Hydrocarbon Hazes on Temperate Sub-Neptune K2-18b Supported by Data from the James Webb Space Telescope
Abstract
K2-18b, a sub-Neptune orbiting in the habitable zone of an M dwarf, has attracted significant interest following observations with the Hubble Space Telescope and, more recently, with the James Webb Space Telescope (JWST), which reveal detectable atmospheric features across the near- and mid-infrared. Using free-chemistry Bayesian retrievals, we investigate whether hydrocarbon hazes can explain the apparent mismatch of spectral feature amplitudes between the JWST/NIRISS and NIRSpec and MIRI low-resolution spectrograph datasets. We additionally assess the impact of stellar parameter uncertainties on the derived bulk properties of the planet and explore how planetary mass uncertainties affect atmospheric retrievals. We find that hazy scenarios can reproduce the combined JWST spectrum and provide a consistent explanation for the reduced NIRISS/NIRSpec feature amplitudes relative to the stronger MIRI features. Across all retrievals, the atmosphere remains consistent with an H 2 -dominated sub-Neptune, with CH 4 and CO 2 as the dominant absorbers. Our hazy models retrieve systematically lower molecular abundances compared to haze-free models, reflecting the degeneracy between haze opacity and mean molecular weight. In addition, we identify strong degeneracies between planetary mass, temperature, and mean molecular weight. The retrieved planetary mass is particularly poorly constrained, with 2 σ uncertainties reaching up to ∼71%. We demonstrate that different mass assumptions can significantly bias the inferred atmospheric properties, with higher masses favouring warmer and lower-mean-molecular-weight atmospheres. Breaking these degeneracies will require improved stellar characterization to obtain more precise mass measurements. More laboratory-focused studies and future JWST observations are essential for interpreting these temperate sub-Neptune atmospheres.
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BibTeX
@article{liu2026hydrocarbon,
title = {{Hydrocarbon Hazes on Temperate Sub-Neptune K2-18b Supported by Data from the James Webb Space Telescope}},
author = {Ruohan Liu and Panayotis Lavvas and Giovanna Tinetti and Jesus Maldonado and Sushuang Ma and Arianna Saba},
journal = {The Astronomical Journal},
year = {2026},
month = jul,
doi = {10.3847/1538-3881/ae7a67},
url = {https://univ-reims.hal.science/hal-05835382v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Hydrocarbon Hazes on Temperate Sub-Neptune K2-18b Supported by Data from the James Webb Space Telescope
AU - Ruohan Liu
AU - Panayotis Lavvas
AU - Giovanna Tinetti
AU - Jesus Maldonado
AU - Sushuang Ma
AU - Arianna Saba
PY - 2026
DA - 2026/07/02
JO - The Astronomical Journal
DO - 10.3847/1538-3881/ae7a67
UR - https://univ-reims.hal.science/hal-05835382v1
AB - K2-18b, a sub-Neptune orbiting in the habitable zone of an M dwarf, has attracted significant interest following observations with the Hubble Space Telescope and, more recently, with the James Webb Space Telescope (JWST), which reveal detectable atmospheric features across the near- and mid-infrared. Using free-chemistry Bayesian retrievals, we investigate whether hydrocarbon hazes can explain the apparent mismatch of spectral feature amplitudes between the JWST/NIRISS and NIRSpec and MIRI low-resolution spectrograph datasets. We additionally assess the impact of stellar parameter uncertainties on the derived bulk properties of the planet and explore how planetary mass uncertainties affect atmospheric retrievals. We find that hazy scenarios can reproduce the combined JWST spectrum and provide a consistent explanation for the reduced NIRISS/NIRSpec feature amplitudes relative to the stronger MIRI features. Across all retrievals, the atmosphere remains consistent with an H 2 -dominated sub-Neptune, with CH 4 and CO 2 as the dominant absorbers. Our hazy models retrieve systematically lower molecular abundances compared to haze-free models, reflecting the degeneracy between haze opacity and mean molecular weight. In addition, we identify strong degeneracies between planetary mass, temperature, and mean molecular weight. The retrieved planetary mass is particularly poorly constrained, with 2 σ uncertainties reaching up to ∼71%. We demonstrate that different mass assumptions can significantly bias the inferred atmospheric properties, with higher masses favouring warmer and lower-mean-molecular-weight atmospheres. Breaking these degeneracies will require improved stellar characterization to obtain more precise mass measurements. More laboratory-focused studies and future JWST observations are essential for interpreting these temperate sub-Neptune atmospheres.
SN - 0004-6256
SN - 1538-3881
ER -