Journal ArticlePhotochemically produced SO2 in the atmosphere of WASP-39bTsai, Shang-Min; Lee, Elspeth K.; Powell, Diana; Gao, Peter; Zhang, Xi; Moses, Julianne; Hébrard, Eric; Venot, Olivia; Parmentier, Vivien; Jordan, Sean; Hu, Renyu; Alam, Munazza K.; Alderson, Lili; Batalha, Natalie M.; Bean, Jacob L.; Benneke, Björn; Bierson, Carver J.; Brady, Ryan P.; Carone, Ludmila; Carter, Aarynn L.; Chubb, Katy L.; Inglis, Julie; Leconte, Jérémy; Line, Michael; López-Morales, Mercedes; Miguel, Yamila; Molaverdikhani, Karan; Rustamkulov, Zafar; Sing, David K.; Stevenson, Kevin B.; Wakeford, Hannah R.; Yang, Jeehyun; Aggarwal, Keshav; Baeyens, Robin; Barat, Saugata; de Val-Borro, Miguel; Daylan, Tansu; Fortney, Jonathan J.; France, Kevin; Goyal, Jayesh M.; Grant, David; Kirk, James; Kreidberg, Laura; Louca, Amy; Moran, Sarah E.; Mukherjee, Sagnick; Nasedkin, Evert; Ohno, Kazumasa; Rackham, Benjamin V.; Redfield, Seth; Taylor, Jake; Tremblin, Pascal; Visscher, Channon; Wallack, Nicole L.; Welbanks, Luis; Youngblood, Allison; Ahrer, Eva-Maria; Batalha, Natasha E.; Behr, Patrick; Berta-Thompson, Zachory K.; Blecic, Jasmina; Casewell, S. L.; Crossfield, Ian J.; Crouzet, Nicolas; Cubillos, Patricio E.; Decin, Leen; Désert, Jean-Michel; Feinstein, Adina D.; Gibson, Neale P.; Harrington, Joseph; Heng, Kevin; Henning, Thomas; Kempton, Eliza M.-R.; Krick, Jessica; Lagage, Pierre-Olivier; Lendl, Monika; Lothringer, Joshua D.; Mansfield, Megan; Mayne, N. J.; Mikal-Evans, Thomas; Palle, Enric; Schlawin, Everett; Shorttle, Oliver; Wheatley, Peter J.; Yurchenko, Sergei N.<title>Abstract</title> <p>Photochemistry is a fundamental process of planetary atmospheres that regulates the atmospheric composition and stability¹. However, no unambiguous photochemical products have been detected in exoplanet atmospheres so far. Recent observations from the JWST Transiting Exoplanet Community Early Release Science Program^2,3found a spectral absorption feature at 4.05 μm arising from sulfur dioxide (SO₂) in the atmosphere of WASP-39b. WASP-39b is a 1.27-Jupiter-radii, Saturn-mass (0.28 <italic>M</italic>_J) gas giant exoplanet orbiting a Sun-like star with an equilibrium temperature of around 1,100 K (ref. ⁴). The most plausible way of generating SO₂in such an atmosphere is through photochemical processes^5,6. Here we show that the SO₂distribution computed by a suite of photochemical models robustly explains the 4.05-μm spectral feature identified by JWST transmission observations⁷with NIRSpec PRISM (2.7<italic>σ</italic>)⁸and G395H (4.5<italic>σ</italic>)⁹. SO₂is produced by successive oxidation of sulfur radicals freed when hydrogen sulfide (H₂S) is destroyed. The sensitivity of the SO₂feature to the enrichment of the atmosphere by heavy elements (metallicity) suggests that it can be used as a tracer of atmospheric properties, with WASP-39b exhibiting an inferred metallicity of about 10× solar. We further point out that SO₂also shows observable features at ultraviolet and thermal infrared wavelengths not available from the existing observations.</p>Nature2023-05-1810469071Nature6177961483 to 4870028-0836https://doi.org/10.1038/s41586-023-05902-22009095; 1945633National Science Foundation