An exoplanet as dense as Earth

Of the more than 350 planets that have been observed to date orbiting alien suns, only a handful have known densities. (For more on exoplanets, see Physics Today, May 2009, page 46 .) Now an international team led by Didier Queloz (Geneva Observatory, Switzerland) has introduced the newest member of that club, Corot-7b, the only exoplanet to have a measured density comparable to that of Earth. The result suggests that Corot-7b, like Earth, has a rocky composition. The planet and its host star Corot-7 were named after the Convection, Rotation, and Planetary Transits satellite, which in 2008 observed a star whose intensity periodically dimmed once every 20.4 hours—the result of a planet partially eclipsing the star. The small degree of dimming and the known size of the star established that Corot-7b has a radius about twice that of Earth’s. To obtain the mass of the planet, Queloz and colleagues turned to the High Accuracy Radial Velocity Planet Searcher, a spectrograph that could precisely measure the sinusoidally modulating Doppler shifts in the light emitted by Corot-7 as it is gravitationally tugged to and fro by the small planet orbiting it. The greater the amplitude of oscillation, the greater the planet’s mass. Obtaining the result required the Queloz team to filter out obscuring noise due to sunspot-like activity on the star. As a bonus, the residual signal included a component with a 3.69-day period. Queloz and colleagues attribute that to a previously unknown second planet, about eight times as massive as Earth. (D. Queloz et al., Astron. Astrophys., in press, http://dx.doi.org/10.1051/0004-6361/200913096 .)