If the planet has a rocky composition like Earth’s, it would be 1.2 to 1.4 times as wide as our own planet, qualifying it as a “super-Earth.” Even more intriguingly, the brightness of the star (much dimmer than our own sun) and Gliese 581g’s orbital distance (0.146 AU, less than half the distance between Mercury and our sun) suggest that the planet’s average surface temperature is between 10 degrees and minus-24 degrees Fahrenheit (-12 to -31 degrees Celsius).
That means Gliese 581g is right in the middle of a planetary zone that is, in the words of the Goldilocks tale, “not too hot and not too cold, but just right” for water to exist in liquid form.
It was predicted only a few weeks ago that an exoplanet would likely be discovered that was about earth sized and orbiting its star in the habitable zone by May 2011. So this announcement seems to fulfill that prediction.
The researchers suspect that Gliese 581g is tidally locked, with one side perpetually facing the star and the other side perpetually turned away. In that case, temperatures at the heart of the day side or the night side might be too hot for life as we know it, but there could be a livable zone around the line between shadow and light.
“Any emerging life forms would have a wide range of stable climates to choose from and to evolve around, depending on their longitude,” Vogt said.