2025AJ....170..257S

K2-18b, a temperate sub-Neptune, has garnered significant attention due to claims of possible biosignatures in its atmosphere. Low-confidence detections of dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS) have sparked considerable debate, primarily around arguments that their absorption features are not uniquely identifiable. Here, we consider all five questions from the astrobiology standards of evidence framework, starting with the following: Have we detected an authentic signal? To answer this, we analyzed publicly available JWST observations of K2-18b using independent data reduction and spectral retrieval methodologies. Our comprehensive set of reductions demonstrates that the MIRI transit spectrum is highly susceptible to unresolved instrumental systematics. Applying different wavelength binning schemes yields a potpourri of planet spectra that then lead to a wide assortment of atmospheric interpretations. Consequently, we offer recommendations to help minimize this previously underappreciated instrument systematic in future MIRI reductions of any exoplanet. While the MIRI binning scheme adopted by N. Madhusudhan et al. (2025) favors the presence of DMS/DMDS in K2-18b, we find that 87.5% of retrievals using our preferred MIRI binning scheme do not. When considering the full 0.7-12 μm transit spectrum, we confirm the detection of CH₄ and favor CO₂ and find the presence of DMS and C₂H₄ to be interchangeable. Moreover, we find that the tentative presence of large features in the MIRI transit spectrum is in tension with the more robust, yet smaller, features observed in the near-IR. We conclude that red noise-rather than an astrophysical signal-plagues the mid-IR data, and there is, as yet, no statistically significant evidence for biosignatures in the atmosphere of K2-18b.