Hydrogen sulfide and metal-enriched atmosphere for a Jupiter-mass exoplanet.

As the closest transiting hot Jupiter to Earth, HD 189733b has been the benchmark planet for atmospheric characterization 1,2,3 . It has also been the anchor point for much of our theoretical understanding of exoplanet atmospheres from composition 4 , chemistry 5,6 , aerosols 7 to atmospheric dynamics 8 , escape 9 and modeling techniques 10,11 . Prior studies of HD 189733b have detected carbon and oxygen-bearing molecules H 2 O and CO 12,13 in the atmosphere. The presence of CO 2 and CH 4 has been claimed 14,15 but later disputed 12,16,17 . The inferred metallicity based on these measurements, a key parameter in tracing planet formation locations 18 , varies from depletion 19,20 to enhancement 21,22 , hindered by limited wavelength coverage and precision of the observations. Here we report detections of H 2 O (13.4 sigma), CO 2 (11.2 sigma), CO (5 sigma), and H 2 S (4.5 sigma) in the transmission spectrum (2.4-5 micron) of HD 189733b. With an equilibrium temperature of ~ 1200K, H 2 O, CO, and H 2 S are the main reservoirs for oxygen, carbon, and sulfur. Based on the measured abundances of these three major volatile elements, we infer an atmospheric metallicity of 3-5 times stellar. The upper limit on the methane abundance at 5 sigma is 0.1 ppm which indicates a low carbon-to-oxygen ratio (<0.2), suggesting formation through the accretion of water-rich icy planetesimals. The low oxygen-to-sulfur and carbon-to-sulfur ratios also support the planetesimal accretion formation pathway 23 .