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Exoplanet clouds out atmospheric models
DR EMILY BALDWIN
ASTRONOMY NOW
Posted: 31 August 2010


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Astronomers studying a young gas giant planet with the W. M. Keck Observatory have found that the planet sports an atmosphere unusually thick with dust clouds.

The planet, HR 8799b, is one of three gas giants orbiting its parent star HR 8799 in the constellation Pegasus, 130 light years away. The system was discovered by direct imaging in 2008.


Keck II image of the young extrasolar planet HR 8799 b, seen as the point source in centre of image. The bright light from the parent star HR 8799 is seen in background in yellow/red and has been removed in an annular region centered on the planet. Image: Brendan Bowler and Michael Liu, IfA/Hawaii.

Astronomers from the University of Hawaii used the W. M. Keck Observatory's adaptive optics system to obtain spectra of the light emitted by the planet to obtain information on the planet's temperature, chemical composition and cloud properties. The presence or absence of gaseous methane can be used as a thermometer, but the team found very little traces of methane. Assessing their spectrum and comparing it with archival data and models, the team estimated the coolest possible temperature as 1,200 kelvin.

But the models were not able to match all the data. Current theoretical models predict the seven Jupiter mass planet should be about 400 Kelvin cooler than they measured, based on the age of the planet and the amount of energy it is currently emitting. The team propose that the planet is much more dusty and cloudy than current models predict.

“Direct studies of extrasolar planets are just in their infancy,” says University of Hawaii astronomy professor Michael Liu. “But even at this early stage, we are learning they are a different beast than objects we have known about previously.”

Just six of the nearly 500 known exoplanets were detected through direct imaging, and three of those are around HR 8799.

“Adaptive optics systems on Keck and other large ground-based telescopes make sharper images than even the Hubble Space Telescope,” says co-author Trent Dupuy. “With adaptive optics, we are learning an incredible amount about objects that are smaller than the lowest-mass stars and larger than the most massive gas-giant planets in our Solar System.”

The results of the study will be published in a forthcoming edition of the Astrophysical Journal.

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