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This year is shaping up to be an important one in the search for planets around other stars. In April astronomers at the Geneva Observatory announced the discovery of the most Earth-like exoplanet yet located, the first rocky world beyond our solar system that could hold liquid water. Just 1.5 times the size of Earth and possessing five times its mass, the planet circles the red dwarf Gliese 581. More such announcements will likely come in the months to follow, as the first space observatory dedicated to hunting exoplanets, called COROT, begins full operation and researchers complete their calculations.
In hunting for other worlds, scientists have proved adept at wringing the most from ground-based observatories. The Geneva astronomers found the rocky, Earth-like planet by first detecting the slight wobbling in the motion of its parent star, as star and planet circled around their common center of gravity. Such wobbling betrays itself as tiny Doppler shifts in the spectral lines present in a star’s light. Although the investigators determined the exoplanet’s mass via the observed movement, they could not directly gauge its size, because it does not transit—that is, pass in between Earth and its parent star. Hence, the team had to derive the exoplanet’s density from planet formation models, team member Stéphane Udry reports.