Knowing where to point our future planet-hunter telescopes in space is crucial, because we'll want to maximize observing time for the most likely stellar candidates. There are various ways to narrow the list, but one involves the study of existing spectroscopic data. Charles Lineweaver (Australian National University) calls it a 'poor man's technique,' an inexpensive way to look at the elements within stars and calculate from their abundance the kind of planets that may have formed in that system. The differences between the rocky terrestrial planets in our own Solar System and the outer gas giants are instructive. We can assume that planets form from the same raw materials as the stars they orbit. But the inner planets lack volatile gases like hydrogen and helium compared to the Sun, while maintaining the same abundances of heavier elements like silica and iron. The latter don't vaporise easily in warmer inner orbits. So a star heavy in iron is likely circled by inner planets...

Those following the Gliese 581 story have been awaiting the results of the MOST observations with great interest. The Canadian mission put the red dwarf under study for six weeks after the recent flurry of speculation regarding a possible habitable planet, Gliese 581 c, in the system. If the planet made a transit, moving across the face of its star as seen from Earth, then we could learn more about its size and makeup. The results are now in, and no transit occurred. But a second issue is a bit more satisfactory. During the observation period, Gliese 581 showed little change in brightness, indicating a level of stability that would prove beneficial to the growth of life, whether on Gliese 581 c or the more distant (and massive) Gl 581 d, which may orbit on the outer edge of the star's habitable zone. Here's Jaymie Matthews (University of British Columbia and a MOST mission scientist) on the matter: "The climate there should not be a wild rollercoaster ride that would make it...

If we're finding planets in places 1500 light years away, as the TrES project just did, why don't we know more about planets in the Alpha Centauri system? One problem is that Centauri A and B are relatively close to each other, with a semimajor axis of 23.4 AU. Leaving Proxima Centauri out of the picture (at 12,000 AU, its short-term effects can be disregarded), it's still true that radial velocity studies have to take the complicated and varying spectra that binaries produce into account. In other words, getting a read on binaries like these in terms of the slight wobbles that signal a planetary presence can consume lots of telescope time. Nonetheless, we do have some data thanks to observations with the Anglo-Australian Telescope. And we've learned this: No planet around either Centauri A or B induces a velocity variation as high as 2 meters per second. The implication is that any planet orbiting either star individually (in what is known as an S-type orbit) has to have a mass less...

Catching up with some older items, I want to be sure to cover a planet recently discovered in the constellation Hercules, because it gives further punch to a fact about exoplanet studies: Off-the-shelf equipment made for amateur astronomers can be effective at detecting new worlds. The planet in question was found by the Trans-Atlantic Exoplanet Survey (TrES) and later observed by the Hungarian Automated Telescope Network (HATNet). Because it blocks out about 2.5 percent of the star's light as it passes in front of it, the transiting TrES-3 readily shows up in these projects' automated surveys. The method is clearly effective. TrES works with wide-field timed exposures, measuring the light from every star in the field to seek out transits. When TrES-3 was discovered with these techniques, it was studied again with one of the 10-meter instruments at the Keck Observatory on Mauna Kea and by the Las Cumbres Observatory in Hawaii, as well as with instruments at Lowell Observatory and the...

Odd planets seem to be sprouting in our data like mushrooms. Take the case of XO-3b. It's got the mass of thirteen Jupiters but orbits its star in less than four days, making it the largest, most massive planet ever found in such a tight orbit. But XO-3b also seizes the attention because its orbit is significantly elliptical rather than circular. Is this evidence for the gravitational effects of another object in the same system? We should be able to learn a lot more about this and other questions because XO-3b is also a transiting world, passing between its star and the Earth. This is the third transiting planet identified by the XO Project, which uses two small telescopes at Haleakala (operated by the University of Hawaii) to identify transit candidates before passing the data on to a network of amateur astronomers for further study. After sufficient evidence is gathered, the work goes back to large telescopes at McDonald Observatory (University of Texas) for confirmation....

What better indication of the success of our planet hunting efforts than the news out of the American Astronomical Society's annual meeting in Honolulu. There, the California & Carnegie Planet Search team announced at least 28 new planets, with four multi-planet systems among them and two borderline cases that need further investigation. That's a bump of 12 percent in the number of known planets over the last year. Behold: With the exoplanet count now not that far from 250, planetary discoveries are coming fast enough that a certain ennui seems to have settled in among press and public. Sure, Gliese 581 c was big news because we thought it was potentially habitable, but finding more and more gas giants probably won't trigger the public imagination, even if GJ 436 b did cause a ripple because of the presence of water. That ripple lasted only long enough for scientists to explain what kind of water they were talking about. Here's Geoff Marcy (UC Berkeley) on the subject: "From the...

New work on Gliese 581's interesting planetary system may prove dismaying for those hoping for a planet in the habitable zone. With two 'super-Earths' and a Neptune class world, this is a system that cries out for close analysis. The Geneva team that detected the super-Earths had calculated surface temperatures on Gliese 581 c at roughly 20 degrees C. What they left out was the likely greenhouse effect of the atmosphere. For habitability -- defined here as the presence of liquid water at the surface -- is not dependent on the central star alone, but also on the properties of the planets circling it. Werner von Bloh (Potsdam Institute for Climate Impact Research) and team tackle the habitability question in terms of atmosphere. From their paper: ...habitability is linked to the photosynthetic activity of the planet, which in turn depends on the planetary atmospheric CO2 concentration, and is thus strongly in?uenced by the planetary dynamics. In principle, this leads to additional...

Brown dwarfs, most of them unobserved, doubtless litter the galaxy. The more we can learn about them and their possible companions, the better for our understanding of how planets form and stars evolve. These minute 'failed stars,' far less massive than the Sun, cannot sustain hydrogen fusion, but they're players in the exoplanet hunt. The brown dwarf 2MASS1207-3932, for example, has a planetary companion of five Jupiter masses, thought to be the first for which an image was obtained. Now we learn that this young star, perhaps eight million years old and surrounded with a protoplanetary disc, is also producing jets of matter. The results, growing out of work at the European Southern Observatory's Very Large Telescope, are surprising. The dwarf's mass is itself only 24 times that of Jupiter, making it the smallest object known to produce such jets. Image: Using ESO's VLT, astronomers found jets coming out from a 24 Jupiter-mass brown dwarf, showing that outflows are rather ubiquituous...

Two gas giants discovered around the star HD 155358 raise again the question of planetary formation and the mechanisms behind it. Most planets detected through radial-velocity methods, which measure the effects unseen companions have on a star's motion, have been found to orbit stars that are high in metal content. 'Metals' in this context means elements higher than hydrogen and helium, and of the two primary models for planetary formation, high metal content seems to favor the one known as the core accretion model, about which more in a moment. What to do about a low metal star whose system is dominated by two massive planets? HD 155358 contains only 20 percent of the metal content of our Sun. Such a finding may favor the rival disk instability model. Here the notion is that the rotating disk of gas and dust in a protoplanetary system becomes unstable not long after it forms, causing it to fragment. As clumps begin to appear, they become large enough to cause their gases to collapse...