HST PanCET Program: A Cloudy Atmosphere for the Promising JWST Target WASP-101b

Wakeford, H. R. and Stevenson, K. B. and Lewis, N. K. and Sing, D. K. and López-Morales, M. and Marley, M. and Kataria, T. and Mandell, A. and Ballester, G. E. and Barstow, J. and Ben-Jaffel, L. and Bourrier, V. and Buchhave, L. A. and Ehrenreich, D. and Evans, T. Matthew and García Muñoz, A. and Henry, G. and Knutson, H. and Lavvas, P. and Lecavelier des Etangs, A. and Nikolov, N. and Sanz-Forcada, J. 2017. HST PanCET Program: A Cloudy Atmosphere for the Promising JWST Target WASP-101b. Astrophysical Journal Letters 835, 1, DOI: 10.3847/2041-8213/835/1/L12

We present results from the first observations of the Hubble Space Telescope (HST) Panchromatic Comparative Exoplanet Treasury program for WASP-101b, a highly inflated hot Jupiter and one of the community targets proposed for the James Webb Space Telescope (JWST) Early Release Science (ERS) program. From a single HST Wide Field Camera 3 observation, we find that the near-infrared transmission spectrum of WASP-101b contains no significant H2O absorption features and we rule out a clear atmosphere at 13 sigma. Therefore, WASP-101b is not an optimum target for a JWST ERS program aimed at observing strong molecular transmission features. We compare WASP-101b to the well-studied and nearly identical hot Jupiter WASP-31b. These twin planets show similar temperature-pressure profiles and atmospheric features in the near-infrared. We suggest exoplanets in the same parameter space as WASP-101b and WASP-31b will also exhibit cloudy transmission spectral features. For future HST exoplanet studies, our analysis also suggests that a lower count limit needs to be exceeded per pixel on the detector in order to avoid unwanted instrumental systematics.

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