Competing chemical signatures in the atmosphere of TOI-270 d: Inference of sulfur and carbon chemistry

Context . Recent JWST measurements allow access to the near-infrared spectrum of the sub-Neptune TOI-270 d, for which two different interpretations, a high-metallicity miscible envelope and a lower metallicity hycean world, are currently in conflict. Aims . Here, we reanalyze published data and reproduce previously retrieved molecular abundances based on an independent data reduction and a different retrieval framework. The aim of this study is to refine the understanding of TOI-270 d and highlight the impact of various choices during JWST data analysis. Particularly, we test the impact of data resolution on atmospheric retrieval calculations. Methods . We reduced one JWST NIRSpec G395H and one NIRISS SOSS GR700XD transit dataset using the Eureka! pipeline and a custom Markov Chain Monte Carlo-based light curve fitting algorithm at the instruments’ native resolutions. The atmospheric composition was estimated with the updated B E AR retrieval code across a grid of retrieval setups and spectral resolutions. Results . Our transit spectrum is consistent with previous studies except at the red end of the NIRISS data. Our retrievals support a H 2 /He-dominated atmosphere with high mean molecular weight for TOI-270 d. We provide refined abundance constraints and find statistically favored model extensions indicating either sulfur-rich chemistry with species such as CS 2 , CS, and H 2 CS or the possible presence of CH 3 Cl or CH 3 F. However, Bayesian inference cannot distinguish between these scenarios due to similar opacities below 4 µm. To obtain physically plausible atmospheric solutions at native resolution, accounting for the instrument’s line spread function is essential. Conclusions . Our analysis reinforces TOI-270 d as a highly interesting warm sub-Neptune for atmospheric studies, with a complex chemistry in a cloud-free upper atmosphere. However, its exact nature remains uncertain and warrants further detailed photochemical modeling and observations.