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 H₂O and CO ^12,13 in the atmosphere. The presence of CO₂ and CH₄ 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 H₂O (13.4 sigma), CO₂ (11.2 sigma), CO (5 sigma), and H₂S (4.5 sigma) in the transmission spectrum (2.4-5 micron) of HD 189733b. With an equilibrium temperature of ~ 1200K, H₂O, CO, and H₂S 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.

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