Gliese 581 continues to occupy the attention, and understandably so. At least three planets orbit this M-dwarf, one of which sprang into the public consciousness with the announcement that it might be in its star's habitable zone. But both Gl 581c and d are interesting from the habitability standpoint, even if subsequent discussions have pointed out just how problematic it is to make such judgments on insufficient data. Ponder how tricky the call can be. For being in a circumstellar habitable zone only means that a terrestrial-size planet can have liquid water on its surface. A new paper by Franck Selsis (Centre de Recherche Astrophysique de Lyon), James Kasting (Pennsylvania State) and colleagues wades right into this morass, pointing out how many other factors could make such a planet remain uninhabitable: Water may not be available A high impact rate may prevent the emergence of life The thus far unknown minimum ingredients for life's formation may not be present Gravity may be...

One of the surprises of the early planet-hunting era has been the discovery of 'hot Jupiters,' giant planets orbiting extremely close to their parent star. That these planets should be prolific in our catalog at present makes sense given the nature (and limitations) of radial velocity detection methods, but before we started finding them, there seemed little reason to believe gas giants would exist at orbits within 0.1 AU. Now we see them as evidence that protoplanets can migrate during the formation period, probably causing havoc as they pass through the inner system. Are hot Jupiters the bane of terrestrial planets? You would think so, given the above scenario, with a gas giant clearing planet-forming materials out of the inner disk during its passage. But Martyn Fogg and Richard Nelson (University of London) think otherwise. Their new paper looks at models of terrestrial planet formation and finds that inner disks survive the passage of the inbound giant, resuming their planetary...

Looking through the list of candidate missions selected by the European Space Agency recently, my attention was immediately drawn to PLATO, a planet-finder spacecraft designed to study transiting exoplanets and to measure the seismic oscillations of the stars they orbit. Although at first reminiscent of COROT, PLATO (Planetary Transits and Oscillations of Stars) is really more like an enhanced version of NASA's upcoming Kepler mission, as I'm reminded by Centauri Dreams regular Vincenzo Liguori, who passed along helpful background information. One immediate difference turns out to be field-of-view, which in PLATO is wide indeed due to the observation strategy involved. Unlike COROT or Kepler, PLATO would put photometric techniques to work in the study of relatively bright stars -- 100,000 of these, with another 400,000 studied down to 14th magnitude. The earlier mission concepts are aimed at surveying fainter and more distant stars in a smaller field. Note the significance of this:...

Alpha Centauri A and B have a mean separation of 23 AU. In Solar System terms, that gives you a spacing a bit further from the Sun than the orbit of Uranus. But with the two stars moving around a common center of mass, the distance between them varies over time. Centauri B is sometimes as far from Centauri A as Pluto is from Sol, while at other times it closes as close as Saturn. From a planet around Centauri B, Centauri A would sometimes shine with the light of 5000 full moons, creating day and night sky scenarios that would be, to say the least, striking. Recent research is making it clear that planets can form in such systems, but binaries are tricky, and we still have much to learn about how such planets would form and where, and under what conditions certain kinds of objects are more likely to occur. Twenty percent of the exoplanet systems thus far found are binary, with the majority of these being wide binaries (separated by 250 to 6500 AU, a far cry from our Centauri stars)....