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

124 light-years out, a planet called K2-18b sits in its star's habitable zone and refuses to stop being interesting.

In 2023, a Cambridge-led team published a paper in the Astrophysical Journal Letters claiming a tentative detection of dimethyl sulfide — DMS — in K2-18b's atmosphere. On Earth, DMS is produced almost exclusively by marine phytoplankton. The team was careful: they called it tentative, flagged the need for follow-up, and framed K2-18b as a candidate "Hycean world" — a class of ocean-covered sub-Neptune planets that might, in theory, support life. The word "biosignature" appeared. The internet did what the internet does.

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.

The evidence

The 2023 claim rested on JWST transmission spectroscopy — genuinely the most powerful tool humanity has for reading exoplanet atmospheres. The instrument is not in question. What's contested is the interpretation of a faint spectral feature in a noisy dataset.

Welbanks et al. (arXiv:2508.05961) applied independent retrieval methods and found the feature isn't robustly there. They explicitly called for additional JWST transit observations before any biosignature claim could be evaluated — which is exactly what Cycle 5 is positioned to provide.

JWST Cycle 5 begins observations around July 2026. K2-18b is among its targets. More transits mean more photons, better signal-to-noise, and a cleaner shot at either confirming or ruling out the DMS feature. The ESA Webb Cycle 5 science overview frames the program as continuing high-priority atmospheric characterization of potentially habitable worlds — K2-18b fits squarely in that category regardless of where the DMS question lands.

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.