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Goldilocks Zone
Life as we know it requires liquid water to evolve and survive. In order for liquid water to exist, a planet must orbit a star at such a distance that water neither freezes nor evaporates on its surface. Astronomers refer to this region, where temperatures are just right for life to evolve, as the Goldilocks zone. This zone is farther out from larger, hotter stars, and closer in to small, cool stars. Gliese 581, a red dwarf star, is about a third as massive and 1.3 percent as luminous as our sun. Gliese 581g orbits much closer to its star than Earth does to the sun.
Tidal Lock
Due to this close orbit, the gravitational pull of Gliese 581 slows down the rotation of the planet, synchronizing the days and years. That is to say, it takes Gliese 581g 37 Earth days to rotate once on its axis, and to make a full orbit around the star. This phenomenon, known as tidal lock, causes one side of the planet to perpetually face its sun, just as one side of our moon always faces Earth. The sunward side experiences extreme heat, while the dark side is cold and unlivable.
Twilight Zone
On such a world, the most likely climate for life is in the twilight band around the edge of the planet. Within this space, with the sun low on the horizon, a wide range of temperate climate zones may exist. The sun never rises or sets, but as you move east or west -- toward the light or dark side of the planet -- the temperatures rise and fall. This variation could potentially produce diverse forms of life.
Odds for Life
As of July 2011, astronomers know only that Gliese 581g is a rocky, terrestrial world on which liquid water could exist. Researchers have yet to determine if water does exist there. If it does, the odd for life are high. Life on Earth, after all, thrives anywhere there is water. Even if Gliese 581g proves to be lifeless, its discovery still gives seekers for extraterrestrial life a reason for optimism. It's only within the past decade that scientists developed technology capable of detecting such planets. The discovery of a Goldilocks world so soon implies either that Butler and Vogt were extremely lucky, or our galaxy is full of temperate, earthlike planets.