Water vapor in the spectrum of the extrasolar planet HD 189733b: 2. The eclipse

TL;DR

This study presents a high-precision emission spectrum of exoplanet HD 189733b, revealing marginal water vapor absorption and suggesting a cloud-free atmosphere with no thermal inversion or potentially sub-solar water abundance.

Contribution

First detailed analysis combining HST and Spitzer data to characterize the water vapor presence and atmospheric composition of HD 189733b.

Findings

Marginal evidence for water vapor absorption in the spectrum.

Best explained by a non-inverted, possibly cloud-free atmosphere.

Data consistent with a nearly solar or sub-solar water abundance.

Abstract

Spectroscopic observations of exoplanets are crucial to infer the composition and properties of their atmospheres. HD 189733b is one of the most extensively studied exoplanets and is a corner stone for hot Jupiter models. In this paper, we report the day-side emission spectrum of HD 189733b in the wavelength range 1.1 to 1.7 $\mu$m obtained with the Hubble Space Telescope Wide Field Camera 3 in spatial scan mode. The quality of the data is such that even a straightforward analysis yields a high precision Poisson noise limited spectrum: the median 1-$\sigma$ uncertainty is 57 ppm per 0.02 $\mu$m bin. We also build a white-light curve correcting for systematic effects and derive an absolute eclipse depth of 96$\pm$39 ppm. The resulting spectrum shows marginal evidence for water vapor absorption, but can also be well explained by a blackbody spectrum. However, the combination of these WFC3 data with previous Spitzer photometric observations is best explained by a day-side atmosphere of HD 189733b with no thermal inversion and a nearly solar or sub-solar H$_2$O abundance in a cloud-free atmosphere. Alternatively, this apparent sub-solar abundance may be the result of clouds or hazes which future studies need to investigate.