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

In 2023, a Cambridge-led team announced something that stopped the room: a tentative detection of dimethyl sulfide — a molecule on Earth produced almost exclusively by marine phytoplankton — in the atmosphere of K2-18b, a sub-Neptune 124 light-years away. Two years later, a NASA-led reanalysis said the signal doesn't hold up. Now JWST Cycle 5, beginning around July 2026, is lined up to take another look. This is what science is supposed to do. It's just rarely this public, or this consequential.

What happened

K2-18b sits in the habitable zone of its star and belongs to a class of worlds called Hycean planets — hypothetically ocean-covered, hydrogen-rich, potentially hospitable to life. Madhusudhan et al. (2023) published a JWST transmission spectrum of the planet and found features they interpreted as carbon dioxide, methane, and — tentatively — dimethyl sulfide (DMS) and dimethyl disulfide (DMDS). The authors were careful. They called it "tentative" and explicitly asked for follow-up. The press was less careful. Headlines ran hot.

The follow-up arrived in 2025. Welbanks et al. (arXiv:2508.05961), a NASA-led team, reanalyzed the same data using updated atmospheric retrieval methods. Their conclusion: the DMS feature is not robustly detected. The signal is consistent with noise and instrument systematics. The data, they found, are equally consistent with a DMS-free atmosphere. The 2023 detection, by their read, does not meet the scientific standard of evidence.

The evidence, such as it is

This isn't a case of one team being sloppy. Transmission spectroscopy — reading a planet's atmosphere by analyzing starlight filtered through it during a transit — is genuinely hard. The signals are small. JWST is the most powerful tool humanity has ever pointed at this problem, and it is still working at the edge of what's physically detectable. The original Madhusudhan team flagged the DMS feature as tentative precisely because they knew this. The Welbanks reanalysis isn't a refutation so much as a more conservative read of ambiguous data — and it explicitly calls for more JWST transit observations before any biosignature claim can be evaluated.

That's where Cycle 5 comes in. JWST Cycle 5 approved programs include K2-18b as a target, with observations expected to begin around July 2026. More transits mean more signal stacked against the noise. The ESA Webb Cycle 5 science overview frames this round of observations as part of a broader push on atmospheric characterization for potentially habitable worlds — K2-18b is among the highest-priority cases.

What the data can and can't tell us

Even a clean DMS detection in Cycle 5 wouldn't confirm life. It would confirm a molecule that, on Earth, is mostly made by marine phytoplankton — but "mostly" isn't "exclusively," and K2-18b is not Earth. Abiotic chemistry on a hydrogen-rich Hycean world could plausibly produce similar molecules. The biosignature case requires ruling out those alternatives, and that work is nowhere near done.

Conversely, a non-detection in Cycle 5 wouldn't rule out life either. It would rule out DMS at detectable concentrations, which is a different thing.

What Cycle 5 can realistically deliver: a much cleaner spectrum, a more definitive answer on whether DMS is present at the level the 2023 paper suggested, and — if the signal is still ambiguous — a better-constrained upper limit that tells future missions what they're working with.

Why this case matters

K2-18b is not confirmed to host life. It may not even have DMS. But it is the clearest example we currently have of the scientific method operating in real time on the most interesting question in astronomy. A claim was made. It was scrutinized. The data were found wanting. More data are being collected. That's the whole process, running in public, on a 124-light-year target, with a space telescope that cost ten billion dollars and took thirty years to build. Whatever Cycle 5 finds — confirmation, contradiction, or a deeper puzzle — it will be the most carefully examined atmospheric spectrum of a potentially habitable world in history.