As planet hunters catalog stellar wobbles and light-curves, some of them are working their way down through the various planetary types aiming at the ultimate discovery, an Earth-like world around another star. And if Lisa Kaltenegger has her way, they'll be able to tell us something about the existence of life on that planet. Kaltenegger faced a Washington DC audience yesterday to announce a new methodology for examining terrestrial worlds. Unable to be there myself, I attended via a much appreciated Webcast. Kaltenegger (Harvard-Smithsonian Center for Astrophysics) and Wesley Traub (JPL and CfA) are looking closely at the history of Earth's atmosphere to understand what happens in the various stages of planetary evolution. The development of life is one of many factors that changed the atmosphere in the past 4.5 billion years. When the day comes that we have spectroscopic data from exoplanets as small as Earth, we'll be able to study the signatures of the gases there to learn...

A Jupiter-sized planet with the density of cork? The idea seems farcical, but it's under discussion as I write at a news conference held by the Harvard-Smithsonian Center for Astrophysics (CfA). The planet, called HAT P-1, revolves around ADS 16402, a star much like our Sun that is part of a binary system some 450 light years away in the constellation Lacerta. The first planet found by the Hungarian Automated Telescope observing network, HAT P-1 may represent a new class of planet entirely. For despite a radius of 1.38 times Jupiter's, HAT P-1 has only half its mass. "This planet is about one-quarter the density of water," said Gaspar Bakos (CfA). "In other words, it's lighter than a giant ball of cork! Just like Saturn, it would float in a bathtub if you could find a tub big enough to hold it, but it would float almost three times higher." Intriguingly, the new world isn't the first planet with oddly low density. Another planet found by the transit method, HD 209458b, is also about...

Following hard on our discussion of HD 3651, a K-class dwarf whose brown dwarf companion was recently imaged, comes news that the Hubble Space Telescope has photographed something smaller still. CHRX 73 B orbits a low-mass red dwarf. Some would consider it a planet, others a brown dwarf; which camp you are in depends on what you use as a planetary marker. If it's mass, then this object, 12 times the mass of Jupiter, would probably be considered a planet. But team leader Kevin Luhman (Pennsylvania State) has other ideas. For if your marker is how the object formed, then a whole new set of criteria swim into view. Luhman argues that to be a planet, an object must have evolved from the gas and dust disk that circles a newly formed star. Whereas brown dwarfs are thought to form like any other star, from the collapse of huge clouds of hydrogen gas. They simply lack the mass to ignite hydrogen fusion in their cores. And Luhman seems to be on firm ground in making this distinction: "The...

Small telescopes doing amazing things. That's the theme of the day in exoplanet hunting, reinforced by the announcement of a new planet discovered by the Trans-Atlantic Exoplanet Survey (TrES). Small, automated equipment and off-the-shelf camera technology went into this work, which spotted the third transiting planet found with the kind of telescopes available to amateurs. "Hunting for planets with amateur equipment seemed crazy when we started the project," says David Charbonneau, an astronomer at the Harvard-Smithsonian Center for Astrophysics, "but with this discovery the approach has become mainstream." Image: A computer-generated simulation of TrES-2 crossing (transiting) the disk of its host star. TrES-2 transits farther from the disk center than any other known transiting planet. The transit of TrES-2 causes a drop in the brightness of its home star of about one and a half percent. This slight dimming of the star's light was noticed and measured by the TrES researchers, who...

Planetary migration -- as when a 'hot Jupiter' moves inward toward its parent star during system formation -- may not be as disruptive as we once thought. In fact, according to a new study led by the University of Colorado at Boulder and Pennsylvania State, solar systems with hot Jupiters may well harbor Earth-like planets covered with deep oceans. The research team's paper, just published in Science, paints a positively benign scenario, one in which the gas giant's migration actually becomes the trigger for the formation of water worlds that could well turn out to be habitable. Let's look at this more closely, because it's quite a shift from earlier studies, which assumed that a hot Jupiter's migrations would eject protoplanetary materials from the system or else absorb them. Working with computer simulations, the Colorado/Penn State researchers now think the hot Jupiters force rocky debris outward in the system, helping the formation of rocky planets. At the same time, and this is...

Putting a brown dwarf into the same stellar system with one or more known planets seems like a dicey proposition, but we know it sometimes occurs. Radial-velocity studies have already detected systems like HD 38529 and HD 168443 that include a brown dwarf and known planet. In both these cases, the brown dwarf involved is known to revolve around the exoplanet host star in an orbit at least ten times wider than that of the planet found there. But there are more unusual possibilities: A brown dwarf around the star HD 202206 actually moves inside the orbit of the known exoplanet there. Look no further if you need a new science fiction setting. Until now, we've never had a direct image of a brown dwarf around an exoplanet host star, but a new paper changes all that. A German team led by Markus Mugrauer (University of Jena) provides just such a detection, as shown in the image below. The star is HD 3651, a K-class dwarf near the boundary between Pegasus and Pisces some 11 parsecs from...

Among the 200-plus exoplanets discovered thus far, the system around the red dwarf Gl 876 stands out. For one thing, it contains the closest thing to a terrestrial-sized planet yet found, with a mass of about six times that of Earth and a tight, two-day orbit around its primary. For another, it houses two gas giants, the only planets of this type known to orbit an M-dwarf, and they exhibit a 2:1 mean-motion resonance -- one of them orbits the star twice in the same amount of time the other makes a single orbit. Such resonances provide clues in the study of how systems like these formed and changed over time. These gas giants, one in a 30-day orbit, the other in a 60, are fascinating in their own right, but the detection of the small inner planet by Doppler techniques shows just how far planetary detection methods have come in the past decade. Now a paper slated for publication in The Astrophysical Journal looks at Gl 876 in terms of planetary transits, for to arrive at the true mass...

Asteroseismologists -- the people who study the oscillations of stars to examine their internal structure -- can tell us a good deal about Alpha Centauri, and specifically about Centauri B, a K-class star which has also been carefully measured with the techniques of long-baseline interferometry. But a problem arises when you compare their mass estimates of the star with radial velocity studies, for the mass estimates of each differ by 28 Jupiter masses, plus or minus 9. Is the discrepancy a data analysis error or an indication of something more interesting? Pondering whether it might show the existence of a companion for Centauri B, a French team has searched for the presence of such an object and, in the process, has built a catalog of background objects detectable with adaptive optics, using the European Southern Observatory's Paranal site in Chile. Image: They look like one huge star in this photograph, but Centauri A and B simply overwhelm the view with their glare. Remember,...

What sort of stars harbor the planetary systems we've thus far identified? The answer is easy: most of the known exoplanets were found through radial velocity surveys, and these focus on nearby Sun-like stars. Thus we're looking at a range of stars between late-F and early-K class dwarfs, and almost all are within 50 parsecs of the Sun. It is also apparent that planetary systems in our scope of observation increase with increasing metallicity of the parent star, a measure of elements higher than hydrogen and helium. Are there other trends we can identify? Perhaps not. As I. Neill Reid (Space Telescope Science Institute) writes in a new paper on the subject, "With the possible exception of a higher mean velocity perpendicular to the Plane, the planetary hosts appear to be unremarkable members of the Galactic Disk." There is not, for example, a correlation we might expect to find in metal-rich stars between the mass of the primary star and the masses of its planetary companions...

Hunting for exoplanets isn't a matter of peering into telescopes and seeing faint specks of light. It's all about combing through data -- reams and reams of data thankfully digitized -- for the telltale signatures of planets. And it's fascinating to reflect that in many cases the signatures we seek are in our possession in the form of already gathered radial velocity data. We must continue to re-examine our growing stellar libraries, which in the case of radial velocities only get richer over time as planetary influences become more pronounced. And so we come to Mu Arae, a G-type dwarf star much like our Sun and catalogued as HD 160691. A new study by Krzysztof Gozdziewski, Andrzej Maciejewski, and Cezary Migaszewski re-examines this already intriguing planetary system to discover yet another planet, the fourth to be found there. These radial velocity measurements were made by the Anglo-Australian Planet Search project and build on the earlier detections of three worlds, two being...