About 120 light-years from Earth, in the constellation Leo, a planet roughly eight times our world’s mass is forcing astronomers into one of the sharpest scientific arguments in years. The planet is K2-18 b, and in May 2025, a University of Cambridge team led by astronomer Nikku Madhusudhan published findings in The Astrophysical Journal Letters claiming that the James Webb Space Telescope (JWST) had picked up chemical fingerprints of dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) in its atmosphere. On Earth, those gases are produced almost exclusively by living organisms, primarily marine phytoplankton. If the detection holds up, it would mark the strongest indirect hint of biological activity ever found beyond our solar system.
Within weeks, a separate research group pushed back hard, arguing the data cannot support that conclusion. The result is a high-stakes scientific standoff that, as of June 2025, remains unresolved.
What the telescope actually found
K2-18 b first became a headline target in September 2023, when JWST observations using its NIRISS and NIRSpec instruments confirmed methane and carbon dioxide in the planet’s atmosphere. A NASA mission update documented those detections and flagged an early, tentative hint of DMS in the same data. That initial signal was too faint to call a detection, but it was enough to justify pointing JWST’s Mid-Infrared Instrument (MIRI) at the planet for a closer look.
The MIRI follow-up came in the form of a preprint titled “New Constraints on DMS and DMDS in the Atmosphere of K2-18 b from JWST MIRI,” posted to the arXiv repository in April 2025. That study used transit spectroscopy, measuring how starlight filters through K2-18 b’s atmosphere during a planetary crossing, to evaluate whether mid-infrared spectral features matched the signatures of DMS and DMDS. The authors applied a retrieval framework, essentially a statistical tool that tests which combination of molecules best explains the observed light, and reported that including DMS and DMDS improved the model’s fit to the MIRI data compared with models that left those gases out.
Madhusudhan’s Cambridge team then went further. In a peer-reviewed paper and an accompanying press release through EurekAlert, they described their results as the “strongest hints yet of biological activity outside the solar system.” They acknowledged the signal was tentative and called for more observations, but the framing was unmistakably bold: this was being presented as a meaningful step toward finding a biosignature on another world.
On the points that are not in dispute, the picture is clearer. K2-18 b orbits a cool red dwarf star. Its mass and radius suggest a deep, hydrogen-rich atmosphere sitting atop either an icy interior or a global water ocean, a configuration some researchers call a “Hycean” world (a term Madhusudhan’s own group coined in 2021). JWST’s infrared instruments have delivered some of the most detailed exoplanet spectra ever recorded for a sub-Neptune world, confirming methane and carbon dioxide while also placing constraints on water vapor and other gases. By any measure, K2-18 b is one of the best-characterized planets in the habitable zone of its star.
Why the finding is under fire
The excitement did not last long unchallenged. A competing preprint led by Savvas Constantinou and colleagues, also posted to arXiv, conducted a joint analysis combining data from all three JWST instruments (NIRISS, NIRSpec, and MIRI) and concluded there is insufficient evidence for DMS or DMDS in K2-18 b’s atmosphere. Their counter-analysis used Bayesian model comparison techniques and found that adding DMS or DMDS to atmospheric models did not improve the fit strongly enough to justify claiming a detection. In their assessment, the supposed signal could not be reliably separated from instrument noise or from overlapping spectral features of more common molecules. They called for additional MIRI transit observations before any detection claim could be considered robust.
This is not a peripheral disagreement. It strikes directly at the headline claim. Both teams are working with data from the same telescope and using broadly similar analytical methods, yet they reach opposite conclusions about whether the signal is real. The split illustrates how sensitive atmospheric retrievals are to modeling choices: assumptions about cloud structure, temperature profiles, which molecules to include in the model, and how to weight data from different instruments can all shift the outcome.
NASA’s own language has stayed carefully neutral. In its earlier communications about K2-18 b, the agency described the possible DMS signal as intriguing but far from confirmed, putting its emphasis on the solid methane and carbon dioxide detections. An Associated Press report on the newer claims echoed that caution, quoting researchers who stressed that non-biological explanations must be thoroughly explored before anyone invokes life.
The problem with calling it biology
Even if future observations confirm that DMS and DMDS are genuinely present in K2-18 b’s atmosphere, a large logical gap remains between detecting those molecules and attributing them to living organisms.
On Earth, the connection is straightforward. DMS is overwhelmingly a product of marine microbes, and DMDS is chemically related and similarly tied to biological processes. But K2-18 b is not Earth. If it truly is a Hycean world, its surface conditions would involve crushing atmospheric pressures, a hydrogen-dominated gas envelope, and ocean chemistry unlike anything in terrestrial experience. Under those exotic conditions, sulfur compounds could potentially form through abiotic pathways, reactions driven by geology or photochemistry rather than biology, that simply do not occur in Earth’s more oxidizing atmosphere.
Scientists who study biosignatures have long warned about this trap. A molecule that is biogenic on Earth might have perfectly ordinary non-biological sources on a planet with different chemistry. Ruling out those alternatives requires detailed models of the planet’s interior, ocean, and atmospheric chemistry, models that do not yet exist for K2-18 b with any precision. In practice, establishing a biological origin would demand multiple independent biosignature candidates, consistent across different wavelengths and observing epochs, alongside abiotic models that fail to reproduce the observations. No current exoplanet observation meets that bar.
Three tiers of evidence, and where each stands
For readers trying to sort signal from noise in this debate, it helps to think about the evidence in layers.
Tier one: methane and carbon dioxide. These detections are well established. Multiple independent analyses and NASA itself treat them as solid. Neither gas alone indicates life. Methane can come from geological processes, and carbon dioxide is common across the solar system. But together, in a hydrogen-rich atmosphere, they tell scientists that K2-18 b has a chemically active envelope worth studying further.
Tier two: DMS and DMDS. The MIRI-based preprint and Madhusudhan’s published paper both report spectral features best explained, within their modeling frameworks, by these sulfur-bearing molecules. These are genuine scientific claims backed by telescope data and statistical inference. But the competing analysis led by Constantinou and colleagues, which draws on a broader dataset spanning all three JWST instruments, found the signal too weak to distinguish from alternative explanations. As of June 2025, the presence of DMS or DMDS on K2-18 b should be treated as an open question, not a settled fact.
Tier three: biological origin. Even the researchers making the detection claim have stopped short of declaring evidence of life. The leap from “we see a molecule associated with biology on Earth” to “biology produced this molecule on another planet” requires eliminating every plausible abiotic source. That work has barely begun for K2-18 b.
What comes next for K2-18 b
The path forward is observational. Additional MIRI transits of K2-18 b, already being discussed within the JWST scheduling community, would increase the signal-to-noise ratio and help settle whether the disputed spectral features are real or artifacts. Emission spectroscopy, which captures light radiated by the planet itself rather than filtered starlight, could provide an independent check on atmospheric composition. And improved theoretical models of Hycean atmospheres and sulfur chemistry will be essential for interpreting whatever the telescope finds.
Further out, NASA’s planned Habitable Worlds Observatory, a next-generation space telescope designed specifically to characterize Earth-like exoplanets, could eventually study worlds like K2-18 b with far greater sensitivity. But that mission is not expected to launch until the late 2030s or early 2040s, meaning JWST will remain the primary tool for this work for at least another decade.
The debate over K2-18 b is not a failure of the science. It is the science working as intended. Two teams looked at the same data, applied rigorous but different analytical choices, and reached different conclusions. Resolving that disagreement through new observations, refined models, and independent replication is exactly how tentative hints either harden into discoveries or fade into footnotes. For now, K2-18 b remains the most tantalizing lead astronomers have in the search for life beyond our solar system, but a lead is not a verdict.
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*This article was researched with the help of AI, with human editors creating the final content.
