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...