We present Hubble Space Telescope near-infrared transit photometry of the nearby hot-Jupiter HD 189733b. The observations were taken with the NICMOS instrument during five transits, with three transits executed with a narrowband filter at 1.87 m and two performed with a narrowband filter at 1.66 m. Our observing strategy using narrowband filters is insensitive to the usual HST intra-orbit and orbit-to-orbit measurement of systematic errors, allowing us to accurately and robustly measure the near-IR wavelength dependance of the planetary radius. Our measurements fail to reproduce the previously claimed detection of an absorption signature of atmospheric H2O below 2 m at a 5 confidence level. We measure a planet-to-star radius contrast of at 1.66 m and a contrast of at 1.87 m. Both of our near-IR planetary radii values are in excellent agreement with the levels expected from Rayleigh scattering by sub-micron haze particles, observed at optical wavelengths, indicating that upper-atmospheric haze still dominates the near-IR transmission spectra over the absorption from gaseous molecular species at least below 2 m.
Transit spectrophotometry of the exoplanet HD 189733b - I. Searching for water but finding haze with HST NICMOS D. K. Sing, J.-M. Désert, A. Lecavelier des Etangs, G. E. Ballester, A. Vidal-Madjar, V. Parmentier, G. Hebrard and G. W. Henry A&A, 505 2 (2009) 891-899 DOI: https://doi.org/10.1051/0004-6361/200912776