Another “Earth” here? I think astronomers might be jumping the gun a little bit. I heard a bit of the discussion on Morning Edition today, but when I got to my favorite space sites to check things out, I was a tad disappointed.
SpaceRef.com gives a bit more information. This planet is identified as the smallest yet discovered outside the solar system. Though earth-hunters would like this to be so, they overlook those three pulsar planets that rained on the opening day of the exoplanet parade back in 1990. Some astronomers have exhibited an almost pouty attitude toward the clearly dead planets orbiting the clearly dead neutron star PSR B1257+12. All three are smaller than Gliese 581’s newly discovered companion–that’s not a fact that radiation storms from a neutron star have been able to blow away.
This is not to say that refining planet detection doesn’t continue to be a stellar achievement as progressive size barriers of Jupiter, Saturn, Neptune, and now “super-Earth” are broken.
Also, finding a planet within a star’s habitable zone is indeed an important achievement. Gliese 581 is the right distance away so that water will not boil away (such as on the planets we’ve found with iron and silicates in the atmosphere) or freeze into a snowball (pick any outer moon of our own system but Io).
But let’s review what we know:
1. This new planet is five times as massive as the Earth and about 50% larger in diameter. We’re getting closer, but this isn’t quite earth-sized yet.
2. Red dwarfs, though very common stars in the universe, have a nasty little habit of regularly blowing their stack. Ordinarily, a planet might be safe tucked under the ruddy red glow of a star signficantly less bright than the sun. But flares from such stars can be detected from Earth, and would be frequent enough and powerful enough to make life unpleasant for Earth settlers attempting a foothold there. Could life develop under such conditions? It’s certainly possible, but given that these flares would be more frequent and potentially more dangerous than asteroid or comet impacts the Earth has suffered, I’m on the doubters’ bandwagon on this one. To me, little red stars look more like a wolf in grandma’s clothing than an innocent little girl in a riding hood.
3. In order to get close enough to a red dwarf’s habitable zone, a planet would most likely be tidally locked, as all of the near moons of giant planets are. This means one hemisphere facing the star, the other facing away. The scientists are right that the average temperature of a planet orbiting seven million miles from Gliese 581 is 0 to 40 degrees Celsius. It’s far more likely that if water is present on this world, it is contained in an ice field covering the night side of the planet. Flares have probably cooked the other side pretty well over the eons. Unless of course the planet has retained a thick atmosphere to balance the extremes of tidal lock.
4. Paul Gilster at Centauri Dreams is more optimistic and he has this post and this one with the basic news plus some other embedded links that talk up the bright side of this find.
Color me a cynic on this one, though it would surely be a fascinating system to study the planets from a geological view.
The European Southern Observatory provided this artist’s eye view, which includes Gliese 581’s Neptune-sized planets in the distance:
Catholic Sensibility 
These are all good points, and we should all remember that what we really have boils down to a planet of a certain minimum mass with a known orbital period inside a star’s habitable zone. After that, we speculate. Of the issues you mention here, Todd, I think the flares are the most significant, with many M stars exhibiting flare activity that life would have to adapt to. I’m less concerned about the tidal locking, because there have been some pretty convincing analyses of the situation that suggest atmospheric circulation could keep atmospheric gases from freezing out on the dark side while maintaining a healthy area on at least part of the sun side. Anyway, this was a good post and I think you’re right that we can’t get too carried away until we have more evidence. But at least we’ve got one planet in a habitable zone, and to me that’s the most significant thing.