J/AJ/170/257ivo://CDS.VizieR/J/AJ/170/257CDSStevenson K.B.Lustig-Yaeger J.May E.M.Kopparapu R.K.Fauchez T.J.,Haqq-Misra J.Limbach M.A.Schwieterman E.W.Sotzen K.S.Tsai S.-M.2026-07-23T14:58:17Z2026-07-23T15:51:42ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frexoplanetsinfrared-astronomyspectroscopyastronomical-modelsstellar-atmospheresK2-18b, 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 0.7-12um transit spectrum, we confirm the detection of CH_4_ and favor CO_2_ and find the presence of DMS and C_2_H_4_ 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.2025AJ....170..257Shttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/AJ/170/257CatalogResearchIsServedByTAP VizieR generic servicerelated-toV/133 : Kepler Input Catalog (Kepler Mission Team, 2009)IV/34 : K2 Ecliptic Plane Input Catalog (EPIC) (Huber+, 2017)IV/39 : TESS Input Catalog version 8.2 (TIC v8.2) (Paegert+, 2021)J/A+A/564/A125 : AGN Torus model comparison of AGN in the CDFS (Buchner+, 2014)J/AJ/155/257 : Photometry & RV follow-up observations of K2-18 (Sarkis+, 2018)J/AJ/156/276 : Exoplanetary atmospheric properties with MIRI (Danielski+, 2018)J/A+A/634/L4 : HST/STIS FUV spectra of K2-18 (dos Santos+, 2020)J/ApJS/251/15 : LAMOST-Kepler/K2 survey (LK-MRS) first year obs. (Zong+, 2020)J/A+A/666/A60 : Stellar Models and Limb Darkening (Kostogryz+, 2022)J/MNRAS/517/5050 : K2-18 HARPS time-series (Radica+, 2022)J/A+A/693/A60 : JWST/NIRSpec reference sample (Heintz+, 2025)J/AJ/169/6 : JWST/MIRI photometry from flux calibration (Gordon+, 2025)https://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/AJ/170/257https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/AJ/170/257http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/AJ/170/257https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/AJ/170/257https://vizier.iucaa.in/viz-bin/votable?-source=J/AJ/170/257http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/AJ/170/257GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapInfrareddefaultJ/AJ/170/257/fig4JWST NIRISS/SOSS and NIRSpec/G395H transit spectrumrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintInstJWST instrument identifier (JWST) (1)meta.id;instrcharWave[0.72/5.15] Central wavelength of binumem.wl.centralfloatWidth[0.01/0.02] Wavelength bin half-widthumspect.binSizefloatDepth[2.5e-3/3.3e-3] Transit depthsrc.var.amplitudedoublee_Depth[2.2e-5/2.6e-4] Lower uncertainty in Depthstat.error;stat.min;src.var.amplitudedoubleE_Depth[2.2e-5/2.8e-4] Upper uncertainty in Depthstat.error;stat.max;src.var.amplitudedoubleJ/AJ/170/257/fig5JWST MIRI/LRS retrieval results for different pixel shift casesrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintShiftShift of starting wavelength in pixel (JWST) (1)pixem.wl;arith.diffintWave[5.25/12.07] Central wavelength of binumem.wl.centralfloatWidth[0.07/0.28] Wavelength bin half-widthumspect.binSizefloatDepth[1.8e-3/3.7e-3] Transit depthsrc.var.amplitudedoublee_Depth[3.2e-5/5.9e-4] Lower uncertainty in Depthstat.error;stat.min;src.var.amplitudedoubleE_Depth[3.2e-5/5.7e-4] Upper uncertainty in Depthstat.error;stat.max;src.var.amplitudedouble