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Combined analysis of the 12.8 and 15 μm JWST/MIRI eclipse observations of TRAPPIST-1 b

Nature Astronomy volume 9, pages 358–369 (2025)Cite this article

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Abstract

The first James Webb Space Telescope/MIRI photometric observations of TRAPPIST-1 b allowed for the detection of the thermal emission of the planet at 15 μm, suggesting that the planet could be a bare rock with a zero albedo and no redistribution of heat. These observations at 15 μm were acquired as part of Guaranteed Time Observer time that included a twin programme at 12.8 μm to obtain measurements inside and outside the CO2 absorption band. Here we present five new occultations of TRAPPIST-1 b observed with MIRI in an additional photometric band at 12.8 μm. We perform a global fit of the ten eclipses and derive a planet-to-star flux ratio and 1σ error of 452 ± 86 ppm and 775 ± 90 ppm at 12.8 μm and 15 μm, respectively. We find that two main scenarios emerge. An airless planet model with an unweathered (fresh) ultramafic surface, that could be indicative of relatively recent geological processes, fits the data well. Alternatively, a thick, pure-CO2 atmosphere with photochemical hazes that create a temperature inversion and result in the CO2 feature being seen in emission also works, although with some caveats. Our results highlight the challenges in accurately determining a planet’s atmospheric or surface nature solely from broadband filter measurements of its emission, but also point towards two very interesting scenarios that will be further investigated with the forthcoming phase curve of TRAPPIST-1 b.

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Fig. 1: Phase-folded JWST/MIRI observations of TRAPPIST-1 b.
Fig. 2: TRAPPIST-1 b’s emission spectrum, compared with bare-surface models.
Fig. 3: Best-fit atmosphere model is a hazy CO2-rich atmosphere.

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Data availability

The data used in this Article are associated with JWST GTO programmes 1177 (PI: T.G.) and GTO 1279 (PI: P.-O.L.) and are publicly available from the Mikulski Archive for Space Telescopes (https://mast.stsci.edu) on this link. Additional source data, tables and figures from this work are archived on Zenodo at https://doi.org/10.5281/zenodo.13385020 (ref. 62). Source data are provided with this paper.

Code availability

This work was performed using the following codes to process, extract, reduce and analyse the data: STScI’s JWST calibration pipeline38, Eureka!21, trafit40, starry63, exoplanet64, PyMC365, emcee66, dynesty46, NumPy67, Astropy68 and matplotlib69.

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Acknowledgements

We thank M. Turbet and F. Selsis for discussion regarding the modelling of a putative atmosphere of TRAPPIST-1 b and its likelihood. We thank B. Charnay for discussion regarding photochemical haze formation processes and comparison with Titan. We thank J. Ih for sharing their bare-surface models published in ref. 25 and for useful discussion regarding the calculation of the Bond albedo. We thank O. Lim for sharing their transmission spectra of TRAPPIST-1 b obtained from the reduction and analysis of JWST/NIRISS data and published in their paper ref. 19. We thank E. Agol for his help on planetary flux estimation equations. Finally, we thank A. Iyer for providing the SPHINX stellar spectrum model extrapolated on TRAPPIST-1’s parameters. This work is based on observations made with the NASA/ESA/CSA JWST. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. These observations are associated with programme 1279. MIRI draws on the scientific and technical expertise of the following organizations: NASA Ames Research Center, USA; Airbus Defence and Space, UK; CEA-Irfu, Saclay, France; Centre Spatial de Liège, Belgium; Consejo Superior de Investigaciones Científicas, Spain; Carl Zeiss Optronics, Germany; Chalmers University of Technology, Sweden; Danish Space Research Institute, Denmark; Dublin Institute for Advanced Studies, Ireland; European Space Agency, The Netherlands; ETCA, Belgium; ETH Zurich, Switzerland; Goddard Space Flight Center, USA; Institut d’Astrophysique Spatiale, France; Instituto Nacional de Técnica Aeroespacial, Spain; Institute for Astronomy, Edinburgh, UK; Jet Propulsion Laboratory, USA; Laboratoire d’Astrophysique de Marseille (LAM), France; Leiden University, The Netherlands; NOVA Opt-IR group at Dwingeloo, The Netherlands; Northrop Grumman, USA; Max-Planck-Institut für Astronomie (MPIA), Heidelberg, Germany; Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA), France; Paul Scherrer Institut, Switzerland; Raytheon Vision Systems, USA; RUAG Aerospace, Switzerland; Rutherford Appleton Laboratory (RAL Space), UK; Space Telescope Science Institute, USA; Toegepast Natuurwetenschappelijk Onderzoek (TNO-TPD), The Netherlands; UK Astronomy Technology Centre, UK; University College London, UK; University of Amsterdam, The Netherlands; University of Arizona, USA; University of Bern, Switzerland; University of Cardiff, UK; University of Cologne, Germany; University of Ghent, Belgium; University of Groningen, The Netherlands; University of Leicester, UK; University of Leuven, Belgium; University of Stockholm, Sweden. The following national and international funding agencies funded and supported the MIRI development: NASA; European Space Agency (ESA); Belgian Science Policy Office (BELSPO); Centre Nationale d’Etudes Spatiales (CNES); Danish National Space Centre; Deutsches Zentrum für Luft- und Raumfahrt (DLR); Enterprise Ireland; Ministerio De Economalia y Competividad; Netherlands Research School for Astronomy (NOVA); Netherlands Organisation for Scientific Research (NWO); Science and Technology Facilities Council; Swiss Space Office; Swedish National Space Agency; UK Space Agency. E.D. acknowledges support from the innovation and research Horizon 2020 programme in the context of the Marie Sklodowska-Curie subvention 945298 as well as from the Paris Observatory–PSL fellowship. M. Gillon is FRS-FNRS Research Director. His contribution to this work was done in the framework of the PORTAL project funded by the Federal Public Planning Service Science Policy (BELSPO) within its BRAIN-be: Belgian Research Action through the Interdisciplinary Networks programme. P.-O.L., C.C., A.D., R.G. and A.C. acknowledge funding support from CNES. T.G. and T.J.B. acknowledge support from NASA in WBS 411672.07.04.01.02. O.A., I.A., B.V. and P.R. thank the ESA and BELSPO for their support in the framework of the PRODEX Programme. D.B. is supported by Spanish MCIN/AEI/10.13039/501100011033 grants PID2019-107061GB-C61 and MDM-2017-0737. L.D. acknowledges funding from the KU Leuven Interdisciplinary Grant (IDN/19/028), the European Union H2020-MSCA-ITN-2019 under grant 860470 (CHAMELEON) and the FWO research grant G086217N. J.P. acknowledges financial support from the UK Science and Technology Facilities Council and the UK Space Agency. G. Ostlin acknowledges support from the Swedish National Space Board and the Knut and Alice Wallenberg Foundation. P.T. acknowledges support by the European Research Council under grant agreement ATMO 757858. E.D. has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie actions grant agreement 945298-ParisRegionFP as well as from the Paris Observatory–PSL fellowship. L.C. acknowledges support by grant PIB2021-127718NB-100 from the Spanish Ministry of Science and Innovation/State Agency of Research MCIN/AEI/10.13039/501100011033. E.F.v.D. acknowledges support from A-ERC grant 101019751 MOLDISK. T.P.R. acknowledges support from the ERC 743029 EASY. G. Olofsson acknowledges support from SNSA. P.P. thanks the Swiss National Science Foundation (SNSF) for financial support under grant number 200020_200399. O.A. is a senior research associate of the Fonds de la Recherche Scientifique—FNRS.

Author information

Author notes

  1. These authors contributed equally: Elsa Ducrot, Pierre-Olivier Lagage.

Authors and Affiliations

  1. LESIA, Observatoire de Paris, CNRS, Université Paris Diderot, Université Pierre et Marie Curie, Meudon, France

    Elsa Ducrot, Anthony Boccaletti & Daniel Rouan

  2. Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, Gif-sur-Yvette, France

    Elsa Ducrot, Pierre-Olivier Lagage, Achrène Dyrek, Alain Coulais & René Gastaud

  3. SRON Netherlands Institute for Space Research, Leiden, The Netherlands

    Michiel Min, Rens Waters & Billy Edwards

  4. Astrobiology Research Unit, University of Liège, Liège, Belgium

    Michaël Gillon

  5. Bay Area Environmental Research Institute, NASA’s Ames Research Center, Moffett Field, CA, USA

    Taylor J. Bell

  6. Space Science and Astrobiology Division, NASA’s Ames Research Center, Moffett Field, CA, USA

    Taylor J. Bell & Thomas Greene

  7. Maison de la Simulation, Université Paris-Saclay, UVSQ, CNRS, CEA, Gif-sur-Yvette, France

    Pascal Tremblin

  8. Max-Planck-Institut für Astronomie (MPIA), Heidelberg, Germany

    Jeroen Bouwman, Thomas Henning, Oliver Krause & Silvia Scheithauer

  9. Department of Astrophysics/IMAPP, Radboud University, Nijmegen, The Netherlands

    Rens Waters

  10. HFML–FELIX, Radboud University, Nijmegen, The Netherlands

    Rens Waters

  11. Department of Astrophysics, University of Vienna, Vienna, Austria

    Manuel Güdel

  12. Institute for Particle Physics and Astrophysics, ETH Zürich, Zurich, Switzerland

    Manuel Güdel, Polychronis Patapis & Adrian M. Glauser

  13. Institute of Astronomy, KU Leuven, Leuven, Belgium

    Bart Vandenbussche, Leen Decin, Ioannis Argyriou & Pierre Royer

  14. STAR Institute, Université de Liège, Liège, Belgium

    Olivier Absil

  15. Centro de Astrobiología (CAB), CSIC-INTA, Villanueva de la Cañada, Spain

    David Barrado & Luis Colina

  16. LERMA, Observatoire de Paris, Université PSL, Sorbonne Université, CNRS, Paris, France

    Alain Coulais

  17. Université Paris-Saclay, CEA, IRFU, Gif-sur-Yvette, France

    René Gastaud & Christophe Cossou

  18. UK Astronomy Technology Centre, Royal Observatory, Edinburgh, UK

    Alistair Glasse & Gillian Wright

  19. European Space Agency, Space Telescope Science Institute, Baltimore, MD, USA

    Sarah Kendrew

  20. Department of Astronomy, AlbaNova University Center, Stockholm University, Stockholm, Sweden

    Goran Olofsson

  21. Space Park Leicester, School of Physics & Astronomy, University of Leicester, Leicester, UK

    John Pye

  22. Department of Astrophysics, American Museum of Natural History, New York, NY, USA

    Niall Whiteford

  23. SRON Netherlands Institute for Space Research, Groningen, The Netherlands

    Fred Lahuis

  24. Leiden Observatory, Leiden University, Leiden, The Netherlands

    Ewine F. van Dishoeck

  25. Oskar Klein Centre, Department of Astronomy, Stockholm University, Stockholm, Sweden

    Göran Ostlin

  26. School of Cosmic Physics, Dublin Institute for Advanced Studies, Dublin, Ireland

    Tom P. Ray

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  2. Pierre-Olivier Lagage
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Contributions

All authors played an important role in one or more of the following: development of the original proposal, management of the project, definition of the target list and observation plan, analysis of the data, theoretical modelling and preparation of this Article. Some specific contributions are listed as follows. P.-O.L. is PI of the JWST MIRI GTO European consortium programme dedicated to JWST observations of exoplanet atmospheres; R.W. is co-lead of the programme. E.D. provided overall programme leadership and management of the TRAPPIST-1 b working group. P.-O.L., T.G., J.B. and T.H. designed the observational programme and set the observing parameters. A.D. generated simulated data for prelaunch testing of the data reduction methods. E.D., M. Gillon, T.J.B. and P.-O.L. reduced the data, modelled the light curves and produced the eclipse depth. M.M. and P.T. generated theoretical models to interpret the data. E.D. led the writing of the Article, and M. Gillon, T.J.B., P.-O.L., T.G. and M.M. made notable contributions to the writing. G.W. is the European PI of the JWST MIRI instrument, P.-O.L., T.H., M. Güdel, B.V., L.C., E.F.v.D., T.P.R. and G. Olofsson are European co-PIs and O.A., A.M.G., L.D., R.W., O.K., J.P., G. Ostlin, D.R. and D.B. are European co-Investigators. T.G. is a US co-Investigator of the JWST MIRI instrument. A.G. led the MIRI instrument testing and commissioning effort.

Corresponding author

Correspondence to Elsa Ducrot.

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Competing interests

The authors declare no competing interests.

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Peer review information

Nature Astronomy thanks Nicolas Cowan, Nestor Espinoza and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Planetary flux of TRAPPIST-1 b versus wavelength, from measurements and models.

The black dot with its error bar is the measured flux by ref. 12 from the analysis of 5 visits at 15μm, the black dot without an error bar is the expected flux in the F1280W filter for TB = 503 K. The planetary flux computed from this work is shown with red dots at 12.8 μm and 15 μm. Fluxes are derived from our “fiducial" joint analysis and the error bar in flux show their 1σ uncertainties. The error bar in wavelength stands for the width of the filter in each band. The blue curve shows the blackbody curve expected for a dayside temperature of 503 K, the green curve shows the blackbody curve expected for 508 K apparent dayside temperature predicted for zero heat redistribution and no internal heating, and the orange curve shows the blackbody curve expected for Teq = 400 K temperature for isotropic redistribution of stellar heating.

Source data

Extended Data Fig. 2 Comparison of the irradiance at the top of the atmosphere for TRAPPIST-1b, Titan and Venus.

For the spectra of Titan and Venus we used the scaled standard Solar irradiance spectrum at air mass zero (AM0). For TRAPPIST-1b the spectrum from the Mega-MUSCLES project is used32. The UV part of the spectrum is indicated by the violet band.

Source data

Extended Data Table 1 Summary of the observations in JWST program GTO 1279
Full size table
Extended Data Table 2 Details of the different data reduction methods
Full size table
Extended Data Table 3 Details of the different data analysis methods
Full size table
Extended Data Table 4 Derived eclipse depth with various approaches and fitting method
Full size table

Supplementary information

Supplementary Information

Supplementary Figs. 1–10 and Table 1.

Source data

Source Data Fig. 1

Raw and binned photometric data to reproduce Fig. 1; model also provided.

Source Data Fig. 2

Observations, blackbody models, bare-surface models and MIRI filter throughput to reproduce Fig. 2.

Source Data Fig. 3

Data, models and best fit to reproduce Fig. 3.

Source Data Extended Data Fig. 1

Observations and models to reproduce Extended Data Fig. 1.

Source Data Extended Data Fig. 2

Models to reproduce Extended Data Fig. 2.

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Ducrot, E., Lagage, PO., Min, M. et al. Combined analysis of the 12.8 and 15 μm JWST/MIRI eclipse observations of TRAPPIST-1 b. Nat Astron 9, 358–369 (2025). https://doi.org/10.1038/s41550-024-02428-z

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