Sulfur dioxide in the mid-infrared transmission spectrum of WASP-39b

Diana Powell^{1

2}, Adina D Feinstein^{3

4}, Elspeth K H Lee⁵, Michael Zhang³, Shang-Min Tsai⁶, Jake Taylor^{7

8

9}, James Kirk¹⁰, Taylor Bell^{11

12}, Joanna K Barstow¹³, Peter Gao¹⁴, Jacob L Bean³, Jasmina Blecic^{15

16}, Katy L Chubb¹⁷, Ian J M Crossfield¹⁸, Sean Jordan¹⁹, Daniel Kitzmann⁴, Sarah E Moran²⁰, Giuseppe Morello^{21

22

23}, Julianne I Moses²⁴, Luis Welbanks²⁵, Jeehyun Yang²⁶, Xi Zhang²⁷, Eva-Maria Ahrer^{28

29}, Aaron Bello-Arufe³⁰, Jonathan Brande¹⁸, S L Casewell³¹, Nicolas Crouzet³², Patricio E Cubillos^{33

34}, Brice-Olivier Demory^{5

35}, Achrène Dyrek³⁶, Laura Flagg^{37

38}, Renyu Hu^{30

39}, Julie Inglis³⁹, Kathryn D Jones⁵, Laura Kreidberg⁴⁰, Mercedes López-Morales⁴¹, Pierre-Olivier Lagage³⁶, Erik A Meier Valdés⁵, Yamila Miguel^{32

42}, Vivien Parmentier⁴³, Anjali A A Piette¹⁴, Benjamin V Rackham^{44

45}, Michael Radica^{8

9}, Seth Redfield^{46

47}, Kevin B Stevenson⁴⁸, Hannah R Wakeford⁴⁹, Keshav Aggarwal⁵⁰, Munazza K Alam¹⁴, Natalie M Batalha⁵¹, Natasha E Batalha⁵², Björn Benneke^{8

9}, Zach K Berta-Thompson⁵³, Ryan P Brady⁵⁴, Claudio Caceres^{55

56

57}, Aarynn L Carter⁵¹, Jean-Michel Désert⁵⁸, Joseph Harrington⁵⁹, Nicolas Iro⁶⁰, Michael R Line²⁵, Joshua D Lothringer⁶¹, Ryan J MacDonald⁶², Luigi Mancini^{33

40

63}, Karan Molaverdikhani^{64

65}, Sagnick Mukherjee⁵¹, Matthew C Nixon⁶⁶, Apurva V Oza³⁰, Enric Palle²², Zafar Rustamkulov⁶⁷, David K Sing^{67

68}, Maria E Steinrueck⁴⁰, Olivia Venot⁶⁹, Peter J Wheatley^{28

29}, Sergei N Yurchenko⁵⁴

Affiliations

¹ Center for Astrophysics | Harvard & Smithsonian, Cambridge, MA, USA. diana.powell@uchicago.edu.
² Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL, USA. diana.powell@uchicago.edu.
³ Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL, USA.
⁴ Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, CO, USA.
⁵ Center for Space and Habitability, University of Bern, Bern, Switzerland.
⁶ Department of Earth Sciences, University of California, Riverside, Riverside, CA, USA.
⁷ Department of Physics, University of Oxford, Oxford, UK.
⁸ Institut Trottier de Recherche sur les Exoplanètes, Université de Montréal, Montréal, Quebec, Canada.
⁹ Département de Physique, Université de Montréal, Montréal, Quebec, Canada.
¹⁰ Department of Physics, Imperial College London, London, UK.
¹¹ Bay Area Environmental Research Institute, NASA Ames Research Center, Moffett Field, CA, USA.
¹² Space Science and Astrobiology Division, NASA Ames Research Center, Moffett Field, CA, USA.
¹³ School of Physical Sciences, The Open University, Milton Keynes, UK.
¹⁴ Earth and Planets Laboratory, Carnegie Institution for Science, Washington, DC, USA.
¹⁵ Department of Physics, New York University Abu Dhabi, Abu Dhabi, United Arab Emirates.
¹⁶ Center for Astro, Particle, and Planetary Physics (CAP3), New York University Abu Dhabi, Abu Dhabi, United Arab Emirates.
¹⁷ Centre for Exoplanet Science, University of St Andrews, St Andrews, UK.
¹⁸ Department of Physics & Astronomy, University of Kansas, Lawrence, KS, USA.
¹⁹ Institute of Astronomy, University of Cambridge, Cambridge, UK.
²⁰ Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA.
²¹ Department of Space, Earth and Environment, Chalmers University of Technology, Gothenburg, Sweden.
²² Instituto de Astrofísica de Canarias (IAC), Tenerife, Spain.
²³ INAF - Palermo Astronomical Observatory, Palermo, Italy.
²⁴ Space Science Institute, Boulder, CO, USA.
²⁵ School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA.
²⁶ Planetary Sciences Section, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA.
²⁷ Department of Earth and Planetary Sciences, University of California, Santa Cruz, Santa Cruz, CA, USA.
²⁸ Centre for Exoplanets and Habitability, University of Warwick, Coventry, UK.
²⁹ Department of Physics, University of Warwick, Coventry, UK.
³⁰ Astrophysics Section, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA.
³¹ School of Physics and Astronomy, University of Leicester, Leicester, UK.
³² Leiden Observatory, University of Leiden, Leiden, The Netherlands.
³³ INAF - Turin Astrophysical Observatory, Pino Torinese, Italy.
³⁴ Space Research Institute, Austrian Academy of Sciences, Graz, Austria.
³⁵ Space and Planetary Sciences, Institute of Physics, University of Bern, Bern, Switzerland.
³⁶ Université Paris-Saclay, CEA, CNRS, AIM, Gif-sur-Yvette, France.
³⁷ Department of Astronomy, Cornell University, Ithaca, NY, USA.
³⁸ Carl Sagan Institute, Cornell University, Ithaca, NY, USA.
³⁹ Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA.
⁴⁰ Max Planck Institute for Astronomy, Heidelberg, Germany.
⁴¹ Center for Astrophysics | Harvard & Smithsonian, Cambridge, MA, USA.
⁴² SRON Netherlands Institute for Space Research, Leiden, The Netherlands.
⁴³ Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Laboratoire Lagrange, French Riviera, France.
⁴⁴ Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA.
⁴⁵ Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA, USA.
⁴⁶ Astronomy Department, Wesleyan University, Middletown, CT, USA.
⁴⁷ Van Vleck Observatory, Wesleyan University, Middletown, CT, USA.
⁴⁸ Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA.
⁴⁹ School of Physics, University of Bristol, Bristol, UK.
⁵⁰ Indian Institute of Technology Indore, Indore, India.
⁵¹ Department of Astronomy and Astrophysics, University of California, Santa Cruz, Santa Cruz, CA, USA.
⁵² NASA Ames Research Center, Moffett Field, CA, USA.
⁵³ Department of Astrophysical and Planetary Sciences, University of Colorado Boulder, Boulder, CO, USA.
⁵⁴ Department of Physics and Astronomy, University College London, London, UK.
⁵⁵ Instituto de Astrofisica, Facultad Ciencias Exactas, Universidad Andres Bello, Santiago, Chile.
⁵⁶ Centro de Astrofisica y Tecnologias Afines (CATA), Santiago, Chile.
⁵⁷ Núcleo Milenio de Formación Planetaria (NPF), Valparaíso, Chile.
⁵⁸ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Amsterdam, The Netherlands.
⁵⁹ Planetary Sciences Group, Department of Physics and Florida Space Institute, University of Central Florida, Orlando, FL, USA.
⁶⁰ Institute of Planetary Research, German Aerospace Center (DLR), Berlin, Germany.
⁶¹ Department of Physics, Utah Valley University, Orem, UT, USA.
⁶² Department of Astronomy, University of Michigan, Ann Arbor, MI, USA.
⁶³ Department of Physics, University of Rome "Tor Vergata", Rome, Italy.
⁶⁴ Universitäts-Sternwarte, Ludwig-Maximilians-Universität München, München, Germany.
⁶⁵ Exzellenzcluster Origins, Garching, Germany.
⁶⁶ Department of Astronomy, University of Maryland, College Park, MD, USA.
⁶⁷ Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA.
⁶⁸ Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD, USA.
⁶⁹ Université de Paris Cité and Université Paris-Est Creteil, CNRS, LISA, Paris, France.

PMID: 38232945
PMCID: PMC10901732
DOI: 10.1038/s41586-024-07040-9

Sulfur dioxide in the mid-infrared transmission spectrum of WASP-39b

Diana Powell et al. Nature. 2024 Feb.

. 2024 Feb;626(8001):979-983.

doi: 10.1038/s41586-024-07040-9. Epub 2024 Jan 17.

Authors

Affiliations

¹ Center for Astrophysics | Harvard & Smithsonian, Cambridge, MA, USA. diana.powell@uchicago.edu.
² Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL, USA. diana.powell@uchicago.edu.
³ Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL, USA.
⁴ Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, CO, USA.
⁵ Center for Space and Habitability, University of Bern, Bern, Switzerland.
⁶ Department of Earth Sciences, University of California, Riverside, Riverside, CA, USA.
⁷ Department of Physics, University of Oxford, Oxford, UK.
⁸ Institut Trottier de Recherche sur les Exoplanètes, Université de Montréal, Montréal, Quebec, Canada.
⁹ Département de Physique, Université de Montréal, Montréal, Quebec, Canada.
¹⁰ Department of Physics, Imperial College London, London, UK.
¹¹ Bay Area Environmental Research Institute, NASA Ames Research Center, Moffett Field, CA, USA.
¹² Space Science and Astrobiology Division, NASA Ames Research Center, Moffett Field, CA, USA.
¹³ School of Physical Sciences, The Open University, Milton Keynes, UK.
¹⁴ Earth and Planets Laboratory, Carnegie Institution for Science, Washington, DC, USA.
¹⁵ Department of Physics, New York University Abu Dhabi, Abu Dhabi, United Arab Emirates.
¹⁶ Center for Astro, Particle, and Planetary Physics (CAP3), New York University Abu Dhabi, Abu Dhabi, United Arab Emirates.
¹⁷ Centre for Exoplanet Science, University of St Andrews, St Andrews, UK.
¹⁸ Department of Physics & Astronomy, University of Kansas, Lawrence, KS, USA.
¹⁹ Institute of Astronomy, University of Cambridge, Cambridge, UK.
²⁰ Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA.
²¹ Department of Space, Earth and Environment, Chalmers University of Technology, Gothenburg, Sweden.
²² Instituto de Astrofísica de Canarias (IAC), Tenerife, Spain.
²³ INAF - Palermo Astronomical Observatory, Palermo, Italy.
²⁴ Space Science Institute, Boulder, CO, USA.
²⁵ School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA.
²⁶ Planetary Sciences Section, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA.
²⁷ Department of Earth and Planetary Sciences, University of California, Santa Cruz, Santa Cruz, CA, USA.
²⁸ Centre for Exoplanets and Habitability, University of Warwick, Coventry, UK.
²⁹ Department of Physics, University of Warwick, Coventry, UK.
³⁰ Astrophysics Section, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA.
³¹ School of Physics and Astronomy, University of Leicester, Leicester, UK.
³² Leiden Observatory, University of Leiden, Leiden, The Netherlands.
³³ INAF - Turin Astrophysical Observatory, Pino Torinese, Italy.
³⁴ Space Research Institute, Austrian Academy of Sciences, Graz, Austria.
³⁵ Space and Planetary Sciences, Institute of Physics, University of Bern, Bern, Switzerland.
³⁶ Université Paris-Saclay, CEA, CNRS, AIM, Gif-sur-Yvette, France.
³⁷ Department of Astronomy, Cornell University, Ithaca, NY, USA.
³⁸ Carl Sagan Institute, Cornell University, Ithaca, NY, USA.
³⁹ Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA.
⁴⁰ Max Planck Institute for Astronomy, Heidelberg, Germany.
⁴¹ Center for Astrophysics | Harvard & Smithsonian, Cambridge, MA, USA.
⁴² SRON Netherlands Institute for Space Research, Leiden, The Netherlands.
⁴³ Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Laboratoire Lagrange, French Riviera, France.
⁴⁴ Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA.
⁴⁵ Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA, USA.
⁴⁶ Astronomy Department, Wesleyan University, Middletown, CT, USA.
⁴⁷ Van Vleck Observatory, Wesleyan University, Middletown, CT, USA.
⁴⁸ Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA.
⁴⁹ School of Physics, University of Bristol, Bristol, UK.
⁵⁰ Indian Institute of Technology Indore, Indore, India.
⁵¹ Department of Astronomy and Astrophysics, University of California, Santa Cruz, Santa Cruz, CA, USA.
⁵² NASA Ames Research Center, Moffett Field, CA, USA.
⁵³ Department of Astrophysical and Planetary Sciences, University of Colorado Boulder, Boulder, CO, USA.
⁵⁴ Department of Physics and Astronomy, University College London, London, UK.
⁵⁵ Instituto de Astrofisica, Facultad Ciencias Exactas, Universidad Andres Bello, Santiago, Chile.
⁵⁶ Centro de Astrofisica y Tecnologias Afines (CATA), Santiago, Chile.
⁵⁷ Núcleo Milenio de Formación Planetaria (NPF), Valparaíso, Chile.
⁵⁸ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Amsterdam, The Netherlands.
⁵⁹ Planetary Sciences Group, Department of Physics and Florida Space Institute, University of Central Florida, Orlando, FL, USA.
⁶⁰ Institute of Planetary Research, German Aerospace Center (DLR), Berlin, Germany.
⁶¹ Department of Physics, Utah Valley University, Orem, UT, USA.
⁶² Department of Astronomy, University of Michigan, Ann Arbor, MI, USA.
⁶³ Department of Physics, University of Rome "Tor Vergata", Rome, Italy.
⁶⁴ Universitäts-Sternwarte, Ludwig-Maximilians-Universität München, München, Germany.
⁶⁵ Exzellenzcluster Origins, Garching, Germany.
⁶⁶ Department of Astronomy, University of Maryland, College Park, MD, USA.
⁶⁷ Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA.
⁶⁸ Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD, USA.
⁶⁹ Université de Paris Cité and Université Paris-Est Creteil, CNRS, LISA, Paris, France.

PMID: 38232945
PMCID: PMC10901732
DOI: 10.1038/s41586-024-07040-9

Abstract

The recent inference of sulfur dioxide (SO₂) in the atmosphere of the hot (approximately 1,100 K), Saturn-mass exoplanet WASP-39b from near-infrared JWST observations^1-3 suggests that photochemistry is a key process in high-temperature exoplanet atmospheres⁴. This is because of the low (<1 ppb) abundance of SO₂ under thermochemical equilibrium compared with that produced from the photochemistry of H₂O and H₂S (1-10 ppm)^4-9. However, the SO₂ inference was made from a single, small molecular feature in the transmission spectrum of WASP-39b at 4.05 μm and, therefore, the detection of other SO₂ absorption bands at different wavelengths is needed to better constrain the SO₂ abundance. Here we report the detection of SO₂ spectral features at 7.7 and 8.5 μm in the 5-12-μm transmission spectrum of WASP-39b measured by the JWST Mid-Infrared Instrument (MIRI) Low Resolution Spectrometer (LRS)¹⁰. Our observations suggest an abundance of SO₂ of 0.5-25 ppm (1σ range), consistent with previous findings⁴. As well as SO₂, we find broad water-vapour absorption features, as well as an unexplained decrease in the transit depth at wavelengths longer than 10 μm. Fitting the spectrum with a grid of atmospheric forward models, we derive an atmospheric heavy-element content (metallicity) for WASP-39b of approximately 7.1-8.0 times solar and demonstrate that photochemistry shapes the spectra of WASP-39b across a broad wavelength range.

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Conflict of interest statement

The authors declare no competing interests.

Figures

**Fig. 1. A sample of spectrophotometric light curves and residuals for the transit of WASP-39b observed with MIRI/LRS.**
a, An exoplanet transit model multiplied by a systematics model (solid black line) was fitted to each light curve. b, The residuals to the best-fit models are shown for each light curve. We report the 1σ scatter in each light curve as the standard deviation of the out-of-transit residuals, with the ratio to the predicted photon noise in parentheses. The reduction is from Eureka!.

**Fig. 2. MIRI/LRS transmission spectra of WASP-39b derived using three independent reduction pipelines.**
a, The spectrum is dominated by broad absorption features from SO₂ at 7.7 and 8.5 μm and H₂O across the entire wavelength coverage of MIRI/LRS. We define our uncertainties as 1σ. b, We present the log of opacities of dominant species in the spectrum in units of cm² mol⁻¹. The opacities were adopted from PLATON using ExoMol line lists^, and assume atmospheric properties pressure, P = 1 mbar, and temperature, T = 1,000 K.

**Fig. 3. Free retrievals of the MIRI/LRS transmission spectrum of WASP-39b.**
a, The spectrum from the Eureka! reduction (with 1σ uncertainties) is compared with the best-fit retrieved spectra and associated 1σ shaded regions from six free-retrieval codes. b, The corresponding posterior probability distributions of the volume mixing ratio (VMR) and associated 1σ uncertainties (points) for the SO₂ abundance. The quoted log(SO₂) ranges from the lowest to the highest 1σ bounds of all six posteriors. We chose the Eureka! reduction owing to its similar reduction steps to previous WASP-39b observations^,,, and the fact that it provides the full-wavelength coverage of the observations. Results from the other two reductions for SO₂ give broadly consistent results and are discussed further in Methods.

**Fig. 4. Comparison of four independent photochemical models with the observed MIRI/LRS transmission spectra of WASP-39b.**
a, Comparison of morning and evening limb-averaged theoretical transmission spectra to the observations assuming a best-fit atmospheric metallicity of 7.5 times solar. b, Limb-averaged SO₂ volume mixing ratio between 10 and 0.01 mbar as a function of metallicity for the four photochemical models. The shaded region represents the 1σ SO₂ constraint from the free retrievals on the Eureka! reduction (Fig. 3). c, Dependence of VULCAN modelled transmission spectrum on atmospheric metallicity, as compared with the Eureka! reduction. The Tiberius reduction prefers a metallicity of 7.5 times solar, whereas the SPARTA reduction prefers 10 times solar (see Extended Data). The VULCAN models suggest that there is only a minor (<0.05%) difference expected for the SO₂ feature at 7.7 μm when assuming a higher atmospheric metallicity, whereas the SO₂ feature at 8.5 μm is more sensitive to subtle changes. The SO₂ feature at 8.5 μm is fit well by the 7.5–10 times solar metallicity models.

**Extended Data Fig. 1. Comparison of the different background modelling and subtraction per each pipeline.**
a, A median out-of-transit image of the MIRI/LRS detector from the jwst pipeline’s Stage 2 processing. b, Background models from Eureka! (1), Tiberius (2) and SPARTA (3). c, Background-subtracted Stage 2 outputs from each pipeline. The smoothly varying background is expected for MIRI/LRS. There are no discrete features or sharp changes in the background at y pixels < 244, corresponding to λ = 10 μm, which has been seen in other observations. All images are given in Data Numbers per second (DN s⁻¹). The Tiberius reduction did not extract spectra as far red as Eureka! and SPARTA, which is the cause of the horizontal bar in panels b2 and c2.

**Extended Data Fig. 2. MIRI/LRS white and spectrophotometric light curves from the three independent reduction pipelines used in this work.**
a, We quote the out-of-transit ppm scatter in each light curve in the figure. We define the out-of-transit time as −0.135 < t (days) < −0.07 and 0.07 < t (days) < 0.14; these times were selected as they ignore the exponential ramp at the beginning of the observations and do not include any data in transit ingress/egress. b, The residuals and errors of the data compared with the best-fit transit model. Errors quoted are 1σ. c, The spectrophotometric light curves are normalized by the out-of-transit flux during the observations. All reductions show consistent out-of-transit scatter in all wavelength bins (Δλ = 0.25 μm). The white spaces in c1 are where values in the light curve are NaN.

**Extended Data Fig. 3. The best-fitting cloudy PICASO grid models (gold lines) are shown with and without SO₂ compared with the JWST MIRI/LRS data (black points) from the Eureka! reduction.**
a, With SO₂. b, Without SO₂. Also shown are the best fits with H₂O (dark teal), SO₂ (red), CH₄ (light teal) and clouds (navy blue) removed from the model, demonstrating which absorbers dominate the opacity of the best-fit model. When SO₂ is not included in the model, excess CH₄ compensates for its absorption in the Eureka! reduction, as shown in the lower panel.

**Extended Data Fig. 4. Retrieved log of SO₂ and H₂O volume mixing ratio posteriors from all six retrieval codes and three data reductions.**
Median values and 1σ uncertainties are given by the coloured points. VMR, volume mixing ratio.

See this image and copyright information in PMC

References

1. JWST Transiting Exoplanet Community Early Release Science Team. Identification of carbon dioxide in an exoplanet atmosphere. Nature614, 649–652 (2023). 10.1038/s41586-022-05269-w - DOI - PMC - PubMed
1. Alderson, L. et al. Early Release Science of the exoplanet WASP-39b with JWST NIRSpec G395H. Nature614, 664–669 (2023). 10.1038/s41586-022-05591-3 - DOI - PMC - PubMed
1. Rustamkulov, Z. et al. Early Release Science of the exoplanet WASP-39b with JWST NIRSpec PRISM. Nature614, 659–663 (2023). 10.1038/s41586-022-05677-y - DOI - PMC - PubMed
1. Tsai, S.-M. et al. Photochemically produced SO2 in the atmosphere of WASP-39b. Nature617, 483–487 (2023). 10.1038/s41586-023-05902-2 - DOI - PMC - PubMed
1. Zahnle, K., Marley, M. S., Freedman, R. S., Lodders, K. & Fortney, J. J. Atmospheric sulfur photochemistry on hot Jupiters. Astrophys. J.701, L20–L24 (2009).10.1088/0004-637X/701/1/L20 - DOI

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Sulfur dioxide in the mid-infrared transmission spectrum of WASP-39b

Affiliations

Sulfur dioxide in the mid-infrared transmission spectrum of WASP-39b

Authors

Affiliations

Abstract

Conflict of interest statement

Figures

References

LinkOut - more resources

Full Text Sources

Research Materials