{"id":"4a33113f-404c-40e5-aa6c-24de4dffb0fd","arxiv_id":"2504.12267","paper_version":1,"verdict":"CONDITIONAL","confidence":"HIGH","novelty_score":7.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":9,"one_line_summary":"A new JWST MIRI spectrum of the exoplanet K2-18 b shows features attributed to DMS and/or DMDS at around 3 sigma, with mixing ratios above 10 parts per million.","lead":"JWST's MIRI instrument observed K2-18 b in the mid-infrared and found spectral features best explained by dimethyl sulfide (DMS) or dimethyl disulfide (DMDS), molecules produced by life on Earth. If confirmed, this is the strongest independent hint of possible biological activity on a habitable-zone exoplanet.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"DMS/DMDS claim rests on STP/N2-broadened cross sections applied to a ~400 K, mbar, H2-rich photosphere; opacity errors could shift both the >10 ppmv abundance and the 3-sigma significance.","rationale":"The reader's weakest assumption identifies exactly the same load-bearing concern: the retrieved abundances and significance inherit the 1 bar, 298 K, N2-broadened DMS/DMDS cross sections. The paper is methodologically strong in its two independent pipelines, GP treatment of time-correlated noise, leave-one-out analysis, and extensive robustness checks. However, all of those checks operate within a fixed opacity model; they do not test the fidelity of the DMS/DMDS cross sections under the H2-rich, low-pressure, higher-temperature conditions probed by the MIRI spectrum. Since the central claim includes a quantitative abundance floor (greater than 10 ppmv) and a 2.9-3.2 sigma significance, the cross-section assumption is genuinely load-bearing. The proposed sensitivity test brackets the unknown broadening and thermal effects using existing computational tools, without requiring new observations, and would show whether the headline result survives. A CONDITIONAL verdict remains appropriate because the paper itself flags the limitation and frames the result as requiring confirmation; no verdict adjustment is needed unless the sensitivity test fails. The multiple-molecule search could also add a look-elsewhere penalty, but the MIRI search was motivated by a prior DMS hint and the hierarchical model selection partially accounts for model complexity, so the cross-section issue is the stronger concern.","tokens_in":32980,"tokens_out":11226,"duration_ms":130479,"concrete_test":"Run a sensitivity battery on the canonical retrieval: replace the DMS and DMDS cross sections with profiles representing H2 broadening at ~400 K and ~1 mbar, e.g., Voigt-convolve or rescale Lorentzian half-widths by factors 0.3, 1, and 3 and shift band intensities by ±50% to bracket thermal-population uncertainties, then recompute the evidence and abundance posteriors. If the combined DMS+DMDS Bayes factor drops below ~3 sigma in any bracketing case, or the 1-sigma lower bound on the favored molecule falls below 10 ppmv, the quantitative claim is not robust. Definitive settlement requires new laboratory measurements of DMS/DMDS cross sections in H2 at 300-500 K and 0.1-10 mbar.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The most load-bearing assumption is that the HITRAN/PNNL DMS and DMDS cross sections used in the canonical retrieval (Appendix B; Sharpe et al. 2004; Gordon et al. 2017), measured at 1 bar, 298 K with N2 broadening, are representative of the atmospheric layers probed by the MIRI transmission spectrum. The retrieval itself places the photosphere at log(P_ref/bar) = -4.32^{+1.15}_{-0.93} and T_1mbar ~ 422^{+141}_{-133} K, in an H2/He-dominated gas (Section 3.2). For large floppy molecules like DMS and DMDS, the dense rovibrational band shapes in the 6-12 um region depend on pressure broadening and on thermal populations of low-lying vibrational states; using 298 K/N2 data can alter band contrasts and relative peak heights. Because the model selection (Section 3.1), the DMS/DMDS Bayes factors (Table 2), and the leave-one-out analysis (Section 3.3) are all computed with these same opacities, an error propagates into the claimed 2.9-3.2 sigma detection and into the conclusion that the features are uniquely attributable to DMS/DMDS. The paper is transparent about this in Section 4.1 ('both the retrieved mixing ratios and temperature are strongly dependent on the available cross-sections'), but transparency does not remove the dependence. The claim that at least one molecule exceeds 10 ppmv shifts if the true cross sections are stronger or weaker at low pressure and higher temperature.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper presents a JWST MIRI LRS transmission spectrum of the sub-Neptune K2-18 b over approximately 6-12 microns, reduced via two independent pipelines (JExoRES and JexoPipe). Atmospheric retrievals with the AURA framework, considering 20 molecular species, find that the spectrum is best explained by a combination of dimethyl sulfide (DMS) and dimethyl disulfide (DMDS), with a combined detection significance of 2.9-3.2 sigma and retrieved mixing ratios of at least about 10 ppmv. The authors perform extensive robustness tests, including different detrending, binning, limb-darkening treatments, GP-based correction for time-correlated noise, and a leave-one-out cross-validation analysis. They explicitly note that the DMS and DMDS opacities used are measured at near-STP with N2 broadening, which may not represent the H2-rich, low-pressure, roughly 400 K photosphere probed.","tokens_in":33469,"tokens_out":9871,"duration_ms":92271,"significance":"If the detection holds, this would be the first mid-infrared transmission spectrum of a habitable-zone sub-Neptune and an independent line of evidence for a potential biosignature gas on K2-18 b, complementing the earlier tentative DMS hint from NIRISS/NIRSpec. The paper's strengths are the dual-pipeline reduction, the thorough robustness campaign, the leave-one-out analysis, and the explicit caveat about cross-section limitations. The circularity burden is low: all abundances are free parameters fitted to the new MIRI data, and prior context is used only for priors and consistency comparisons. However, the headline significance and abundance claims rest on spectroscopic parameters whose applicability to the retrieved conditions is unquantified, and the mapping from Bayesian evidence to quoted sigma is not specified. These issues must be addressed before the central claim can be considered secure.","major_comments":[{"comment":"The retrieval adopts HITRAN/PNNL cross sections for DMS and DMDS (Sharpe et al. 2004; Gordon et al. 2017) measured at 1 bar and 298 K with N2 broadening, whereas the retrieved photosphere is at log(P_ref/bar) = -4.32(+1.15/-0.93) and T ~ 422(+141/-133) K in an H2-dominated atmosphere (Section 3.2). As the authors state in Section 4.1, the retrieved mixing ratios and temperature are strongly dependent on these cross sections. Since the model selection, the Bayes factors in Table 2, and the leave-one-out analysis in Section 3.3 all use the same opacities, an unquantified error in the cross-section temperature/broadening could shift the >10 ppmv abundance claim and the 2.9-3.2 sigma significance. I request a quantitative sensitivity test, such as repeating key retrievals with cross sections that are scaled or convolved with a pressure-broadened line shape appropriate for H2 at millibar pressures, or, failing that, a revised abstract that explicitly conditions the detection significance and abundances on the STP/N2-broadened cross sections.","section":"Section 4.1 and Appendix B"},{"comment":"The paper reports detection significances in sigma, such as DMS (2.9 sigma) and DMDS (3.2 sigma), derived from the Bayes factor ln(B), but the conversion formula is not specified. Using the common nested-model approximation sigma = sqrt(2 ln B) yields 2.4 sigma for the DMS-only case (ln(B)=2.86) and 2.8 sigma for the DMDS-only case (ln(B)=3.81), both lower than the quoted values. Please specify the exact relation used, including any effective-number-of-parameters correction, or report significance based on the posterior probability of the abundance exceeding a threshold, so that the headline 3-sigma claim is reproducible.","section":"Section 3.1 and Table 2"}],"minor_comments":[{"comment":"The caption states 'The individual spectral contributions of these molecules are shown in Figure 3,' which is self-referential; it should reference Figure 14 or be removed.","section":"Figure 3 caption"},{"comment":"The text says 'The offset is retrieved to be 12+51/-58 ppm,' but Table 3 gives delta_MIRI = -12(+51/-58) ppm; please check the sign convention and ensure consistency between the text and the table.","section":"Section 3.2"},{"comment":"The text contains the typo 'a model that includes DMDS and DMDS'; this should read 'DMS and DMDS.'","section":"Section 3.3"},{"comment":"Benneke et al. 2019a and 2019b are listed with identical journal, page, and DOI (ApJL 887, L14, 10.3847/2041-8213/ab59dc); please correct the bibliographic data for one of the two entries.","section":"References"},{"comment":"In the row 'DMS + DMDS JExoRES (Exp+Linear1)', the DMS abundance is given as '1 < -2.44', which appears to be a formatting artifact; the upper limit should be displayed consistently with other rows.","section":"Table 2"},{"comment":"The sentence 'the model without DMS and DMDS is only marginally favoured over a flat spectrum below 2-sigma significance' is ambiguous; please specify whether 'the model' refers to the maximal model or the canonical model with DMS and DMDS removed.","section":"Section 3.1"}],"recommendation":"major_revision","confidential_remarks":"This paper will attract wide attention given the earlier DMS debate on K2-18 b. The main scientific risk is the unquantified dependence of the detection significance and abundance on STP/N2-broadened cross sections applied to H2-rich, low-pressure, ~400 K conditions. The authors are transparent about this, but a quantitative sensitivity test or a more cautious framing of the abstract is needed. The missing sigma-conversion formula is also a straightforward reproducibility issue that should be fixed. I do not see a fundamental flaw in the data reduction or retrieval methodology; the paper is appropriate for the journal pending these revisions."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Colleague,\n\nRead this one because it is the first mid-infrared transmission spectrum of a habitable-zone sub-Neptune, and it is careful work. The new JWST MIRI LRS spectrum (6-12 um) of K2-18 b is genuinely new: an independent instrument and bandpass from the earlier NIRISS/NIRSpec data, and the paper is the first to consider DMDS alongside DMS in this context. The spectra from two independent pipelines (JExoRES and JexoPipe) agree, and the robustness testing is thorough—GP treatment of correlated noise, binning choices, trend prescriptions, limb-darkening, and a leave-one-out analysis. The data and posterior products are public. That is real, reproducible effort, and the spectrum itself looks like a solid new measurement.\n\nThe interpretive claim is shakier. The evidence for DMS and/or DMDS is at 2.9-3.2 sigma, which is marginal for any biosignature claim, and the two molecules are strongly degenerate in this band. The larger concern is the cross-section foundation: the retrievals use HITRAN/PNNL cross-sections taken at 1 bar, 298 K with N2 broadening, applied to a photosphere that the retrieval itself places near log P ~ -4.3 bar and T ~ 420 K in an H2-dominated atmosphere. For floppy molecules like DMS and DMDS, band shapes and relative peak heights will shift with temperature and broadening gas, so the retrieved abundances and even the detection significance are hostage to that assumption. The authors say exactly this in Section 4.1—\"both the retrieved mixing ratios and temperature are strongly dependent on the available cross-sections\"—but acknowledging a load-bearing uncertainty does not move it. The 10 ppmv abundance floor and the 3-sigma significance both move if the true low-pressure, H2-broadened cross-sections differ.\n\nI do not think this is fatal. The paper is transparent, the new spectrum is a real contribution, and the detection is worth taking seriously as a tentative result. But the DMS/DMDS claim should be read as conditional on cross-section behavior that no one has measured yet, not as a robust biosignature detection.\n\nWho is this for? Anyone tracking K2-18 b or MIRI transmission spectroscopy of temperate exoplanets. I would bring it to our reading group, and I would cite the spectrum itself. It deserves a serious referee—the right referee will push on the cross-section sensitivity and on whether a few data points drive the detection. Recommend sending to peer review, with eyes open that the central claim may not survive a lab measurement.","headline":"First MIRI LRS spectrum of K2-18 b is a real step forward, but the 3-sigma DMS/DMDS claim rests on STP/N2 cross-sections that may not hold at the planet's low-pressure, H2-rich, ~400 K photosphere.","tokens_in":33989,"tokens_out":2151,"would_cite":true,"duration_ms":22353,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"An independent JWST mid-infrared spectrum of K2-18 b shows spectral features that fit the sulfur molecules DMS and/or DMDS at about 3-sigma significance.","keywords":["K2-18 b","exoplanet atmospheres","transmission spectroscopy","JWST MIRI","dimethyl sulfide","dimethyl disulfide","biosignatures","hycean worlds"],"falsifier":"Measure DMS and DMDS absorption cross sections at 1-10 mbar in an H2-dominated gas mixture at 300-600 K and rerun the canonical retrieval; if the Bayesian evidence for the molecules falls below about 2σ or the best-fit mixing ratios drop below 10 ppmv, the central claim would be refuted. A further useful test is a second MIRI transit or a near-infrared 3-5 µm observation that fails to reproduce the DMS/DMDS pattern.","tokens_in":32831,"feed_emoji":"🪐","tokens_out":9285,"duration_ms":80600,"temperature":0.7,"pith_summary":"This paper reports the first mid-infrared transmission spectrum of a habitable-zone sub-Neptune, the candidate hycean world K2-18 b (a proposed ocean-bearing planet with a hydrogen-rich atmosphere), taken with JWST's MIRI LRS instrument over roughly 6-12 µm. The spectrum contains several features that a retrieval of 20 plausible molecules cannot attribute to most species, but that are well described by dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS), with a combined significance of 2.9-3.2 sigma. If the interpretation is right, at least one of these sulfur molecules is present at a mixing ratio above 10 parts per million by volume, making it a possible biosignature in a hydrogen-rich atmosphere. The result provides an independent check on an earlier near-infrared hint of DMS and strengthens the case that K2-18 b's atmosphere is consistent with a hycean world.","feed_headline":"New mid-IR spectrum finds DMS or DMDS on K2-18 b","feed_subtitle":"JWST MIRI data independently back the earlier DMS hint, with at least one sulfur gas above 10 ppmv.","key_machinery":"The load-bearing element is the MIRI LRS transmission spectrum itself, binned at 0.2 µm with time-correlated noise accounted for, combined with a hierarchical Bayesian retrieval that scans opacities of 20 molecules. The discriminating pattern is a double-peak feature between 6.8-8 µm plus a broad peak near 9.8 µm for DMS and near 10.5 µm for DMDS, with features of about 300-400 ppm amplitude. The retrieval uses laboratory absorption cross sections for DMS and DMDS measured at Earth-like conditions (1 bar, 298 K, nitrogen broadening), and a leave-one-out analysis shows that the detection is driven by several spectral points rather than a single outlier.","core_discovery":"The central claim is that the 6-12 µm JWST MIRI LRS transmission spectrum of K2-18 b is inconsistent with a featureless spectrum at 3.4-sigma significance and can only be explained, among the molecules considered, by DMS and/or DMDS, at 2.9-3.2 sigma. When both molecules are included in the retrieval, DMDS is the preferred carrier of the features while DMS is largely unconstrained; removing either molecule lets the other absorb the same spectral bands because their mid-infrared features overlap. Single-molecule retrievals yield DMS at 2.9-3.0 sigma and DMDS at 3.0-3.2 sigma, with log10 volume mixing ratios near -3.4 and -3.2 respectively, and the paper concludes that at least one of the two molecules exceeds 10 ppmv. The paper treats this as new independent evidence for a possible sulfur biosignature on K2-18 b, complementary to the earlier near-infrared detections of methane and carbon dioxide.","pith_inferences":["The DMS-DMDS overlap means the current data cannot cleanly separate the two molecules; observations in wavelength regions where their bands differ more, or high-resolution spectroscopy, would be needed to decide which molecule is actually present.","If laboratory measurements show that hydrogen-broadened DMS and DMDS cross sections at low pressure differ from the nitrogen-broadened Earth-like data, the retrieved mixing ratios and photospheric temperature would shift, and the biosignature interpretation might need revision.","The paper's false-positive discussion points to abiotic production of DMS and DMDS from CH4 and H2S in laboratory discharge and UV experiments; photochemical models that include the measured high CO2 abundance could predict whether such abiotic routes can sustain the inferred concentrations, giving a testable abiotic alternative."],"forward_implications":["The MIRI data provide an independent detection path from the near-infrared NIRISS/NIRSpec observations, so the DMS/DMDS signal does not depend on the detector-offset systematics that weakened the earlier DMS hint.","At the retrieved abundances, steady-state levels above 10 ppmv of DMS or DMDS would require a sustained source; the photochemical models the paper discusses imply biogenic ocean fluxes more than about 20 times Earth's, which would make the molecules a plausible biosignature if the detection holds.","One to three additional MIRI transits, roughly 8-24 hours of JWST time, should raise the combined significance to 4-5 sigma and test whether the features repeat.","The MIRI upper limits on methane and carbon dioxide are consistent with the roughly 1 percent abundances measured earlier, so the new spectrum does not conflict with the previously reported composition."],"supporting_citations":[{"why":"Supplies the previous near-infrared JWST spectrum of K2-18 b, the detections of CH4 and CO2, and the tentative DMS hint that this MIRI observation independently tests.","marker":"Madhusudhan et al. 2023b"},{"why":"Provides the laboratory absorption cross sections for DMS and DMDS, used at 1 bar and 298 K with nitrogen broadening to model the mid-infrared features.","marker":"Gordon et al. 2017"},{"why":"Photochemical modeling that shows DMS and DMDS can accumulate to the retrieved mixing ratios under high biogenic fluxes, making the inferred abundances physically plausible.","marker":"Tsai et al. 2024"},{"why":"Supplies the atmospheric retrieval framework used to fit transmission spectra with molecular opacities, clouds, and hazes, and to compute Bayesian evidences.","marker":"Pinhas et al. 2019"},{"why":"Provides the stellar and planetary parameters and ephemeris adopted for the light-curve fits and radius scaling.","marker":"Benneke et al. 2019a"},{"why":"Provides the limb-darkening parameterization and priors used in the white light curve and spectral light curve fits.","marker":"Kipping 2013"},{"why":"Implements the nested-sampling algorithm used to estimate Bayesian evidences and detection significances.","marker":"Feroz et al. 2009"}],"fun_headline_variants":["JWST MIRI detects DMS or DMDS on K2-18 b at 3-sigma","Sulfur biosignature gases DMS and DMDS found on K2-18 b","New JWST data back DMS hint on exoplanet K2-18 b","K2-18 b's mid-IR spectrum shows DMS or DMDS at 3-sigma","MIRI spectrum independently confirms sulfur gas on K2-18 b"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The abundance and detection rest on the assumption that the laboratory absorption cross sections for DMS and DMDS, measured at Earth-like pressure and temperature in nitrogen, are accurate for the low-pressure hydrogen-rich gas probed in transmission; if they are not, the same spectral features could shift, weaken, or belong to different molecules.","fun_headline_variants_meta":{"raw":{"variants":["JWST MIRI detects DMS or DMDS on K2-18 b at 3-sigma","Sulfur biosignature gases DMS and DMDS found on K2-18 b","New JWST data back DMS hint on exoplanet K2-18 b","K2-18 b's mid-IR spectrum shows DMS or DMDS at 3-sigma","MIRI spectrum independently confirms sulfur gas on K2-18 b"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000555,"raw_usage":{"total_tokens":2726,"prompt_tokens":1111,"completion_tokens":1615,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":727,"completion_tokens_details":{"reasoning_tokens":1498}},"tokens_in":727,"tokens_out":1615,"duration_ms":11266,"temperature":1.0,"reasoning_tokens":1498,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-16T12:33:19.362605+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure DMS and DMDS absorption cross sections at 1-10 mbar in an H2-dominated gas mixture at 300-600 K and rerun the canonical retrieval; if the Bayesian evidence for the molecules falls below about 2σ or the best-fit mixing ratios drop below 10 ppmv, the central claim would be refuted. A further useful test is a second MIRI transit or a near-infrared 3-5 µm observation that fails to reproduce the DMS/DMDS pattern.","supporting_citations":[],"review_version":1}