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Published February 1, 2009 | Published
Journal Article Open

Transit infrared spectroscopy of the hot Neptune around GJ 436 with the Hubble Space Telescope


The nearby transiting system GJ 436b offers a unique opportunity to probe the structure and atmosphere of an extrasolar 'hot Neptune'. In this Letter, we present the main results of observations covering two transit events with the Near Infrared Camera and Multi Object Spectrograph (NICMOS) camera on the Hubble Space Telescope (HST). The data consist in high-cadence time series of grism spectra covering the 1.1–1.9 µm spectral range. We find R_pl= 4.04 ± 0.10 R_⊕ and R_*= 0.446 ± 0.011 R_⊙ for the planet and star radius, confirming and improving earlier measurements with a ground-based photometry and a Spitzer light curve at 8 µm, as opposed to a much higher value obtained with the Fine Guidance Sensor on the HST. We measure no departure from strict periodicity in the transits to the level of ∼7s. This strongly disfavours the proposed explanation of the orbital eccentricity of GJ 436b in terms of the perturbation by another close-by planet. We measure a flat transmission spectrum at the level of a few parts per 10 000 in flux, with no significant signal in the 1.4-µm water band to a level comparable to the maximum amplitude of the effect predicted by planetary atmosphere models.

Additional Information

© 2008 The Authors. Journal compilation © 2008 RAS. Accepted 2008 October 29. Received 2008 October 29; in original form 2008 August 18. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555. The authors wish to thank Eliza Miller-Ricci and Sara Seager for atmosphere models provided in advance of publication and insightful comments, Michel Mayor for support with the observing proposal, Daniel Fabrycky for helpful discussions about transit timing variations in the GJ 436 system and the anonymous referee for numerous helpful comments.

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Published - MNRAS-2009-Pont-L6-L10.pdf


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