Investigating aerosols as a way to reconcile K2-18 b JWST MIRI and NIRISS/NIRSpec observations

TL;DR

This work tackles the apparent tension between JWST NIRISS/NIRSpec and MIRI transmission spectra of the temperate sub-Neptune K2-18 b. Using TauREx 3 retrievals that incorporate free chemistry, non-equilibrium chemistry, and aerosol models with laboratory-derived haze refractive indices, the authors test whether photochemical hazes can reconcile the two datasets. They find that CH4-dominated, nitrogen-poor tholins (exo1) can explain the NIRISS slope through scattering and the MIRI 7 μm absorption feature, while simultaneously reducing the inferred metallicity from $19.63$ to $0.29$ and lowering CH4 abundance from $6.3\times 10^{-2}$ to $3.0\times 10^{-4}$, illustrating strong degeneracies between metallicity, composition, and aerosol properties. However, none of the models are decisively preferred over a flat spectrum for the combined data, underscoring the importance of aerosol absorption in temperate sub-Neptune atmospheres and the need for further JWST observations and laboratory haze measurements to break these degeneracies.

Abstract

Recent JWST observations of the temperate sub-Neptune K2-18 b with NIRISS SOSS/NIRSpec G395H and MIRI LRS have yielded apparently inconsistent results: the MIRI spectra exhibit spectral features nearly twice as large as those seen at shorter wavelengths, challenging the high-metallicity, CH4-rich non-equilibrium model that fits the NIRISS/NIRSpec data. We perform a suite of atmospheric retrievals on both datasets, including free-chemistry, non-equilibrium, and aerosol models, using laboratory-derived complex refractive indices for a variety of photochemical haze analogues. Free retrievals systematically return lower metallicities than inferred by self-consistent chemical disequilibrium models, and the inclusion of absorbing aerosols, especially CH4-dominated, nitrogen-poor tholins, can further reduce the inferred metallicity by over an order of magnitude. These hazes reproduce the observed NIRISS slope through scattering and match MIRI features via C-H bending absorption near 7 um, while yielding particle properties consistent with photochemical production in H2-rich atmospheres. Although their inclusion improves the joint fit and reduces tension between datasets, it also significantly lowers the retrieved CH4 abundance, highlighting degeneracies between metallicity, composition, and aerosol properties. Our results underscore the importance of aerosol absorption in interpreting temperate sub-Neptune spectra, and motivate future JWST observations and laboratory work to break these degeneracies.

Figures (8)

• Figure 1: Combined spectra of NIRISS SOSS/NIRSpec G395H low resolution Madhusudhan2023 and MIRI LRS Eureka! data reduction Luque2025 (error bars) are shown alongside the retrieved tholins contributions (solid lines) of the different tholins data for each corresponding combine retrieval.
• Figure 2: The $\chi^2$ of the best non-equilibrium model from Jaziri2025 is shown as a function of the MIRI LRS wavelength shift, based on the combined JWST observations from NIRISS SOSS, NIRSpec G395H Madhusudhan2023, and MIRI LRS Luque2025. Blue cross markers indicate the computed values, and a second-order polynomial fit is shown as a black solid line, with its minimum value (160 ppm) marked by a black dot.
• Figure 3: Spectrum from the MIRI LRS Eureka! data reduction Luque2025 (error bars) is compared to the best non-equilibrium model from Jaziri2025 (black solid line), which fits the NIRISS SOSS/NIRSpec G395H observations. They are also compared to the two MIRI LRS non-equilibrium retrieval solutions (light black solid line for low metallicity and blue solid line for high metallicity), as well as to the best MIRI LRS model (light blue solid line), all shown at a resolution of 200. Key spectral features are highlighted in their respective colors.
• Figure 4: Corner plot of the non-equilibrium retrieval model with FRECKLL on the MIRI LRS Eureka! data reduction Luque2025.
• Figure 5: Combined spectra of NIRISS SOSS/NIRSpec G395H low resolution Madhusudhan2023 and MIRI LRS Eureka! data reduction Luque2025 (error bars) are shown alongside the best model (black solid line) and its contributions. It correspond to the retrieval including tholins exo1.