Abstract

Infrared radiation emitted from a planet contains information about the chemical composition and vertical temperature profile of its atmosphere1,2,3. If upper layers are cooler than lower layers, molecular gases will produce absorption features in the planetary thermal spectrum4,5. Conversely, if there is a stratosphere—where temperature increases with altitude—these molecular features will be observed in emission6,7,8. It has been suggested that stratospheres could form in highly irradiated exoplanets9,10, but the extent to which this occurs is unresolved both theoretically11,12 and observationally3,13,14,15. A previous claim for the presence of a stratosphere14 remains open to question, owing to the challenges posed by the highly variable host star and the low spectral resolution of the measurements3. Here we report a near-infrared thermal spectrum for the ultrahot gas giant WASP-121b, which has an equilibrium temperature of approximately 2,500 kelvin. Water is resolved in emission, providing a detection of an exoplanet stratosphere at 5σ confidence. These observations imply that a substantial fraction of incident stellar radiation is retained at high altitudes in the atmosphere, possibly by absorbing chemical species such as gaseous vanadium oxide and titanium oxide.

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Acknowledgements

This work is based on observations with the NASA/ESA HST, obtained at the Space Telescope Science Institute (STScI) operated by AURA, Inc. This work is also based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. 336792 and is supported by the ERC Horizon 2020 research and innovation programme (grant agreement no. 724427). Support for this work was provided by NASA through grants under the HST-GO-14767 “Panchromatic Comparative Exoplanetary Treasury (PanCET)” programme from the STScI. J.G. acknowledges support from a Leverhulme Trust Research Project Grant. H.R.W acknowledges support from the NASA Postdoctoral Program, administered by Universities Space Research Association through a contract with NASA. M.S.M. acknowledges support from the NASA Exoplanets Research Program. J.K.B. acknowledges support from a Royal Astronomical Society Fellowship. D.E. and V.B. acknowledge the financial support of the National Centre for Competence in Research “PlanetS” supported by the Swiss National Science Foundation (SNSF). A.L.E. acknowledges support from CNES and the French Agence Nationale de la Recherche (ANR), under programme ANR-12-BS05-0012 “Exo-Atmos”. J.S.-F. acknowledges support from the Spanish MINECO through grant AYA2014-54348-C3-2-R. G.W.H. acknowledges support from NASA, NSF, Tennessee State University, and the State of Tennessee through its Centers of Excellence programme. L.B.-J. and P.L. acknowledge support from CNES (France) under project PACES. P.T. and D.S.A. acknowledge funding from the European Research Council under the European Union Seventh Framework Program: grant 247060-PEPS.

Author information

Affiliations

  1. Astrophysics Group, School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK

    • Thomas M. Evans
    • , David K. Sing
    • , Jayesh Goyal
    •  & Nikolay Nikolov
  2. NASA Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, California 91109, USA

    • Tiffany Kataria
  3. NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA

    • Hannah R. Wakeford
    •  & Avi M. Mandell
  4. Department of Astronomy, University of Maryland, College Park, Maryland 20742, USA

    • Drake Deming
  5. NASA Ames Research Center, MS 245-5, Moffett Field, California 94035, USA

    • Mark S. Marley
  6. Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10025, USA

    • David S. Amundsen
  7. NASA Goddard Institute for Space Studies, New York, New York 10025, USA

    • David S. Amundsen
  8. Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, USA

    • Gilda E. Ballester
  9. Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK

    • Joanna K. Barstow
  10. Sorbonne Universités, UPMC Université Paris 6 and CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis boulevard Arago, F-75014 Paris, France

    • Lotfi Ben-Jaffel
    •  & Alain Lecavelier des Etangs
  11. Observatoire de l’Université de Genève, 51 chemin des Maillettes, 1290 Sauverny, Switzerland

    • Vincent Bourrier
    •  & David Ehrenreich
  12. Centre for Star and Planet Formation, Niels Bohr Institute and Natural History Museum, University of Copenhagen, DK-1350 Copenhagen, Denmark

    • Lars A. Buchhave
  13. Lowell Center for Space Science and Technology, University of Massachusetts, Lowell, Massachusetts 01854, USA

    • Ofer Cohen
  14. Zentrum für Astronomie und Astrophysik, Technische Universität Berlin, D-10623 Berlin, Germany

    • Antonio García Muñoz
  15. Center of Excellence in Information Systems, Tennessee State University, Nashville, Tennessee 37209, USA

    • Gregory W. Henry
  16. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA

    • Heather Knutson
  17. Groupe de Spectrométrie Moléculaire et Atmosphérique, UMR 7331, CNRS, Université de Reims Champagne-Ardenne, Reims 51687, France

    • Panayotis Lavvas
  18. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA

    • Nikole K. Lewis
  19. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA

    • Mercedes López-Morales
  20. Centro de Astrobiología (CSIC-INTA), ESAC Campus, Camino bajo del Castillo, E-28692 Villanueva de la Cañada, Madrid, Spain

    • Jorge Sanz-Forcada
  21. Maison de la Simulation, CEA, CNRS, Université Paris-Sud, UVSQ, Université Paris-Saclay, 91191 Gif-sur-Yvette, France

    • Pascal Tremblin
  22. Bay Area Environmental Research Institute, Moffett Field, California 94035, USA

    • Roxana Lupu

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Contributions

T.M.E. and D.K.S. designed the HST observations of WASP-121. D.K.S. and M.L.-M. led the HST Treasury programme, with support provided by all authors. T.M.E. led the HST data analysis with contributions from N.N., H.R.W. and D.D. D.D. proposed and designed the Spitzer observations and analysed the data. D.K.S. led the retrieval analysis. T.K., J.G., M.S.M., A.L.E. and P.T. provided additional theoretical interpretation of the data. R.L. provided molecular absorption cross-sections for the theoretical interpretation. T.M.E. wrote the manuscript along with D.K.S., T.K., M.S.M. and A.L.E. All authors discussed the results and commented on the paper. The author list ordering is alphabetical after M.S.M.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Thomas M. Evans.

Reviewer Information Nature thanks K. Heng and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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    Supplementary Data

    This file contains an archive of ASCII files containing reduced HST data products and models used in the paper.

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https://doi.org/10.1038/nature23266

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