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No thick atmosphere around TRAPPIST-1 b and c from JWST thermal phase curves

Nature Astronomy (2026)Cite this article

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Abstract

The survival of dense secondary atmospheres around temperate rocky planets orbiting low-mass red dwarfs remains an important open question. Here we show that the thermal phase curves of TRAPPIST-1 b and TRAPPIST-1 c, measured with the James Webb Space Telescope at 15 μm, are consistent with bare rocky surfaces rather than thick atmospheres. TRAPPIST-1 b exhibits a high dayside brightness temperature (490 ± 17 K), no significant nightside emission and no phase offset—features indicative of a dark, airless surface. TRAPPIST-1 c shows a cooler dayside (369 ± 23 K) and a similarly cold nightside, consistent with either a tenuous, oxygen-rich atmosphere or an equally airless, more reflective surface. Models with surface pressures above ~1 bar are strongly disfavoured for both planets. These results suggest divergent evolutionary pathways or atmospheric loss processes despite their similar compositions.

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Fig. 1: Detrended programme 3077 light curve.
Fig. 2: Posterior PDFs for the dayside and nightside fluxes.
Fig. 3: Range of best-fit phase curve and eclipse models for TRAPPIST-1 b and c.
Fig. 4: Phase-folded phase curves and emission spectra of TRAPPIST-1 b and c.
Fig. 5: Temperature maps computed for four distinct GCM simulations along with a low-albedo, airless-planet case.

Data availability

The JWST programme 3077 data used in this work are available on the MAST online database (https://mast.stsci.edu/portal/Mashup/Clients/Mast/Portal.html). Source data are provided with this paper.

Code availability

The code Trafit used to analyse the light curves is a Fortran 2003 code that can be obtained from M.G. on reasonable request. The generic planetary climate model (and documentation on how to use the model) can be downloaded from the SVN repository https://svn.lmd.jussieu.fr/Planeto/trunk/LMDZ.GENERIC/. More information and documentation on the model are available on http://www-planets.lmd.jussieu.fr. The N-body code Posidonius and its documentation are available from https://github.com/revolal/posidonius/tree/kaula_v2.

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Acknowledgements

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 paper25. 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 programmes 1177, 1279, 2304 and 3077. M.G. is F.R.S-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 Interdisciplinary Networks programme. We thank the Belgian Federal Science Policy Office (BELSPO) for the provision of financial support in the framework of the PRODEX Programme of the European Space Agency (ESA) under contract number 4000142531. T.J.B. and T.P.G. acknowledge funding support from the NASA Next Generation Space Telescope Flight Investigations programme (now JWST) via WBS 411672.07.05.05.03.02. T.J.B. also acknowledges funding support from STScI provided with GO 3077 through JWST-GO-03077.015-A. C.D. and Z.H. acknowledge funding support from NASA Exoplanets Research Program (grant number 80NSSC23K1115), the Space Telescope Science Institute (GO 3077; JWST-GO-03077.019-A) and the Alfred P. Sloan Research Fellowship. X.L. and D.D.B.K. acknowledge support from the National Natural Science Foundation of China (NSFC) under grants 12473064 and 42250410318. M.T. acknowledges support from the Tremplin 2022 programme of the Faculty of Science and Engineering of Sorbonne University. M.T., A.M. and J.L. thank the generic planetary climate model team for the teamwork development and improvement of the model. M.T. acknowledges support from the high-performance-computing resources of Centre Informatique National de l’Enseignement Supérieur (CINES) under the allocations numbers A0120110391 and A0140110391 made by Grand Èquipement National de Calcul Intensif (GENCI). V.S.M., A.P.L. and E.A. are part of the Virtual Planetary Laboratory team, which is a member of the NASA Nexus for Exoplanet System Science, and financed through NASA Astrobiology Program grant 80NSSC18K0829. V.S.M. and A.P.L. made use of the advanced computational, storage and networking infrastructure provided by the Hyak supercomputer system at the University of Washington. B.-O.D. acknowledges support from the Swiss State Secretariat for Education, Research and Innovation (SERI) under contract number MB22.00046. A.R. and E.B. acknowledge the financial support of the SNSF (grant numbers 200021_197176 and 200020_215760). This work has been carried out within the framework of the NCCR PlanetS supported by the Swiss National Science Foundation under grants 51NF40_182901 and 51NF40_205606. The N-body computations including tides with Posidonius were performed at the University of Geneva on the Baobab and Yggdrasil clusters. The GO 3077 team offers a special thanks to S. Kendrew and the JWST Science Operations’ Planning and Scheduling Team at the Space Telescope Science Institute for their dedicated efforts to optimize and schedule the long MIRI observation that made the entirety of this work possible.

Author information

Author notes

  1. These authors contributed equally: Michaël Gillon, Elsa Ducrot.

Authors and Affiliations

  1. Astrobiology Research Unit, Université de Liège, Liège, Belgium

    Michaël Gillon, Elsa Ducrot & Laetitia Delrez

  2. LIRA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, Meudon, France

    Elsa Ducrot & Benjamin Charnay

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

    Elsa Ducrot, Achrène Dyrek & Pierre-Olivier Lagage

  4. Bay Area Environmental Research Institute, NASA Ames Research Center, Moffett Field, CA, USA

    Taylor J. Bell

  5. Space Science and Astrobiology Division, NASA Ames Research Center, Moffett Field, CA, USA

    Taylor J. Bell & Thomas P. Greene

  6. AURA for the European Space Agency (ESA), Space Telescope Science Institute, Baltimore, MD, USA

    Taylor J. Bell

  7. Department of Astronomy, Boston University, Boston, MA, USA

    Ziyu Huang & Chuanfei Dong

  8. Department of Astronomy and NASA Nexus for Exoplanet System Science, Virtual Planetary Laboratory Team, University of Washington, Seattle, WA, USA

    Andrew P. Lincowski, Eric Agol & Victoria S. Meadows

  9. Eastern Wyoming College, Torrington, WY, USA

    Andrew P. Lincowski

  10. Department of Atmospheric and Oceanic Sciences, Peking University, Beijing, China

    Xintong Lyu & Daniel D. B. Koll

  11. Sorbonne Universités, UPMC Université Paris 6 et CNRS, UMR 7095, Institut d’Astrophysique de Paris, Paris, France

    Alice Maurel

  12. Laboratoire de Météorologie Dynamique/IPSL, CNRS, Sorbonne Université, Ecole Normale Supérieure, Université PSL, Ecole Polytechnique, Institut Polytechnique de Paris, Paris, France

    Alice Maurel & Martin Turbet

  13. Département d’astronomie de l’Université de Genève, Chemin Pegasi, Sauverny, Switzerland

    Alexandre Revol & Émeline Bolmont

  14. Centre sur la Vie dans l’Univers, Université de Genève, Geneva, Switzerland

    Alexandre Revol & Émeline Bolmont

  15. NASA Goddard Space Flight Center, Greenbelt, MD, USA

    Thomas J. Fauchez & Aishwarya Iyer

  16. Sellers Exoplanet Environment Collaboration (SEEC), NASA Goddard Space Flight Center, Greenbelt, MD, USA

    Thomas J. Fauchez

  17. Integrated Space Science and Technology Institute, Department of Physics, American University, Washington DC, USA

    Thomas J. Fauchez

  18. Laboratoire d’astrophysique de Bordeaux, Université de Bordeaux, CNRS, B18N, Pessac, France

    Jérémy Leconte, Franck Selsis & Martin Turbet

  19. Center for Space and Habitability, University of Bern, Bern, Switzerland

    Brice-Olivier Demory

  20. Space Research and Planetary Sciences, Physics Institute, University of Bern, Bern, Switzerland

    Brice-Olivier Demory

  21. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA

    Aaron Householder, David Berardo & Julien de Wit

  22. Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA, USA

    Aaron Householder

  23. Center for Astrophysics, Harvard & Smithsonian, Cambridge, MA, USA

    Sebastian Zieba

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

    Billy Edwards

  25. IPAC, Caltech, Pasadena, CA, USA

    Thomas P. Greene

  26. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

    Renyu Hu

  27. Department of Astronomy & Astrophysics, The Pennsylvania State University, University Park, PA, USA

    Renyu Hu

  28. Center for Exoplanets and Habitable Worlds, The Pennsylvania State University, University Park, PA, USA

    Renyu Hu

  29. Institute for Computational and Data Science, The Pennsylvania State University, University Park, PA, USA

    Renyu Hu

  30. Institut für Planetenforschung, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Berlin, Germany

    Nicolas Iro

  31. Max Planck Institut für Astronomie, Heidelberg, Germany

    Laura Kreidberg

  32. Johns Hopkins Applied Physics Laboratory, Laurel, MD, USA

    Jacob Lustig-Yaeger

Authors
  1. Michaël Gillon
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  2. Elsa Ducrot
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  3. Taylor J. Bell
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  4. Ziyu Huang
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  5. Andrew P. Lincowski
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  6. Xintong Lyu
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  7. Alice Maurel
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  8. Alexandre Revol
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  9. Eric Agol
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  10. Émeline Bolmont
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  11. Chuanfei Dong
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  12. Thomas J. Fauchez
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  13. Daniel D. B. Koll
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  14. Jérémy Leconte
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  15. Victoria S. Meadows
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  16. Franck Selsis
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  17. Martin Turbet
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  18. Benjamin Charnay
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  19. Laetitia Delrez
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  20. Brice-Olivier Demory
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  21. Aaron Householder
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  22. Sebastian Zieba
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  23. David Berardo
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  24. Achrène Dyrek
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  25. Billy Edwards
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  26. Julien de Wit
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  27. Thomas P. Greene
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  28. Renyu Hu
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  29. Nicolas Iro
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  30. Laura Kreidberg
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  31. Pierre-Olivier Lagage
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  32. Jacob Lustig-Yaeger
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  33. Aishwarya Iyer
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Contributions

M.G. and E.D. co-led the project. Both M.G. and E.D. wrote the JWST proposal of programme 3077, interacted with STScI to set up observations, performed an analysis of the data and wrote an important part of the paper. T.J.B. contributed one of the independent analyses using the Eureka! pipeline and contributed substantially to the writing and editing of the paper. X.L. and D.D.B.K. designed the strategy and experiments for the bare-rock models and space weathering simulations of TRAPPIST-1 b and TRAPPIST-1 c, and wrote the corresponding parts of the paper. V.S.M. and A.P.L. designed the strategy and experiments for day–night climate–photochemical spectral modelling of the TRAPPIST-1 b and TRAPPIST-1 c planets. A.P.L. conducted the day–night climate–photochemical simulations, and prepared the associated secondary eclipse spectra and phase curves of TRAPPIST-1 b and TRAPPIST-1 c. V.S.M. and A.P.L. interpreted the results and wrote the corresponding parts of the paper. E.A., A.P.L., V.S.M. and J.L-Y. developed the calculation of the confidence of phase-curve model fits to the data. A.M. and M.T. performed the 3D GCM simulations of TRAPPIST-1 b and TRAPPIST-1 c for the JWST proposal of programme 3077 and the present paper. A.M., M.T., T.J.F. and J.L. computed the associated eclipses, phase curves and transit spectra for the proposal and the present paper. A.M., M.T., F.S., J.L. and T.J.F. designed the strategy for 3D climate modelling of the TRAPPIST-1 b and TRAPPIST-1 c planets, and wrote the corresponding parts of the paper. A.H and D.B performed a data reduction and analysis of the photometric time series. B.-O.D. conducted an independent reduction of the data and analysed the photometric time series. T.P.G. contributed to the design of the observations and contributed the JWST programme 1177 observations used in the joint analyses. Z.H. and C.D. performed an independent data reduction and analysis of the JWST/MIRI data and wrote the corresponding part of the paper. A.R. performed the N-body simulations of TRAPPIST-1 including tides with Posidonius to estimate the rotation of the planets. A.R. and E.B. performed estimations of the tidal heating that planets b and c are experiencing, within the framework of a realistic dissipation response. A.R., E.B. and F.S. wrote the corresponding part of the paper. B.C., L.D., S.Z., A.D., B.E., J.d.W., R.H., N.I., L.K., P.-O.L. and A.I. contributed to the writing of the observing proposal and to the writing and critical review of the paper.

Corresponding authors

Correspondence to Michaël Gillon or Elsa Ducrot.

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The authors declare no competing interests.

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Nature Astronomy thanks Mark Hammond 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 Program 3077 raw light curve and external parameters evolution.

a, raw light curve obtained by MG with a photometric aperture of 4 pixels radius (see text for details). b, evolution of the external parameters: x and y positions of the target’s point-spread function (PSF) center, background, PSF’s full-width at halfmaximum in the x and y directions, and noise-pixel (as defined in Deming et al. 2015). Blue points are those discarded before the MCMC analysis. The vertical red line shows the gap between visit 1 and 2.

Extended Data Fig. 2 Flares in the light curve of program 3077.

a, double flare-like structure in the light curve of program 3077. The black points are flux measurements binned per 7.2 minutes. b, Stack of the four low-amplitude flare-like structures in the residuals of MG’s analysis #1, binned per 7.2 minutes. For both panels, the error bars are the averages of the individual errors divided by the square root of the number of points within the bin, and the best-fit flare models are overimposed in red.

Extended Data Fig. 3 Additional transit-like structure in the light curve of Program 3077.

a, detrended 3077 program light curve obtained by MG in a version of analysis #1 not assuming the transit of a putative planet i, with the best-fit planet model deduced from the same analysis superimposed in red. b, residuals of that version of analysis #1. c, modified Allan diagram showing the ratio of the measured standard deviations of the residuals of the best-fit model and of the one expected for a fully white noise as a function of the sampling (that is for diXerent binnings), without (black) and with the inclusion of a low-amplitude transit in the global model (green). The red line shows what the ratio should be for a fully white noise, that is 1. d, Best-fit phase curve for planet b from that version of analysis #1. The data points are phase-folded and corrected from the systematics and the contribution of the other planets. The best-fit phase curve model is superimposed in red. e, same for planet c. For panels a, b, d and e, the data points are binned per 60 minutes, each binned point is the average value of the individual points within the bin, and the error bar is the average of the individual errors divided by the square root of the number of points within the bin. The purple arrows show the location of the putative transit of a putative planet i.

Extended Data Fig. 4 Transit spectra and key gas profiles of 1.5D atmospheric models that fit the TRAPPIST-1 b phase curve measurements within 3σ.

All features shown here are significantly smaller than the 1σ( ~ 100ppm) error bars from currently observed spectra, due to the stellar source noise issues. Shown are model transit spectra and gas profiles for TRAPPIST-1 b. The zero point for the transit depth is the planetary surface.

Extended Data Fig. 5 Emission and transmission spectra of TRAPPIST-1b.

Upper panel: Occultation depth measurements compared to models of TRAPPIST-1 b occultation depth spectra computed for five distinct GCM simulations along with a low albedo, airless planet case. Data points (in red) were taken from Greene et al. (2023) and Ducrot et al. (2024). The six spectra have been selected here for their good agreement with the observed occultations at 12.8 and 15 μm. Lower panel: Transit spectra computed for the TRAPPIST-1b atmospheric scenarios fitting all available infrared MIRI data points (0.01 bar N2 with 1 ppm of CO2 and Haze high cases). The transit spectrum has been offset by 103 ppm for the Haze high case so that the minimum value is zero. For comparison, we added the transmission spectrum from Lim et al. (2023), not corrected from stellar contamination, with an offset of 7150 ppm.

Extended Data Fig. 6 TRAPPIST-1 b’s dayside spectrum for different geologically fresh surface materials (left) and a space weathered basalt surface (right).

Left: Models for the planet’s dayside emission spectrum based on geologically fresh materials (curves), compared to the contrast ratio at 15μm from this work (black point) and the reported contrast ratio at 12.8μm (from Ducrot et al. 2024). Right: Space weathering darkens the surface and could improve the match to the observed flux at 15μm. Progressively darker curves correspond to increasing space weathering via the addition of graphite.

Extended Data Fig. 7 TRAPPIST-1 b’s phase curve for different geologically fresh surface materials (left) and a space weathered basalt surface (right) compared to data from MG Analysis #2.

Left: Models for the planet’s dayside emission spectrum based on geologically fresh materials (curves), compared to the 15μm phase curve from this work (black point). An ultramafic is preferred over all other fresh materials shown here. Right: Space weathering reduces the planet’s albedo at short wavelengths and increases the phase curve amplitude at 15 μm.

Extended Data Table 1 Prior probability distribution functions (PDFs) used in the MCMC analyses of MG
Full size table

Supplementary information

Supplementary Information (download PDF )

Supplementary Figs. 1–17 and Tables 1–8.

Source data

Source Data Fig. 1 (download CSV )

Photometry (binned and unbinned) + model for Fig. 1.

Source Data Fig. 2 (download CSV )

Occultation depth measurements posterior distribution functions for planets b and c, for Fig. 2.

Source Data Fig. 3 (download CSV )

Photometry + model for Fig. 3.

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Gillon, M., Ducrot, E., Bell, T.J. et al. No thick atmosphere around TRAPPIST-1 b and c from JWST thermal phase curves. Nat Astron (2026). https://doi.org/10.1038/s41550-026-02806-9

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