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...

The Spitzer Space Telescope has peered into the Orion nebula with striking results: nearly 2300 planet-forming disks in the overall Orion cloud complex, a star-forming region some 1450 light years from Earth. This is where infrared truly shines, for such disks are too small to be seen with visible-light telescopes. But Spitzer is made to order for picking up the infrared signature of warm dust, giving us an unprecedented look at solar system formation in the aggregate. The image below gives a glimpse, but be sure to click to enlarge the photograph for a bit more detail. Thomas Megeath (University of Toledo, OH) likens the research to a census of potential solar systems, saying "...we want to know how many are born in the cities, how many in small towns, and how many out in the countryside." Megeath and colleagues discovered that 60 percent of the disk-bearing stars in the Orion cloud complex are found in clusters of hundreds of stars, while 15 percent exist in much smaller groupings,...

Centauri A and B continue to stand out as likely venues for terrestrial planets. What a change since the days when it was thought orbits in binary systems like this one would be completely unstable. Today we believe that both the major Centauri stars could support small, rocky worlds within about 4 AU, and that such planets are as likely, if not more so, to form there as around our own Sun. The latter insight emerges directly from the work of Elisa Quintana and Jack Lissauer. Add to that two other factors: At UC-Santa Cruz, Greg Laughlin and Jeremy Wertheimer have shown that Proxima Centauri could perturb the debris disk surrounding the Centauri stars enough to deliver volatiles to inner worlds there. Laughlin has been arguing the Centauri case for some time now, discussing not just the Proxima factor but pointing as well to the metallicity of Alpha Centauri, which is high enough to provide the kind of materials needed to form planets analogous to Earth. Keen on detecting such a...

Gravitational microlensing is a fascinating way to find exoplanets, provided you're not worried about nearby targets. For the best way to do microlensing of this sort is to work with a crowded starfield, which means looking toward galactic center, where stars are numerous and distant enough that their lensing events can be studied. You're hoping to find a star that passes in front of another as seen from Earth, in which case the gravity of the foreground star sets up the gravitational lensing effects that magnify the light of the background star. Thus OGLE-2003-BLG-235L, which displayed a planetary companion that was discovered in 2003 using ground-based observations. The oddity here is that while the planet produced an additional brightening of the background star, thus confirming its existence, astronomers weren't sure about the identity of the star it circled. It has taken two years and new observations by the Hubble Space Telescope to pin down OGLE-2003-BLG-235L -- the foreground...

We know all too little about planetary-mass objects -- planemos, as Centauri Dreams is learning to call them -- that are not associated with a star. But we've learned a bit more with the discovery of an unusual binary system. Discovered in optical imagery taken by the European Southern Observatory's 3.5 meter instrument in La Silla (Chile), Oph162225-240515 (Oph1622 for short) is a 14-Jupiter mass planemo apparently orbited by a companion of about half that mass. "This is a truly remarkable pair of twins - each weighing some hundred times less than our sun," says Ray Jayawardhana, an associate professor of astronomy and astrophysics at the University of Toronto. "Their mere existence is a surprise, and their origin and fate a bit of a mystery." Following up the find with optical spectra and infrared work, Jayawardhana and ESO's Valentin Ivanov established that both members of the pair are at the same distance from the Sun and far too cool to be stars. They're also young, perhaps a...

Centauri Dreams sometimes muses that we know all too little about nearby space. Ponder that it is only within the last decade that we have begun to characterize the whole category of objects known as 'brown dwarfs,' while our understanding of M-class dwarfs is evolving so rapidly that we're now seeing them as potential havens for terrestrial worlds. That makes both kinds of dwarfs interesting as mission targets once we've created the technologies to make such journeys. And it also means that we have to develop a better census of red and brown dwarf stars in our own neighborhood. It is within the realm of possibility, for example, that there may be a brown dwarf closer to us than the Centauri stars, and the discovery of a target, say, one light year away would give powerful impetus to interstellar propulsion studies. Even M-class stars are readily overlooked in a crowded sky, and the boundary between them and brown dwarfs can be tricky to establish. As witness a most interesting...

For Centauri Dreams, the most exciting part of the exoplanet hunt is the refinement of our models. We know, for example, of numerous planetary systems dominated by gas giants. Now we're trying to figure out which of these may contain smaller, rocky worlds, and that means learning more about solar system dynamics. A step in the right direction emerges from a June paper that analyzes what happens to moon-sized protoplanets as they evolve in systems with gas giants. Based on computer simulations, the work assumes a giant planet the size of Jupiter and manipulates the position and mass of the protoplanets in these settings over time, testing four systems with known planets: 55 Cancri, HD 38529, HD 37124 and HD 74156. The most interesting result is the ready formation of terrestrial worlds around 55 Cancri, often with orbits in the habitable zone. HD 38529 also produced a rocky world, one about the size of Mars, and showed conditions favorable to an asteroid belt as well. No further...

One of the beauties of the Spitzer Space Telescope is that it can pinpoint the swirling dust disks around distant stars. Such dust, heated by the star, puts out an infrared signature that Spitzer can analyze to a degree hitherto unattainable. Now a team of astronomers has observed some 500 young T Tauri stars in the star-forming regions of the Orion nebula. They've been looking at how young stars spin, and the effects that dusty disks have on slowing their rotation. T Tauri stars are ideal for this kind of work. They're young objects (less than 10 million years old) that are still in the process of gravitational contraction. Such stars often show large accretion disks, but a variant called weak-lined T Tauri stars have little or no disk. Figuring out the various phases of T Tauri formation and how they relate to planets is thus a substantial challenge. The answers Spitzer has provided are intriguing even if they leave many questions unanswered. Slow-spinning stars are five times more...

Hard to believe that it's been over ten years now since the discovery of 51 Peg B, the first exoplanet found around a main sequence star. So much has happened since, including over 160 exoplanet candidates identified within 200 parsecs, with most of these discovered through Doppler search methods examining radial velocities (although the nearby planet TrES-1 was found via transit methods). The last published list of exoplanets appeared in 2002, which is why the just published "Catalog of Nearby Exoplanets" is so welcome. Appearing in The Astrophysical Journal, the catalog confines itself to the 200 parsec limit for good reasons. Yes, we have located exoplanets far beyond it, including some in the direction of galactic center found by the OGLE survey and several found through microlensing projects elsewhere. But within 200 pc, high resolution imaging and stellar spectroscopy allows satisfactory follow-up work, and we're also working with stars whose parallax has been studied through...

We've recently discussed Greg Laughlin and Jeremy Wertheimer's work on the possible role of Proxima Centauri in destabilizing the Centauri A and B debris disk and bringing volatiles to the inner system. Our deepening knowledge of the Centauri system is one of the most energizing aspects of the exoplanet hunt, for its proximity inexorably makes Alpha Centauri of high astrobiological interest. And no one has done more significant work on planet formation around binary stars than Elisa Quintana and Jack Lissauer (NASA Ames). The two have examined the possibilities of terrestrial worlds around Centauri A and B and are continuing with the study of other binary scenarios. Now they have extended their analysis to binary systems whose stars are much closer to each other than Centauri A and B. Their new paper is significant for planet hunters because more than half of all main sequence stars are in binary or multiple systems, whereas our basic models for planet formation have been based on...

We may not have images of terrestrial planets around another star yet, but many things can be learned about such worlds by computer simulation. A team of British astronomers, for example, has examined known exoplanetary systems in hopes of isolating those in which Earth-like worlds could exist in stable and habitable orbits. This is tricky business, because the massive planets present in almost every exoplanetary system we know about could disrupt such orbits long before life might have a chance to form on any worlds there. It's also tricky because to determine which systems could have life-bearing planets requires you to figure out the location of the habitable zone in each. Researchers Barrie Jones, Nick Sleep and David Underwood (Open University, Milton Keynes, UK) here use the classical definition of habitable zone: the distances from a star where water at the surface of an Earth-like planet would be in liquid form. Not surprisingly, they find that the question of planetary...

A starshade shaped like a daisy? Centauri Dreams remains entranced with the concept, known as New Worlds Imager and now getting renewed attention thanks to the efforts of astronomer Webster Cash (University of Colorado, Boulder). We've seen before that Cash is hoping to land a NASA Discovery-class mission for a starshade that would block the light of a nearby star to reveal the planets around it. The starshade would work in tandem with a telescope mounted on a separate spacecraft 15,000 miles away, with the shade being moved as needed to place it into the line of sight of stars of interest. The result: the ability to image planetary systems including terrestrial worlds, and even to analyze exoplanetary atmospheres. Cash's latest thoughts on the subject appear in the July 6 issue of Nature, where he describes a starshade some 50 yards in diameter and its associated space telescope. Both could be launched into an orbit roughly a million miles from Earth, where shade and telescope could...

So much good material has run lately on planet-hunter Gregory Laughlin's systemic site that Centauri Dreams feels seriously remiss in returning to it so infrequently. So there is catching up to do, but we focus today on Laughlin's new work, with UCSC graduate student Jeremy Wertheimer, on an intriguing question about Proxima Centauri. Is the tiny star in fact gravitationally bound to Centauri A and B? Surprisingly, much recent work has suggested otherwise, including a 1993 paper by Robert Matthews and Gerard Gilmore that set the tone for Proxima research in that decade. But Laughlin notes that the European Space Agency's Hipparcos satellite has firmed up our knowledge of the position, distance and velocity of nearby stars, enough to demand a new look at this question. After all, Proxima is roughly 15,000 AU from the Centauri binaries, and shows only a small velocity relative to them. It would seem unlikely these stars would not be bound into a triple system, and Laughlin and...

Back in 2003, while researching Centauri Dreams, I interviewed physicist Geoffrey Landis at Glenn Research Center in Cleveland. At that time, Landis' office was packed with Mars images, apropos for a man who had done so much work on rover technology. I asked him whether, after all this study, Mars had taken on the aspect of a real place to him, like Cleveland. Not surprisingly, he said that it had, and he credited 3-D images from Mars Pathfinder for that. Wearing glasses, Landis said, "It was as if you were standing on Mars. You could see ups and downs, ridges and valleys. That changed the view of Mars from another planet to a place you could go out and walk around." We're a long way from 3-D close-ups, but I suspect some astronomers are starting to feel that way about Beta Pictoris, a young star some 63 light years away in the southern constellation Pictor that first drew attention to itself because of excess infrared radiation. A warm circumstellar disk was surely the cause, and...

Beta Pictoris, an A5 dwarf star some 63 light years from the Earth, is well known to exoplanet hunters, some of whom have been studying its circumstellar dust disk since its discovery by the Infrared Astronomical Satellite (IRAS). That disk was first detected way back in 1983, and is thought to be perhaps 1100 AU wide and much more massive than the disk from which our own Solar System formed. The disk and possible planetary formation going on there has always been tantalizingly like our own system's, but now we get a surprise. For as a new paper in Nature suggests, this young system (between eight and twenty million years old) contains much more carbon gas than expected. This work comes courtesy of the Far Ultraviolet Spectroscopic Explorer satellite (FUSE), along with Hubble's imaging spectrograph. The presence of carbon may solve at least one Beta Pictoris mystery: why didn't the star's radiation reduce the gas orbiting it? A hidden mass of hydrogen had been suspected as blocking...

It's shaping up to be a good week for exoplanet findings, with yesterday's intriguing work on 'planemos' and their disks and now, also presented at the AAS Calgary meeting, word of new findings on planetary migration. This is a significant issue, because so many of the exoplanets we know about are huge 'hot Jupiters' in tight orbits around their star. The effects such planets would have on smaller worlds in the habitable zone could be devastating if the gas giants migrated through that region early in the system's life. And migration is assumed to be what happens. The assumption is that such planets form a long way from their stars, as much as 20 AU out, and move to their present positions as the planet interacts tidally with the surrounding gas disk. But migration is tricky business, implying that most planets would fall into their stars within a million years. Preserving a solar system with gas giants and low-mass terrestrial worlds becomes challenging business (and recall that it...