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 1-3 . 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 modelling techniques 10,11 . Previous studies of HD 189733b have detected carbon and oxygen-bearing molecules H 2 O and CO (refs.  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 (13.4σ), CO 2 (11.2σ), CO (5σ) and H 2 S (4.5σ) in the transmission spectrum (2.4-5.0 μm) of HD 189733b. With an equilibrium temperature of about 1,200 K, 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 main volatile elements, we infer an atmospheric metallicity of three to five times stellar. The upper limit on the methane abundance at 5σ 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 .