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Astronomers use an old trick to open new window on extrasolar planets

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All it needed was the right combination of technology and a target close enough and bright enough to pursue. Tau Bootis b fit the latter requirement. For hardware, both teams used a new, exquisitely sensitive near-infrared spectrometer bolted to the back end of one of four eight-meter telescopes that make up the Very Large Telescope array. The facility, run by the European Southern Observatory, is located on a mountaintop in the Atacama Desert of northern Chile.

Hunting for the tell-tale signs of chemicals that are revealed as the star’s light interacts with the atmosphere, the teams found that the planet's atmosphere appears to be dominated by carbon monoxide. The spectrometer revealed carbon monoxide's spectral bar code with enough precision to uncover the spectrum’s wobble as the planet orbits. From the magnitude of the wobble, the teams calculated the planet's so-called radial velocity – the pace at which it appears to move back and forth. Both teams had a good handle on the star's mass. Knowing the radial velocity for the planet and the star, and knowing the star's mass, the team estimated the planet's mass with math a junior high-school student would recognize.

Dr. Rodler’s team found that tau Bootis b tipped the scales at 5.6 times Jupiter's mass. Dr. Brogi's team's estimate yielded 5.95 times Jupiter's mass. One reason for the discrepancy might be traced to the amount of telescope time each team received – 18 hours for Brogi's team versus six hours for Rodler’s group. Typically, the more time the spectrometer has to build up the data, the more precise the measurement.

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