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

Two teams of astronomers used a technique for finding extrasolar planets to directly measure one such planet. The approach could allow the study of more exoplanets' atmospheres than ever before.

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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.

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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.

In addition to using the radial-velocity technique to derive mass with a precision once the province of the transit approach, Brogi's team also used the spectra to build a picture of changes in the atmosphere's temperature with height.

Observations of a handful of other extrasolar planet atmospheres with the Spitzer Space Telescope have shown that temperatures are hottest at the top of the atmospheres of so-called hot Jupiters – gas giants orbiting very close to their host stars.

Tau Bootis b, which orbits its star once every 3.3 days, seems to be an oddball. Brogi's team's analysis suggests no such heating at the top of the atmosphere. Theorists have suggested this could happen if the planet is orbiting a star prone to frequent energetic outbursts. Frequent exposure to increased levels of high-energy radiation from the star could strip away heat-absorbing elements, including titanium oxide, from the top layers of a planet's atmosphere.

Tau Bootis, at roughly 1.3 times the mass of the sun, seems to be one such active star.

Brigi's team's data also yielded an inclination to the planet's orbit – something that has been a subject of debate. The answer: 44.5 degrees off an imaginary line padding through the star's equator.

The team's efforts collectively represent "a great result," says Dimitar Sasselov, a researcher studying extrasolar planets at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

"The success of the CO technique, from the ground no less, is a welcome addition to our exoplanet study toolbox," he says. "Tau Bootis b has always been somewhat of an odd beast. I think it tells us something about the secret lives of super-massive planets."

Brogi's team's results are set for publication in Thursday's issue of the journal Nature. Rodler’s team's work was published June 18 in Astrophysical Journal Letters.


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