We’d all like Gliese 581 c to be as Earth-like as possible, but not everyone puts high odds on the planet being even potentially habitable. In an e-mail discussion circulating among space professionals, Gerald Nordley took issue with the ‘terrestrial world’ concept and pointed out how the results of Stephane Udry and the Geneva exoplanet team shouldn’t be taken too far. Nordley, a retired Air Force astronautical engineer, is a familiar name to those who follow interstellar studies from his work in the Journal of the British Interplanetary Society as well as his essays in venues like Analog. He is also the author of numerous science fiction stories.

Here are Nordley’s comments, reprinted with permission:

Udry et al., make a good case for a planet being there, but the rest looks speculative at best. The planet has a minimum mass of 5 Earths, the “1.5 Earth radius” is based on a density assumption with no data behind it, and the planet’s insolation is about 2.44 times the Earth’s (L/a2 = 0.013/.0732). The effective temperatures calculated didn’t reference any atmosphere model. A similar calculation for Earth gets you about 256K (-17C), depending on albedo. They used a Venus-like albedo to get down to 273K — actually not bad for the Venusian upper atmosphere. Of course, we all know what the surface of Venus is like.

If an awful lot of things break the right way, well, maybe a terrestrial planet. But in my crystal ball G 581c is a rather hot mini-Uranus.

The next planet out has an insolation of 20% Earth’s. If it (big if!) were of similar density to the Earth, it would have a surface radius and gravity roughly twice as high as high as Earth’s. And even if the top of the atmosphere were much colder, if it were a few bars deep, the lapse rate would produce a liquid water surface.

Nordley’s thoughts come at a time when Greg Laughlin (UC-Santa Cruz) has pegged the odds on Gliese 581 c harboring “a clement surface or a temperate ocean-atmospheric interface” at a thousand to one against. Which is not to downplay the significance of the Gliese 581 c discovery, but only to point out that there is a wide gap between the actual facts we have on this planet and the speculation they have provoked. We can learn more through continuing observations — and we can’t rule out the possibility of a transit, which would help immensly.

So just what is the significance of Gliese 581 c? Whether or not it turns out to be habitable, this planet represents the first time we’ve ever looked at a world where all the pieces could fall into place. And the new planet should energize further investigation, quickening the pulse not only of those already involved in the search, but in a public that shows signs of becoming excited again about deep space exploration. That makes the outlook for the next few years in exoplanet studies more promising than ever, because Gliese 581 c is only the first of many terrestrial world candidates to come.