Astronomers have found more than 700 planets beyond our solar system, and thousands more await confirmation by follow-up observations. Many of these alien worlds are too hot or too cold to support life as we know it, but researchers have found a few that appear to be much more hospitable.
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:
1. Gliese 581g
This rocky world was announced in September 2010 and has been controversial ever since, with some researchers casting doubt on its existence and its discoverers remaining firmly behind their find.
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.
2. Gliese 667Cc
Gliese 667Cc, which was discovered in February 2012 by the same core team that spotted Gliese 581g, orbits a red dwarf 22 light-years away, in the constellation Scorpius (The Scorpion).
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 was spotted by NASA's planet-hunting Kepler space telescope in December 2011. It's a super Earth about 2.4 times as wide as our planet. If the greenhouse effect operates on Kepler-22b like it does on Earth, the alien world would have an average surface temperature of 72 degrees Fahrenheit (22 degrees Celsius), researchers have said.
Kepler-22b is found about 600 light-years away, and it orbits a star very much like our own sun.
4. HD 85512b
HD 85512b is another super Earth, one that's thought to be 3.6 times as massive as our planet. The alien world is found about 35 light-years from us, in the direction of the constellation Vela (The Sail).
Astronomers announced the discovery of HD 85512b in September 2011. The planet's estimated surface temperature is 77 degrees Fahrenheit (25 degrees Celsius).
5. Gliese 581d
This world, which is about seven times as massive as Earth, orbits a bit farther out than its planetary sibling Gliese 581g.
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.