Gliese 581 is an utterly maddening star, one that continues to tantalize us with potential habitability. The case of Gl 581g, examined here yesterday, is only the latest wrinkle, but it's in some ways the most frustrating. We're studying planets we cannot actually see, inferring their presence through the tiniest of variations in starlight caused by the planets' gravitational effect on their star. Adjust planetary orbits here in the direction of eccentricity and you can make Gl 581g disappear. Assume circular orbits and you can produce a habitable zone super-Earth. I think we'll still be arguing about this one for some time, but in the interim the question will lose a lot of its force. Remember, the reason we're so excited about Gl 581g is that it would become the closest planet with the possibility of liquid water at the surface (possibly joined by Gl 581d in the same system). But Jean Schneider's Exoplanet.eu catalog shows 777 confirmed planets this morning, and Kepler has pulled...

Not long ago, while making a presentation about possible destinations for an interstellar probe, I called Gl 581d the most likely candidate for habitability yet discovered among nearby stars. I knew the planet was problematic, perhaps too far on the outer edge of the habitable zone to be a realistic candidate, although this seems to depend on a variety of factors including atmospheric modeling. But what I had really been pondering in deciding whether or not to include Gl 581d in the talk was whether its purported sister world, Gl 581g, should be brought into play. Steven Vogt (UC-Santa Cruz) and colleagues were getting ready to distribute their new paper making a further case for a super-Earth in the habitable zone, one that seemed to be ideally placed for liquid water to exist on the surface. Bring that into the discussion? I decided against it, because the controversy over this world continues and Centauri Dreams seems a better venue than a short public talk to get into the...