As if we didn't already have enough trouble defining what a planet is, astronomers have now discovered a brown dwarf only eight times the mass of Jupiter. Surrounded by a dusty disk, the object is actually smaller than a number of planets already known to be orbiting other stars. Any miniature solar system that formed around the brown dwarf would be roughly 100 times smaller than our own. All of which raises the question of what to call objects that might be found around this tiny dwarf: planets or moons? The question has obvious resonance in an era marked by repeated discoveries in the Kuiper Belt that could be considered of planetary size. And another sign of the ambiguity in definition is that worlds like Titan, Ganymede and Callisto are large enough in their own right to qualify as planets, if we overlook the inconvenient fact that they orbit massive planets of their own. The question may seem insignificant, but how we define things is ultimately a measure of how extensive our...

M-class red dwarfs have never figured prominently in the SETI search. The reason for this is apparent: such stars, of which Proxima Centauri, Earth's nearest stellar neighbor, is one, are flare stars. The intense radiation from solar flares should cleanse a planetary surface of life, especially given the close proximity of such a planet to its star. Remember, the habitable zone around a red dwarf is going to occur well inside the orbit of Mercury. And there's a second reason. By virtue of having to orbit the host star so tightly, a planet around a red dwarf is going to be tidally locked. One side would be baked, the other frozen, which makes the odds on liquid water look slim. But assumptions are made to be questioned, which is why work at Ames Research Center in the late 1990s remains so interesting. One implication of the Ames work, for example, is that there are conceivable weather patterns that could circulate heat to the dark side of a tidally locked world, keeping it warm...

If you're going to have a conference on exoplanets, there is no better venue than l'Observatoire de Haute-Provence. It was here, just ten years ago, that Mayor and Queloz discovered the first planet orbiting a main sequence star outside our own Solar System. The star was 51 Pegasi, a name that will surely be recalled for generations as the first confirmation that planets exist around other stars. And attendees at a late August conference celebrating the discovery had much to say about the course of future exoplanetary developments. David Charbonneau (Harvard-Smithsonian Center for Astrophysics) summarizes the conference findings in his paper "Hot Jupiters: Lands of Plenty," now available on the arXiv site. A major issue stands out: the huge leap in precision for radial velocity observations of the sort that bagged 51 Pegasi's planet, allowing researchers to monitor a wider group of stars than the F, G, K and early M-class dwarfs that have been the focus heretofore. The new precision...

M-class red dwarf stars are of increasing interest in terms of astrobiology. If we can devise weather models that allow for regions of relative stability, a planet locked tidally to its star at a fraction of the distance from Mercury to the Sun could produce conditions suitable for life. The National Geographic show 'Extraterrestrial,' shown again the other night, projects just such an environment, and imagines life forms that might evolve there. But red dwarfs are tricky because they're flare stars. In their early lives, they spin more quickly than they will when they enter their dotage; the rapid spin can produce magnetic fields that, in turn, create flares. Life on a planet circling a younger red dwarf would have to adapt to flares that can double the star's brightness within a matter of seconds. Some believe this makes Proxima Centauri an unlikely candidate for life-bearing planets. And what about Barnard's Star, so tantalizingly close (5.9 light years) to our own Sun? Study a...