TY - JOUR
T1 - Diversity in the haziness and chemistry of temperate sub-Neptunes
AU - Roy, Pierre-Alexis
AU - Benneke, Björn
AU - Fournier-Tondreau, Marylou
AU - Coulombe, Louis-Philippe
AU - Piaulet-Ghorayeb, Caroline
AU - Lafrenière, David
AU - Allart, Romain
AU - Cowan, Nicolas B.
AU - Dang, Lisa
AU - Johnstone, Doug
AU - Langeveld, Adam B.
AU - Pelletier, Stefan
AU - Radica, Michael
AU - Taylor, Jake
AU - Albert, Loïc
AU - Doyon, René
AU - Flagg, Laura
AU - Jayawardhana, Ray
AU - MacDonald, Ryan J.
AU - Turner, Jake D.
N1 - Funding: 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. 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.
PY - 2025/12/12
Y1 - 2025/12/12
N2 - 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.
AB - 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.
U2 - 10.1038/s41550-025-02723-3
DO - 10.1038/s41550-025-02723-3
M3 - Article
SN - 2397-3366
VL - AOP
JO - Nature Astronomy
JF - Nature Astronomy
ER -