The atmospheric composition of the sub-Neptune K2-18b and insights into its formation

Unlocking the atmospheres of sub-Neptune planets is among the revolutionary accomplishments of JWST. However, such observations require complex data analysis methodologies, which have a strong impact on the derived conclusions. Here, we present an independent re-analysis of the JWST transmission spectrum of the temperate sub-Neptune K2-18 b, aimed at assessing the robustness of previously claimed atmospheric detections, exploring the planet’s possible parameter space, and determining the implications behind its formation. The NIRISS/SOSS and NIRSpec/G395H observations were reduced using a combination of public and customised pipelines. We produced a total of 12 different versions of the transmission spectrum by varying key steps: spectral binning, limb-darkening treatment, and the application of a novel correction for an occulted stellar spot, as well as error inflation and instrumental offsets. We performed atmospheric retrievals using TauREx 3, comparing models of varying complexity. We robustly detected CH 4 at a >3σ significance across the majority of reduction and retrieval setups. Hints of CO 2 appear in most configurations, but the evidence is weak and strongly model-dependent. The tentative detection of dimethyl sulphide, reported in previous studies, vanishes in our most comprehensive retrieval models. We find that correcting the stellar spot in the NIRISS transit is a critical step, introducing a uniform offset that primarily drives the inference of a lighter atmosphere characterised by a lower mean molecular weight. Furthermore, the assumed complexity of the retrieval model itself introduces significant biases; including more molecules systematically increases the retrieved CH 4 abundance and atmospheric mean molecular weight, even for species without spectral features. The data are consistent with a hydrogen-rich (i.e. primordial) atmosphere with an elevated O abundance and an even more elevated C abundance, leading to a C/O ratio that is significantly greater than solar. We show that the physical properties of the K2-18 system planets (i.e. the innermost planet, K2-18 c, and K2-18 b) are consistent with those expected by the in situ formation theory of inside-out planet formation. Furthermore, these properties predict a temperature of K2-18 b at the time of formation of ≳500 K. This is much warmer than the current equilibrium temperature and just interior to the carbon ‘soot’ line, where an elevated C/O ratio and a correspondingly volatile-poor (dry) atmospheric composition is expected to be inherited from the protoplanetary disk, arguing against an Hycean world scenario.