What kind of stars are most likely to have planets? Narrowing the search is crucial if the goal is to build a target list for space-based missions, especially when we're looking for terrestrial worlds. So learning that planet-bearing stars have higher metal contents -- the elements above hydrogen and helium, presumably as a relic of their protoplanetary disks -- would winnow the target list nicely, at least among Sun-type stars. And most of the extrasolar planets found thus far have been in orbit around stars of the spectral types F, G and K (our Sun is a G-class star). For various reasons, these are the usual targets for radial-velocity surveys, and they're also stars that can be readily analyzed for metallicity. But M dwarfs are the most common stellar type. We need to go to work on their parameters too, especially in the case of dwarfs that have known planetary companions. A stride in that direction is taken by Jacob Bean (University of Texas), Fritz Benedict and Michael Endl...

The COROT mission, to be launched in December, promises to move us to the next level of planetary detection. Devoted to studying exoplanet transits, in which a planet crosses the face of its star as seen from Earth, the space telescope will probably detect numerous 'hot Jupiters.' But an even more interesting possibility is rocky worlds in close orbit around their stars. And the thinking is that planets only a few times larger than Earth -- and perhaps even smaller than that -- will be within its reach. Any star with a transiting planet will provide a telltale drop in light that, depending on the size of the planet and the distance of the star, may be measurable. But COROT (the acronym stands for Convection Rotation and planetary Transits) is most sensitive to rocky worlds in orbits of 50 days or less. If that sounds dismaying in terms of habitability, consider that a planet in such an orbit around a dim red dwarf could be located ideally within the star's habitable zone. And red...

How could you possibly study the interior of a giant planet orbiting another star? Especially when that planet is so drowned in its star's light that we can't even see it? Various methods suggest themselves, including transits, those cases wherein the exoplanet happens to pass between us and the star it circles. A transit gives you the chance to measure both mass and size. Throw in inferences based on slowly evolving planetary models and you can draw some tentative conclusions. You also wind up with even more questions. And as Tristan Guillot would probably point out, we now have twenty gas giants whose mass and size can be determined, including those within our own Solar System. Guillot, who works in one of the most celestially beautiful places on Earth (he's at the Observatoire de la Cote d'Azur in Nice), makes it his business to compare and contrast what we see around Sol with the rising number of giant planets we're finding around other stars using the transit method. Several...

Watching the snowline descend to ever lower elevations as fall deepens into winter is one of the great pleasures of the Canadian Rockies, an area better suited to train travel than any on Earth. And an image of snow-topped mountains in Alberta came back to me as soon as I read about another kind of snowline, the boundary between the inner regions of a solar system, where rocky planets tend to form, and the outer depths, which become the domain of cold, gaseous worlds. The snowline holds clues to how 'super-Earth' planets form. The paper, by Grant Kennedy (Mt. Stromlo Observatory, Australia) and colleagues, takes a hard look at M-class red dwarfs and contrasts them to solar-type stars. The latter show a relatively constant luminosity during planet formation, meaning conditions change little during this era. But red dwarfs fade dramatically as they evolve toward maturity, dimming to the point where what had been a warm inner disk begins to freeze. And that has implications for...

Imagine being able to measure day and night temperatures on a planet circling another star. That's just what the Spitzer Space Telescope has managed, homing in on the atmosphere of Upsilon Andromedae b, a 'hot Jupiter' orbiting its parent star every 4.6 days. The results are, as you might expect for a planet this close to its star, extreme. The temperature difference between the two sides of this world is a whopping 1400 degrees Celsius (2550 degrees Fahrenheit). We're looking at a planet that seems to be tidally locked to the star it circles, but unlike other tidally locked objects like our Moon, this one has a thick atmosphere that could be circulating faster than the interior. The scientists behind this work are essentially doing meteorology, as witness this remark by Joe Harrington (University of Central Florida): "This planet has a giant hot spot in the hemisphere that faces the star. The temperature difference between the day and night sides tells about how energy flows in the...

The new planet discovered around the red dwarf GJ 849 isn't just another footnote in the unfolding story of exoplanet discoveries. This world, a gas giant about 80 percent as massive as Jupiter, promises to teach us more about planet formation around M-class stars, by far the most common stellar objects in the galaxy (with the possible exception of brown dwarfs). And the more we learn about the so-called core accretion model, the more we'll understand what to expect as we point our telescopes in the direction of other red dwarfs. Not bad for a planet circling one of the 152 stars within 200 parsecs of the Sun known to have planets. But bear this in mind: most of the stars around which we've found these planets have been like our Sun, in the range of 0.7 to 1.3 times its mass. We've studied over 200 M-class dwarf stars with planet detection in mind, but until now had discovered planets around only three. GJ 436 shows a Neptune-class world, as does GJ 581, while GJ 876 seems to be a...

Epsilon Eridani was a magic name in my childhood, so convinced was I that the nearby star (10.5 light years from Earth) was orbited by planets. Now the Hubble Space Telescope has weighed in with definitive evidence for the existence of at least one of those worlds, a Jupiter-class gas giant in a 6.9 year orbit around the star. The planet was originally detected in 2000 by radial velocity measurements, but there was still the faint possibility that turbulence on the star itself might mimic a planet's effects. Now we know that wasn't so. G. Fritz Benedict and Barbara McArthur (University of Texas at Austin) led the team that announced the result. Those stellar wobbles tell an unmistakable story when observed over time. Here's Benedict on the matter: "You can't see the wobble induced by the planet with the naked eye. But Hubble's fine guidance sensors are so precise that they can measure the wobble. We basically watched three years of a nearly seven-year-long dance of the star and its...

Centauri Dreams has often discussed red dwarf stars and the question of habitability. This time let's focus in on a nearby candidate called AU Mic, an M-class dwarf some 10 parsecs (roughly 32 light years) from Earth. At twelve million years old, this is one of the nearest young dwarfs, and it's known to possess a dusty debris disk. In fact, what we see around AU Mic looks like the late stages of planet formation, with planet-sized objects disturbing smaller disk materials and creating something like our own Kuiper Belt. The lesson of AU Mic seems clear: planet formation around M-class dwarfs is probably common, even though we see few debris disks around older stars of this class. That may simply be the result of the sensitivity of our search technologies, and in any case the smallest exoplanets we've yet found orbiting main sequence stars orbit red dwarfs (consider the rocky world around Gliese 876). Let's be clear on this: we're likely to get solid confirmation of smaller,...

"Hot Jupiters?" read a recent e-mail. "Who cares about hot Jupiters? What I'm after are planets like Earth, rocky terrestrial worlds." My correspondent probably feels the same way today, after NASA's announcement of sixteen new extrasolar planet candidates, all of which fall into the hot Jupiter category and some into an even more bizarre niche -- Ultra-Short-Period Planets, or USPPs. One of these star-hugging worlds, called SWEEPS-10, orbits its parent star once every ten hours. But not so fast. Sure, getting those first terrestrial world detections, presumably through the transit method, is going to be phenomenal, but the steps leading up to that breakthrough are hugely significant. The results announced today didn't involve nearby stars but were focused on 180,000 stars in the Milky Way's central bulge, fully 26,000 light years from Earth. And what can be extrapolated from these sixteen planets is that planet-formation isn't a local phenomenon limited to regions out in the spiral...

We've recently looked at the effects of massive stars on the debris disks surrounding them. Now the Spitzer Space Telescope has shed new light on just how problematic such environments can be. The huge O-type stars studied by a team of scientists from the University of Arizona's Steward Observatory (Tucson) are pouring ultraviolet light and powerful solar winds into the protoplanetary disks around Sun-like stars that have the misfortune of being too near to them. The result: Disruption of the disk through a process called photoevaporation. An O star can be as much as 100 times more massive than the Sun, able to heat a nearby star's disk to the point that gas and dust boil off. With the disk unable to hold together, the evaporated material is eventually blown away by solar winds. The result creates what researchers are calling a 'cometary structure' -- the photoevaporation that causes it is something like what happens when a comet forms its tail in its swing through the inner solar...

The disks of gas, dust and debris that surround young stars are breeding grounds for planets, a premise that every new exoplanet detection seems to confirm. But we know little about the disks themselves, and a key area of uncertainty continues to be the nature of disks around stars more massive than the Sun. What effect, for example, does their luminosity have on the disk, and how do the processes of large star formation affect planetary systems? The European Southern Observatory's Very Large Telescope is providing data that will shape a more refined view of these disks. At the heart of these new studies is HD 97048, a star some 600 light years away in the stellar spawning ground known as the Chameleon 1 dark cloud. HD 97048 is two and a half times as massive as the Sun, and fully forty times more luminous, making it ideal for such study. Image: Artist's impression of a flared proto-planetary disc, similar to what has been deduced from VISIR observations on ESO's Very Large Telescope...

The transit method has now bagged its 13th and 14th planets, both of them 'hot Jupiters' so close to their stars that their orbits are two and two and one-half days respectively. That makes for temperatures well over 1800 degrees Celsius, and adds more data points in our improbable collection of massive planets that all but skim their stars as they race around their orbits. One of the new planets, called WASP-1b, is in the constellation Andromeda, and is thought to be 1000 light years distant. WASP-2b, in Delphinius, is some 500 light years away. Behind the discovery is the UK consortium called SuperWASP -- Wide Angle Search for Planets. The astronomers involved are surveying millions of stars from robotic observatories in the Canary Islands and in South Africa. Each observatory uses eight wide-angle cameras, with a field of view 2000 times greater than a conventional astronomical telescope. The goal is to detect the faint dimming of starlight that flags a planetary transit, visible...

Brown dwarfs are clearly commonplace in the galaxy, but we know all too little about them. Thus the excitement about the recent imaging of a brown dwarf that orbits its star along with a planet. More information about that dwarf, HD 3651 B, has now surfaced thanks to a preprint passed along by Massimo Marengo (Harvard-Smithsonian Center for Astrophysics), a member of the team that discovered this interesting object. As we saw several days ago in these pages, an independent team led by Markus Mugrauer (University of Jena) has also submitted a later paper announcing the same object. Clearly, this faint brown dwarf has been the subject of much scrutiny, and deservedly so. For the primary, HD 3651, has already been subjected to radial velocity analysis, turning up a planetary companion of somewhat less mass than Saturn. Located in the constellation Pisces, HD 3651 is a bit less massive than our Sun, and the behavior of its known planet is unusual -- its orbit is highly elliptical, a...

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...