JWST Cycle 5: The Next Shot at K2-18b's Alleged Biosignature

What happened

K2-18b is a sub-Neptune about 2.6 times Earth's radius, orbiting in the habitable zone of a red dwarf star 124 light-years away. JWST observed it in transit — measuring which wavelengths of starlight the atmosphere filters out — and the 2023 Madhusudhan et al. analysis found a spectral feature consistent with DMS and its chemical cousin dimethyl disulfide (DMDS). The authors were explicit that the detection was tentative and called for additional observations before drawing conclusions.

That follow-up arrived in 2025. A NASA-led team, Welbanks et al., published a reanalysis on arXiv using updated atmospheric retrieval methods. Their finding: the DMS feature does not meet the scientific standard of detection. The signal, they concluded, is consistent with noise and instrument systematics. A DMS-free atmosphere fits the data just as well.

What's still open

This is the scientific method running in public, which is rarer and more interesting than it sounds. A team finds a tentative signal. An independent team reanalyzes and says the signal doesn't hold up. A third round of data is incoming. Nobody is hiding anything. Nobody has lied. The question is just genuinely hard.

A few things worth sitting with:

DMS on Earth comes almost entirely from marine biology. If it were robustly detected on K2-18b, that would be extraordinary. It has not been robustly detected.
The reanalysis doesn't prove DMS is absent — it proves the existing data can't confirm it's present. Those are different claims.
K2-18b's Hycean-world status is itself a model, not an established fact. Whether it has a liquid water ocean is still an open question.
Cycle 5 data could sharpen the picture significantly. It could also produce another ambiguous result. That's how this works.

The honest summary: one team saw something interesting, another team looked harder and wasn't convinced, and the telescope is going back for another look. That's not a failure. That's the job.

What is dimethyl sulfide (DMS) and why does it matter for K2-18b?

Dimethyl sulfide is a molecule produced almost exclusively by marine phytoplankton on Earth, which makes it a candidate biosignature — a chemical that might indicate biological activity. A 2023 Cambridge-led team reported a tentative spectral detection of DMS in K2-18b's atmosphere using JWST data. If confirmed, it would be extraordinary; so far, a 2025 NASA-led reanalysis found the signal doesn't meet the scientific standard of detection.

What did the Welbanks et al. 2025 reanalysis actually conclude?

The NASA-led Welbanks et al. team (arXiv:2508.05961) applied updated atmospheric retrieval methods to the same JWST data and found the DMS feature is not robustly detected — the signal is consistent with noise and instrument systematics. They concluded that a DMS-free atmosphere fits the data equally well. Their recommendation: more JWST transit observations before any biosignature claim can be evaluated.

What will JWST Cycle 5 observations of K2-18b actually tell us?

Cycle 5, beginning around July 2026, will add more transit observations of K2-18b, improving signal-to-noise in its transmission spectrum. More data could either strengthen the DMS spectral feature, rule it out more definitively, or — entirely plausibly — produce a result that still requires interpretation. Additional transits are the agreed-upon next step by both the original team and the reanalysis authors.

Does the reanalysis mean there's definitely no life on K2-18b?

No — it means the current data can't confirm the DMS detection that prompted the biosignature discussion in the first place. The reanalysis rules out the specific claim, not the broader possibility. Whether K2-18b has a liquid water ocean, let alone biology, remains an open question that the existing data can't settle.

What is a Hycean world, and is K2-18b definitely one?

A Hycean world is a theoretical class of sub-Neptune exoplanet with a hydrogen-rich atmosphere and a global liquid water ocean — conditions that might support life. K2-18b has been proposed as a Hycean candidate based on its size, orbit, and atmospheric composition, but that classification is itself a model, not an established fact. Whether it actually hosts a liquid ocean is still unresolved.

JWST Cycle 5: The Next Shot at K2-18b's Alleged Biosignature

Unresolved

What happened

The K2-18b Biosignature Fight

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