Transit spectrophotometry of the exoplanet HD189733b. I. Searching for water but finding haze with HST NICMOS

摘要

We present Hubble Space Telescope near-infrared transit photometry of thenearby hot-Jupiter HD189733b. The observations were taken with the NICMOSinstrument during five transits, with three transits executed with a narrowbandfilter at 1.87 microns and two performed with a narrowband filter at 1.66microns. Our observing strategy using narrowband filters is insensitive to theusual HST intra-orbit and orbit-to-orbit measurement of systematic errors,allowing us to accurately and robustly measure the near-IR wavelengthdependance of the planetary radius. Our measurements fail to reproduce theSwain et al. absorption signature of atmospheric water below 2 microns at a5-sigma confidence level. We measure a planet-to-star radius contrast of0.15498+/-0.00035 at 1.66 microns and a contrast of 0.15517+/-0.00019 at 1.87microns. Both of our near-IR planetary radii values are in excellent agreementwith the levels expected from Rayleigh scattering by sub-micron haze particles,observed at optical wavelengths, indicating that upper-atmospheric haze stilldominates the near-IR transmission spectra over the absorption from gaseousmolecular species at least below 2 microns.