Diversity In The Haziness And Chemistry Of Temperate Sub-Neptunes

Recent transit observations of K2-18b and TOI-270d revealed strong molecular absorption signatures, lending credence to the idea that temperate sub-Neptunes (T_eq=250-400K) 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 0.7-5.4μm transmission spectrum of a third temperate sub-Neptune, the 2.4R_⊕ planet LP 791-18c (T_eq=355K), which is even more favorable for atmospheric characterization thanks to its small M6 host star. Intriguingly, despite LP 791-18c’s radius, mass, and equilibrium temperature being in between those of K2-18b and TOI-270d, we find a drastically different transmission spectrum.

While we also detect methane on LP 791-18c, 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×solar) for LP 791-18c, with a CO₂-to-CH₄ ratio smaller than 0.07 (at 2σ), indicating less H₂O in the deep envelope of LP 791-18c 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.

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 lightcurve 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 bestfitting 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. — astro-ph.EP

Pierre-Alexis Roy, Björn Benneke, Marylou Fournier-Tondreau, Louis-Philippe Coulombe, Caroline Piaulet-Ghorayeb, David Lafrenière, Romain Allart, Nicolas B. Cowan, Lisa Dang, Doug Johnstone, Adam B. Langeveld, Stefan Pelletier, Michael Radica, Jake Taylor, Loïc Albert, René Doyon, Laura Flagg, Ray Jayawardhana, Ryan J. MacDonald, Jake D. Turner

Comments: Paper published in Nature Astronomy at this https URL. This preprint is the original submitted version before peer-review
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2512.10876 [astro-ph.EP] (or arXiv:2512.10876v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2512.10876
Focus to learn more
Related DOI:
https://doi.org/10.1038/s41550-025-02723-3
Focus to learn more
Submission history
From: Pierre-Alexis Roy
[v1] Thu, 11 Dec 2025 18:04:43 UTC (8,382 KB)
https://arxiv.org/abs/2512.10876
Astrobiology, Exoplanet,