Potentially Habitable Planets

Over 700 planets have been discovered outside our solar system, but very few exist inside the habitable zone. A handful have been discovered that are in the "just right" temperature range where it is possible for liquid water to exist. This image shows 6 exoplanets ranked according to their similarity to the Earth, with Mars shown for comparison.

6. Gliese 581d is likely a rocky planet with a mass about 7x that of the Earth, orbiting the red dwarf Gliese 581 that lies only 20 light years away. When Gliese 581d was first discovered, it was determined to be outside the habitable zone. It is tidally locked (one side is always facing the star it orbits) and receives only 1/3 of the stellar energy that the Earth does. However, atmospheric modeling studies revealed that this exoplanet may possess a dense CO2 atmosphere that would generate enough of a greenhouse effect to support liquid water.
Source: http:///jStGUA

5. Gliese 163c lies about 50 light years from earth in the Dorado constellation. Like #6 it lies just on the edge of its parent star's habitable zone, and also weighs in at about 7 times the mass of the Earth. Researchers do not know for sure if it is a terrestrial planet or a small gas giant. There is a 2% chance that Gliese 163c will pass across the face of its parent star, which will allow scientists to conduct more in-depth studies on its nature.
Source: http:///SEgiaO

4. With an orbital period of 54 days and a mass about 3.6x that of the Earth, HD 85512b orbits the star HD 85512 about 35 light-years from Earth in the direction of the constellation Vela (the Sail). Further studies are needed to determine the structure and atmospheric composition of this world, but researchers estimate an agreeable temperature of 25 degrees Celsius at the top of its atmosphere (note: temperatures nearer the surface would be much different).
Source: http:///r18CK6

3. Discovered by the planet hunting Kepler space telescope, Kepler 22b orbits a star slightly smaller and cooler than our own sun about 600 light years away. It is 2.4 times the radius of the Earth, but the planet's composition is unknown. If Kepler 22b possesses a similar greenhouse effect to Earth's, its surface temperature would be about 22 degrees Celsius.
Source: http://1.usa.gov/u6RCcn

2. Gliese 667 is a triple-star system that lies 22 light-years in the constellation of Scorpius. Gliese 667C-c orbits the smallest star in this system at a distance of only 0.12 AU (1 AU = distance between Earth and Sun). However, since Gliese 667C's mass is only 31% that of our Sun's, it is likely that the temperature on Gliese 667C-c could be favorable for supporting life. The planet's thermal conditions are not affected by the other two stars in the system.
Source: http:///zdvEwU

1. Though its very existence is a matter of debate, Gliese 581g might possibly be the best candidate for an Earth-like environment. If it does indeed exist, the exoplanet is about 3 to 4 times the mass of the Earth and, unlike Gliese 581d above, lies smack in the middle of the habitable zone. It orbits its parent star once every 37 days and its mass indicates that it is likely a rocky planet.
Source: http://1.usa.gov/ayEj62

It's important to note the uncertainty surrounding conditions on these planets--in some cases, certain characteristics are yet to be determined through observation, and because Earth is the only life-sustaining planet that we know, "habitability" can only be extrapolated from our home planet's environment. Though these exoplanets could possibly play host to microbial life forms or even future human colonists, further research is necessary to understand their respective characteristics as well as the mechanisms necessary to support life.

-RLO - The Universe

Image credit: PHL @ UPR Arecibo