and it completes an orbit every 28 days. At least one other planet circles the star Gliese 667C, which is part of a triple-star system.
Kepler-22b is found about 600 light-years away, and it orbits a star very much like our own sun.
Astronomers announced the discovery of HD 85512b in September 2011. The planet's estimated surface temperature is 77 degrees Fahrenheit (25 degrees Celsius).
When 581d was first discovered in 2007, many scientists regarded it as too cold to be potentially habitable. In the years since, however, atmospheric-modeling studies have suggested that the planet may indeed be able to support life as we know it — provided 581d is warmed by a greenhouse effect.