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Published December 11, 2010 | Published
Journal Article Open

Water in the atmosphere of HD 209458b from 3.6–8 μm IRAC photometric observations in primary transit


The hot Jupiter HD 209458b was observed during primary transit at 3.6, 4.5, 5.8 and 8.0 μm using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We describe the procedures we adopted to correct for the systematic effects present in the IRAC data and the subsequent analysis. The light curves were fitted including limb-darkening effects and fitted using Markov Chain Monte Carlo and prayer-bead Monte Carlo techniques, obtaining almost identical results. The final depth measurements obtained by a combined Markov Chain Monte Carlo fit are at 3.6 μm, 1.469 ± 0.013 and 1.448 ± 0.013 per cent; at 4.5 μm, 1.478 ± 0.017 per cent; at 5.8 μm, 1.549 ± 0.015 per cent; and at 8.0 μm, 1.535 ± 0.011 per cent. Our results clearly indicate the presence of water in the planetary atmosphere. Our broad-band photometric measurements with IRAC prevent us from determining the additional presence of other molecules such as CO, CO_2 and methane for which spectroscopy is needed. While water vapour with a mixing ratio of 10^(-4) to 10^(-3) combined with thermal profiles retrieved from the day side may provide a very good fit to our observations, this data set alone is unable to resolve completely the degeneracy between water abundance and atmospheric thermal profile.

Additional Information

© 2010 The Authors. Journal compilation © 2010 RAS. Accepted 2010 February 10. Received 2010 February 3; in original form 2009 September 1. Article first published online: 17 Nov 2010. We are very grateful to Tommi Koskinen, Alan Aylward Steve Miller, Jean-Pierre Maillard and Giusi Micela for their insightful comments. GT is supported by a Royal Society University Research Fellowship; DMK by STFC; RJB by the Leverhulme Trust; GC by Ateneo Federato della Scienza e della Tecnologia – Universitá di Roma 'La Sapienza', Collegio univ. 'Don N. Mazza' and LLP-Erasmus Student Placement. We acknowledge the support by ANR-06-BLAN-0416 and the 'Programme Origine des Planètes et de la Vie'. This paper is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA.

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