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...

Just what might we find on Gliese 581 c, the potentially habitable planet announced yesterday? Much depends on where the planet formed in its circumstellar system. For that kind of information I listen to Greg Laughlin (University of California at Santa Cruz), whose work on planetary formation via core accretion seems to gain stature with every new planetary find. Here's Laughlin's take from his systemic weblog: The planet probably migrated inward to its current location from beyond the “snowline” in GL 581’s protostellar disk, and so its composition likely includes a deep ocean, probably containing more than an Earth’s mass worth of water. Atmospheric water vapor is an excellent greenhouse gas, so the conditions at the planet’s atmosphere-ocean boundary are probably pretty steamy. It’s also possible, however, that the planet formed more or less in-situ. If this is the case, it would be made from iron and silicates and would be fairly dry. It’s unlikely, but not outside the realm of...

This is a big one, and it happens several years earlier than I had expected. A planet of about five times Earth mass, one whose radius is only 1.5 times that of our own world. Moreover, a planet that's smack in the middle of its star's habitable zone, with a mean temperature estimated at between 0 and 40 degrees Celsius. The models in question say that this is a rocky world, and its temperatures tell us that oceans could exist there. The first detection of a planet where carbon-based life could conceivably exist makes this one a find for the history books. The star is Gliese 581, already known to be home to a planet of Neptune mass and a possible third world about eight times as massive as Earth. It's an M-class red dwarf, far smaller and cooler than the Sun. The new planet, the smallest found up to this point, orbits it in 13 days. Gliese 581, it should be noted, is comparatively close to our own Solar System, about 20.5 light years away in the constellation Libra. Radial velocity...

Investigating brown dwarfs is not for the faint of heart. With a mass below what's needed to sustain hydrogen-burning fusion, they're hard to see, and they may be far more numerous than we've previously estimated. Nor do we have a good handle on how they function, to judge by a new study showing that these objects can possess powerful magnetic fields. It turns out that the 'failed star' designation may be a bit inapt. Sure, they're dim objects but in recent years, it has been discovered that some brown dwarfs put out beams of radio waves that are thousands of times brighter than any detected from our Sun. Brown dwarfs, in other words, are actually pretty lively. If you think about other sources of flashing radio signals, pulsars come immediately to mind. Their powerful magnetic fields and incredibly fast rates of rotation make figuring out how they produce their signals a difficult matter. Now we find brown dwarfs emerging as a second class of stellar objects that produce such...