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Diversity in the haziness and chemistry of temperate sub-Neptunes

Nature Astronomy volume 10, pages 371–384 (2026) Cite this article

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Abstract

Recent transit observations of K2-18 b and TOI-270 d revealed strong molecular absorption signatures, lending credence to the idea that temperate sub-Neptunes (equilibrium temperature Teq = 250–400 K) have upper atmospheres mostly free of aerosols. These observations also indicated higher-than-expected CO2 abundances on both planets, implying bulk compositions with high water mass fractions. However, it remains unclear whether these findings hold true for all temperate sub-Neptunes. Here we present the JWST NIRSpec/PRISM 0.7–5.4-μm transmission spectrum of a third temperate sub-Neptune, the 2.4 R⊕ planet LP 791-18 c (Teq = 355 K), which is even more favourable for atmospheric characterization thanks to its small M6 host star. Intriguingly, despite the radius, mass and equilibrium temperature of LP 791-18 c being between those of K2-18 b and TOI-270 d, we find a drastically different transmission spectrum. Although we also detect methane on LP 791-18 c, its transit spectrum is dominated by strong haze scattering and there is no discernible CO2 absorption. Overall, we infer a deep metal-enriched atmosphere (246–415 times solar) for LP 791-18 c, with a CO2-to-CH4 ratio smaller than 0.07 (at 2σ), indicating less H2O in the deep envelope of LP 791-18 c and implying a relatively dry formation inside the water-ice line. These results show that sub-Neptunes that are near analogues in density and temperature can show drastically different aerosols and envelope chemistry and are intrinsically diverse beyond a simple temperature dependence.

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Fig. 1: JWST/NIRSpec PRISM broadband and spectrophotometric light-curve fits of the LP 791-18 c transit observation.
The alternative text for this image may have been generated using AI.
Fig. 2: JWST NIRSpec/PRISM transmission spectra of the temperate sub-Neptune LP 791-18 c.
The alternative text for this image may have been generated using AI.
Fig. 3: Measured atmospheric properties of LP 791-18 c.
The alternative text for this image may have been generated using AI.
Fig. 4: Diversity in sub-Neptune transmission spectra.
The alternative text for this image may have been generated using AI.

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Data availability

The data used in this work are publicly available in the Mikulski Archive for Space Telescopes (https://archive.stsci.edu/) under guaranteed time observation 1201 (principal investigator D.L.) and can be easily accessed via https://doi.org/10.17909/c8z7-r590. Data products needed to reproduce all figures are also available via Zenodo at https://doi.org/10.5281/zenodo.17316562 (ref. 92).

Code availability

The code needed to reproduce all figures is available via Zenodo at https://doi.org/10.5281/zenodo.17316562 (ref. 92). This research made use of the Astropy93,94,95, Matplotlib96, NumPy97 and SciPy98 Python packages. The open-source codes that were used throughout this work are listed below: batman (https://github.com/lkreidberg/batman), emcee (https://emcee.readthedocs.io/en/stable/), nestle (http://kylebarbary.com/nestle/), Eureka! (https://eurekadocs.readthedocs.io/en/latest/), ExoTEDRF (https://exotedrf.readthedocs.io/en/latest/) and VULCAN (https://github.com/exoclime/VULCAN).

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Acknowledgements

P.-A.R. and B.B. acknowledge financial support from the Natural Sciences and Engineering Research Council (NSERC) of Canada and from the Canadian Space Agency (Grant No. 23JWGO2A05). P.-A.R. and L.-P.C. acknowledge support from the University of Montreal and the Trottier Institute for Research on Exoplanets. B.B. also acknowledges financial support from the Fond de Recherche Québécois-Nature et Technologie. M.F.-T. acknowledges financial support from the Clarendon Fund Scholarship and the Fonds de Recherche du Québec–Nature et technologies. R.A. acknowledges support from the Swiss National Science Foundation (SNSF) under the Post-Doc Mobility grant P500PT_222212 and the support of the Institut Trottier de Recherche sur les Exoplanètes. N.B.C. acknowledges support from an NSERC Discovery Grant, a Tier 2 Canada Research Chair and an Arthur B. McDonald Fellowship. We thank the Trottier Space Institute and the Trottier Institute for Research on Exoplanets for their financial support and dynamic intellectual environment. L.D. is a Banting and Trottier Postdoctoral Fellow and acknowledges support from NSERC and the Trottier Family Foundation. D.J. is supported by NRC Canada and by an NSERC Discovery Grant. This project has been carried out within the framework of the National Centre of Competence in Research Planets supported by the SNSF (Grant No. 51NF40_205606). S.P. acknowledges financial support from the SNSF. R.J.M. is supported by NASA through the NASA Hubble Fellowship (Grant No. HST-HF2-51513.001), also awarded by the STScI, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA (Contract NAS 5-26555). J.T. acknowledges funding support from the TESS Guest Investigator Program G06165. C.P.-G. is a E. Margaret Burbridge Postdoctoral Fellow. R.A. is an SNSF Postdoctoral Fellow.

Author information

Authors and Affiliations

  1. Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, Los Angeles, CA, USA

    Pierre-Alexis Roy & Björn Benneke

  2. Department of Physics and Trottier Institute for Research on Exoplanets, Université de Montréal, Montreal, Quebec, Canada

    Pierre-Alexis Roy, Björn Benneke, Marylou Fournier-Tondreau, Louis-Philippe Coulombe, Caroline Piaulet-Ghorayeb, David Lafrenière, Romain Allart, Lisa Dang, Stefan Pelletier, Michael Radica, Loïc Albert & René Doyon

  3. Department of Physics, University of Oxford, Oxford, UK

    Marylou Fournier-Tondreau & Jake Taylor

  4. Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL, USA

    Caroline Piaulet-Ghorayeb & Michael Radica

  5. Department of Physics, McGill University, Montreal, Quebec, Canada

    Nicolas B. Cowan

  6. Department of Earth & Planetary Sciences, McGill University, Montreal, Quebec, Canada

    Nicolas B. Cowan

  7. Department of Physics and Astronomy and Waterloo Centre for Astrophysics, University of Waterloo, Waterloo, Ontario, Canada

    Lisa Dang

  8. NRC Herzberg Astronomy and Astrophysics, Victoria, British Columbia, Canada

    Doug Johnstone

  9. Department of Physics and Astronomy, University of Victoria, Victoria, British Columbia, Canada

    Doug Johnstone

  10. Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD, USA

    Adam B. Langeveld & Ray Jayawardhana

  11. Department of Astronomy and Carl Sagan Institute, Cornell University, Ithaca, NY, USA

    Adam B. Langeveld & Jake D. Turner

  12. Observatoire Astronomique de l’Université de Genève, Université de Genève, Versoix, Switzerland

    Stefan Pelletier

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

    Laura Flagg

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

    Laura Flagg

  15. Department of Astronomy, University of Michigan, Ann Arbor, MI, USA

    Ryan J. MacDonald

  16. School of Physics and Astronomy, University of St Andrews, St Andrews, UK

    Ryan J. MacDonald

Authors
  1. Pierre-Alexis Roy
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  2. Björn Benneke
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  3. Marylou Fournier-Tondreau
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  4. Louis-Philippe Coulombe
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  5. Caroline Piaulet-Ghorayeb
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  6. David Lafrenière
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  7. Romain Allart
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  8. Nicolas B. Cowan
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  9. Lisa Dang
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  10. Doug Johnstone
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  11. Adam B. Langeveld
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  12. Stefan Pelletier
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  13. Michael Radica
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  14. Jake Taylor
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  15. Loïc Albert
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  16. René Doyon
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  17. Laura Flagg
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  18. Ray Jayawardhana
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  19. Ryan J. MacDonald
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  20. Jake D. Turner
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Contributions

P.-A.R. and B.B. led the writing of this paper. P.-A.R. carried out the data reduction. P.-A.R. performed the light-curve fitting with contributions from M.F.-T. The atmospheric analyses were performed by P.-A.R. C.P.-G. created the grid of stellar models used for the TLS analyses and provided important help and expertise regarding data reduction and TLS retrievals. B.B. supervised P.-A.R. in this project. D.L. led the observation programme and planned the observations presented in this work. All co-authors provided comments on and suggestions regarding the paper.

Corresponding author

Correspondence to Pierre-Alexis Roy.

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Peer review

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Nature Astronomy thanks the anonymous reviewers for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Summary diagnostics measurements for the transit observation of LP 791-18 c.

a. Median-normalized white light curve of the transit observation of LP 791-18 c. The raw observations are shown in light grey with a binned version in black. b. Measured centre of the trace on the NIRSpec detector (light grey) with a binned version in blue. c. Measured width of the trace on the NIRSpec detector (light grey) with a binned version in orange. d. Measured displacement of the trace in the dispersion direction (light grey) with a binned version in green. e. Width of the cross-correlation peak for the measurement of the displacement in the dispersion direction (light grey) with a binned version in purple. f. Integrated Hα flux (in arbitrary flux units) over the time series (light grey) with a binned version in red. All diagnostics measurements are well-behaved.

Extended Data Fig. 2 Effect of different data reduction methods on the transmission spectrum of LP 791-18 c.

a. Zoom on the saturated region of the transmission spectra of the sub-Neptune LP 791-18 c (median values with 1σ error bars) obtained when considering the standard STScI saturation flags (red) or when expanding the flags to previous groups and neighboring pixels (grey). The number of groups (N) used as a function of wavelength is shown by the grey regions (in the standard STScI flags case). b. Transmission spectra of the sub-Neptune LP 791-18 c (median values with 1σ error bars) obtained when running the complete analysis from a standard stage 1 reduction with (blue) and without (orange) the NIN correction step. Residuals in the transit spectrum between the reductions with and without the NIN correction are shown in the bottom panel. The NIN correction step rectifies a wide trend in the transmission spectrum which is most important around 2 μm and decays towards longer wavelengths. The effect is similar to that observed in the case of TRAPPIST-1 g (Benneke et al., under review). c. Transmission spectra of the sub-Neptune LP 791-18 c (median values with 1σ error bars) obtained when using the light curves produced by the Eureka! (orange) and ExoTEDRF (green) reductions. Residuals in the transit spectrum between the Eureka! and ExoTEDRF reductions. The NIN correction is turned off in the Eureka! reduction (to isolate the difference between the two reduction pipelines). Both transmission spectra are in agreement.

Extended Data Fig. 3 Comparison of the treatment of the spot-crossing event.

a. Example light-curve fit using the Gaussian spot model. The top panel shows the systematics-corrected light-curve for the 0.90-0.92 μm bin with the best-fitting transit model with (red) and without (dotted black) the Gaussian spot model. The bottom panel shows the residuals with the best fitting model. The data is binned per 16 second increments for visual clarity. b. White-light-curve fit using the spotrod model. The top panel shows the systematics-corrected white-light curve with the best-fitting model, including the modelled spot crossing event. The bottom panel shows the residuals of the fit. Again, the data is shown in 16 s bins for clarity. c. Graphical representation of the spot crossing event inferred from the spotrod fit shown in b. d. Transmission spectra obtained from both methods using the same R=50 spectroscopic bins. Because of a slightly different set of orbital parameters used in the Spotrod spectroscopic fit, it is offset by 50 ppm. Both spectra are fully consistent within their displayed 1σ error bars, and show no systematic discrepant trends.

Extended Data Fig. 4 Parameter inference from the free chemistry retrieval.

Joint and marginalized posterior distributions of the atmosphere parameters obtained from the free chemistry retrieval.

Extended Data Fig. 5 Parameter inference from the chemically consistent retrieval with stellar contamination.

Joint and marginalized posterior distributions of the atmosphere parameters obtained from the chemically consistent retrieval with stellar spots contamination.

Extended Data Fig. 6 Calibrated stellar spectrum of LP791-18c and stellar models used for the TLS analyses.

a. Observed and flux-calibrated spectral energy distribution of LP 791-18 c (median values with 1σ error bars, the error bars are too small to see) compared to a Sphinx model (blue) and a Phoenix model (orange) adopting the stellar properties of LP 791-18. The Phoenix model is taken directly from the grid of Phoenix models used when evaluating the TLS contamination in the atmosphere retrievals. We find good overall agreement between the recovered stellar spectrum and the models. b. Residuals from the spectra shown in panel a. The overall agreement is good, but there are some discrepancies in the saturated region (grey) and at short wavelengths. c. Same as in panel a, but now showing the range of stellar models considered for the spots and faculae contamination in the atmosphere retrievals. The models used to assess TLS effects surround the observed SED of LP 791-18 (and largely cover the saturated/short-wavelength discrepancies between the observations and the models) and thus provide a reliable assessment of stellar contamination.

Extended Data Fig. 7 Grid exploration of the miscible envelope sub-Neptune scenario for LP791-18c.

a. Carbon dioxide-to-methane abundance ratio at 10 mbar for a grid of convective-radiative SCARLET models of LP 791-18 c for which vertical mixing was modelled using the VULCAN framework with a Kzz of 104 cm2/s. The colorbar is cut at log10CO2/CH4 values of +2.5 and − 2.5 in order to help visualize the transition from methane dominated to carbon dioxide dominated regimes. The 2σ upper bound on the carbon dioxide-to-methane abundance ratio derived from the free chemistry retrieval is shown as the black line and arrows. b. Mixing ratios of a sample model of the grid shown on the left for the case of 0.0 Bond albedo and 300 × solar metallicity. The temperature-pressure profile is shown as the black dashed line with the corresponding temperature axis at the top. At the temperature regime of LP 791-18 c, CH4 strongly dominates over CO2 unless the metallicity is increased above 500 × solar.

Extended Data Table 1 Stellar and planetary parameters for LP791-18c used or derived in this work
Full size table
Extended Data Table 2 Bayesian model comparison results from our SCARLET atmosphere retrievals in the free chemistry settings
Full size table
Extended Data Table 3 Measured atmospheric properties of LP791-18c for multiple retrievals
Full size table

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Roy, PA., Benneke, B., Fournier-Tondreau, M. et al. Diversity in the haziness and chemistry of temperate sub-Neptunes. Nat Astron 10, 371–384 (2026). https://doi.org/10.1038/s41550-025-02723-3

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