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 at a >3σ significance across the majority of reduction and retrieval setups. Hints of 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 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 gtrsim500: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.
Fernández-Rodríguez et al. (Tue,) studied this question.