Summary
rafael-luque et al. publish in Astronomy & Astrophysics the first joint analysis of the full panchromatic (0.6–12 µm) jwst spectrum of k2-18b, combining NIRISS, NIRSpec, and MIRI observations using three independent data-reduction pipelines and two retrieval codes. The study finds insufficient statistical evidence for dimethyl sulfide (DMS) or dimethyl disulfide (DMDS) in K2-18b’s atmosphere. This directly contradicts src-jwst-k2-18b-biosignature-2025, in which nikku-madhusudhan’s team reported a three-sigma DMS/DMDS detection using MIRI alone — the strongest potential biosignature claim on any exoplanet. The non-biological molecule ethane (C₂H₆) produces statistically indistinguishable fits, introducing a false-positive pathway.
Key Claims
- No statistical evidence for DMS/DMDS: Bayesian model comparison (δ ln Z < 1) shows no preference for DMS/DMDS models when using the JExoRES data reduction
- Contradicts Madhusudhan et al.: The three-sigma DMS/DMDS claim does not survive the first panchromatic joint analysis; the detection was an artifact of limited molecular model sets and single-pipeline sensitivity
- Ethane as alternative: Non-biological ethane (C₂H₆) fits the data equally well, introducing a false-positive pathway specific to DMS/DMDS
- Pipeline dependence: Results from JExoRES, exoTEDRF, SPARTA, and Eureka! pipelines diverge, demonstrating the DMS/DMDS preference lacks robustness across independent reductions
- MIRI features implausibly large: Mid-infrared spectral features attributed to DMS/DMDS appear implausibly large relative to shorter-wavelength observations, suggesting noise contamination
- ~25 more MIRI transits needed: Even under the most favorable DMS/DMDS model, approximately 25–26 additional MIRI transits would be required for a 3σ detection
Notable Quotes
“The first joint analysis of the panchromatic (0.6-12 µm) spectrum of K2-18 b finds insufficient evidence for the presence of DMS and/or DMDS in the atmosphere of the planet.” — Luque et al.
“We find that any marginal preferences are the result of limiting the number of molecules considered in the model and oversensitivity to small changes between data reductions.” — Luque et al.
Contradiction Note
This paper is in direct tension with src-jwst-k2-18b-biosignature-2025, which reported DMS and DMDS detection at three-sigma significance. The Luque et al. reanalysis — using broader wavelength coverage, more pipelines, and more complete molecular models — concludes that the earlier claims were premature and not statistically robust. The k2-18b DMS/DMDS biosignature, previously described as “heavily disputed” (src-k2-18b-technosignature-null-2026), is now formally rebutted by a peer-reviewed panchromatic analysis. This also weakens the hycean-worlds biosignature narrative, though the hycean atmospheric structure hypothesis itself is not directly tested.