Hydrocarbon Hazes on Temperate sub-Neptune K2-18b supported by data from the James Webb Space Telescope (2509.10947v1)

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 (HST) and, more recently, with the James Webb Space Telescope (JWST) that reveal detectable atmospheric features. Previous studies have examined a wide range of possible compositions, focusing primarily in the near-infrared (0.8-5.2 $\mu$m) or mid-infrared (5-12 $\mu$m) wavelengths. We present a new interpretation of K2-18b's JWST transit spectra, combining an independent reduction of MIRI LRS data with previously published NIRISS/NIRSpec observations. We assess the impact of stellar parameter uncertainties on the inferred planetary properties and, using revised stellar parameters, derive a planetary density of $\rho_P = 3.34 \pm 1.44$ g cm$^{-3}$. We consider scattering and absorption from laboratory-produced haze analogues and perform free-chemistry Bayesian retrievals informed by equilibrium chemistry. Our results are consistent with an H$_2$-dominated mini-Neptune atmosphere with a mean molecular weight of $\mu \sim$2.4 Daltons, and support the presence of hydrocarbon hazes across 0.85-12 $\mu$m without requiring instrumental offsets. Our retrieved CH$_4$ and CO$_2$ abundances are broadly consistent between models but systematically lower than in haze-free studies, suggesting that haze reduces the need for high-$\mu$ solutions. While our retrievals tend to favour atmospheric temperatures $\sim$100-200 K warmer than previously reported, cooler solutions ($\sim$250 K) remain viable if the planetary mass is reduced towards the lower end of its uncertainty. We emphasise the need for follow-up self-consistent photochemical and microphysical modelling, alongside further mid-infrared observations to constrain key hydrocarbon species.