The Broadband Infrared Emission Spectrum of the Exoplanet HD 189733b

TL;DR

This study measures the infrared emission spectrum of exoplanet HD 189733b using Spitzer data, revealing a spectrum inconsistent with a simple blackbody and supporting models without atmospheric temperature inversion.

Contribution

First detailed infrared emission spectrum of HD 189733b across multiple bands, challenging previous assumptions and supporting non-inverted atmospheric models.

Findings

Detected emission near 4 microns consistent with theoretical models

Spectrum inconsistent with a Planck blackbody spectrum

Supports classification of hot Jupiters into two atmospheric types

Abstract

We present Spitzer Space Telescope time series photometry of the exoplanet system HD 189733 spanning two times of secondary eclipse, when the planet passes out of view behind the parent star. We estimate the relative eclipse depth in 5 distinct bands and find the planet-to-star flux ratio to be 0.256 +/- 0.014% (3.6 microns), 0.214 +/- 0.020% (4.5 microns), 0.310 +/- 0.034% (5.8 microns), 0.391 +/- 0.022% (8.0 microns), and 0.598 +/- 0.038% (24 microns). For consistency, we re-analyze a previously published time series to deduce a contrast ratio in an additional band, 0.519 +/- 0.020% (16 microns). Our data are strongly inconsistent with a Planck spectrum, and we clearly detect emission near 4 microns as predicted by published theoretical models in which this feature arises from a corresponding opacity window. Unlike recent results for the exoplanet HD 209458b, we find that the emergent spectrum from HD 189733b is best matched by models that do not include an atmospheric temperature inversion. Taken together, these two studies provide initial observational support for the idea that hot Jupiter atmospheres diverge into two classes, in which a thermal inversion layer is present for the more strongly irradiated objects.