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

Abstract

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 composition⁴, chemistry^5,6, aerosols⁷ to atmospheric dynamics⁸, escape⁹ and modelling techniques^10,11. Previous studies of HD 189733b have detected carbon and oxygen-bearing molecules H₂O and CO (refs. ^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 locations¹⁸, 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 (13.4σ), CO₂ (11.2σ), CO (5σ) and H₂S (4.5σ) in the transmission spectrum (2.4-5.0 μm) of HD 189733b. With an equilibrium temperature of about 1,200 K, H₂O, CO and H₂S 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 pathway²³.