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

TL;DR

This study used HST NICMOS to observe HD189733b's transit in near-infrared, finding haze dominates the atmosphere's transmission spectrum over water absorption features below 2 microns.

Contribution

First robust near-IR transit measurements of HD189733b using narrowband filters, challenging previous water absorption detections and highlighting haze effects.

Findings

No water absorption signature detected at 5-sigma confidence.

Near-IR radii consistent with Rayleigh scattering by haze.

Haze dominates the near-IR transmission spectrum below 2 microns.

Abstract

We present Hubble Space Telescope near-infrared transit photometry of the nearby hot-Jupiter HD189733b. The observations were taken with the NICMOS instrument during five transits, with three transits executed with a narrowband filter at 1.87 microns and two performed with a narrowband filter at 1.66 microns. 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 Swain et al. absorption signature of atmospheric water below 2 microns at a 5-sigma confidence level. We measure a planet-to-star radius contrast of 0.15498+/-0.00035 at 1.66 microns and a contrast of 0.15517+/-0.00019 at 1.87 microns. 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 microns.