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...