Journal Article

Thermal emission at 4.5 and 8 μm of WASP-17b, an extremely large planet in a slightly eccentric orbit

D. R. Anderson, A. M. S. Smith, A. A. Lanotte, T. S. Barman, A. Collier Cameron, C. J. Campo, M. Gillon, J. Harrington, C. Hellier, P. F. L. Maxted, D. Queloz, A. H. M. J. Triaud and P. J. Wheatley

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 416, issue 3, pages 2108-2122
Published in print September 2011 | ISSN: 0035-8711
Published online September 2011 | e-ISSN: 1365-2966 | DOI:
Thermal emission at 4.5 and 8 μm of WASP-17b, an extremely large planet in a slightly eccentric orbit

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We report the detection of thermal emission at 4.5 and 8 μm from the planet WASP-17b. We used Spitzer to measure the system brightness at each wavelength during two occultations of the planet by its host star. By combining the resulting light curves with existing transit light curves and radial-velocity measurements in a simultaneous analysis, we find the radius of WASP-17b to be 2.0RJup, which is 0.2RJup larger than any other known planet and 0.7RJup larger than predicted by the standard cooling theory of irradiated gas giant planets. We find the retrograde orbit of WASP-17b to be slightly eccentric, with 0.0012 < e < 0.070 (3σ). Such a low eccentricity suggests that, under current models, tidal heating alone could not have bloated the planet to its current size, so the radius of WASP-17b is currently unexplained. From the measured planet–star flux-density ratios we infer 4.5 and 8 μm brightness temperatures of 1881 ± 50 and 1580 ± 150 K, respectively, consistent with a low-albedo planet that efficiently redistributes heat from its day side to its night side.

Keywords: methods: data analysis; techniques: photometric; occultations; planets and satellites: atmospheres; planets and satellites: individual: WASP-17b; stars: individual: WASP-17

Journal Article.  10042 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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