NANDKUMAR M. KAMAT
This article is about a major astronomical claim in April 2025 that stunned both scientists and the public when a research team led by Professor Nikku Madhusudhan at the University of Cambridge published new data from the James Webb Space Telescope, suggesting possible evidence of life-related chemistry on the exoplanet K2-18 b. The planet, about 120 light-years away in the constellation Leo, has long intrigued exoplanet hunters. With a mass about 8.6 times that of Earth and a radius 2.6 times larger, it orbits a cool red dwarf star in its habitable zone. The planet had already drawn attention after detecting methane and carbon dioxide in its atmosphere, both considered necessary for biological or prebiological chemistry. Now, in a deeper investigation using JWST’s Mid-Infrared Instrument, the Cambridge team believes they have detected dimethyl sulfide and dimethyl disulfide in the atmosphere of K2-18 b—molecules that, on Earth, are mostly produced by microbial life in oceans.
The implications, if confirmed, would be profound. Dimethyl sulfide and dimethyl disulfide are sulfur-bearing compounds known for their biological origin on Earth. Their identification was based on spectral absorption features between six and 12 microns, a region of light ideal for identifying complex organics. The team used the JWST’s MIRI spectrograph and two data pipelines to cross-check their results. They found overlapping features consistent with these molecules and ruled out most other plausible explanations. The retrieval models suggest abundances of dimethyl disulfide or dimethyl sulfide at volume mixing ratios on the order of ten parts per million or higher. While the signal is statistically significant at just above the 3 sigma level—a 99.7% probability that the signal is real—it still falls short of the gold standard of 5 sigma required for confirmation in physics and astronomy. Even the team leading the discovery emphasised caution, noting that more observations and better molecular cross-section data are needed.
On the surface, the new observations appear to bolster the idea that K2-18 b is a hycean world—a term coined by Madhusudhan to describe planets with hydrogen-rich atmospheres and global oceans. Their lower density and extended atmospheres make such planets easier to study with telescopes like JWST. The discovery of methane and carbon dioxide without ammonia and carbon monoxide further strengthened the hycean model.
However, the excitement is tempered by deep scepticism among many astronomers and planetary scientists. Independent analyses from research teams, including Stephen Schmidt at Johns Hopkins and Laura Kreidberg at the Max Planck Institute, have raised concerns about the reliability of the detection. Some suggest that the data is close to the detection limit of JWST and may be a statistical fluctuation. Others argue that DMS and DMDS can be created through abiotic means, for example, on comets or in prebiotic chemistry, and therefore do not constitute strong biosignatures unless all other explanations are ruled out.
This debate touches the heart of exoplanet science today. The tension between hope and scepticism is not new. In this case, the criticism centres on several fronts. First, K2-18 b is a sub-Neptune, a category of planets with uncertain interiors. Models suggest that such planets may lack a solid surface or even a distinct ocean-atmosphere boundary. Any life-supporting ocean might lie beneath a thick, high-pressure hydrogen layer, making it inaccessible to light and thus unlikely to support photosynthetic life as we know it. Second, critics point to the low signal-to-noise ratio in the spectral data. While the Cambridge group reported that the signal cannot be explained by a featureless spectrum at the 3.4 sigma level, other groups have attempted to reanalyse the same datasets and found no robust signatures.
Third, DMS and DMDS are not unambiguous biosignatures. While their primary source on Earth is biological, laboratory experiments and comet studies have shown that such molecules can form without life. This has been echoed by chemist Eleanor Browne, who noted that laboratory synthesis of DMS under prebiotic conditions is feasible and that detection of such compounds must be carefully contextualised. The planetary environment matters as much as the chemistry itself. A molecule considered a biosignature on one planet could be entirely abiotic on another.
The MIRI observations did not detect water vapour, although the original 2023 data hinted at its presence. For K2-18 b to be habitable, its cloud cover and atmospheric reflectivity must allow for surface temperatures where liquid water remains stable. Some models suggest that if the cloud albedo is too low, the surface may be too hot for life, entering a supercritical state where water cannot exist as a distinct liquid. The Cambridge team addressed this, pointing out that retrieved photospheric temperatures around 422 Kelvin—roughly 150 degrees Celsius—could still be tolerable for extremophiles. On Earth, microbes have been found thriving in similar environments like deep-sea hydrothermal vents. The presence of methane and carbon dioxide at one percent each, alongside these sulfur compounds, fits certain models of biogenic fluxes, particularly under low UV radiation environments found around red dwarfs.
Despite the excitement, the findings remain tentative. The Cambridge team hopes to obtain more JWST time to refine their measurements and improve the statistical certainty. Independent confirmation from other research teams and analysis pipelines will be essential to establishing the robustness of the claim. In parallel, laboratory experiments must test the stability and formation pathways of DMS and DMDS under simulated planetary conditions. These efforts could help determine whether the observed molecules could form through volcanic or photochemical processes without life.
Even if the current claim does not stand up to scrutiny, the work serves an important purpose. It pushes the boundaries of what JWST can achieve and helps scientists refine the tools and techniques necessary for future life detection. More broadly, it brings attention to the most abundant class of planets in our galaxy—sub-Neptunes and mini-Neptunes—whose atmospheric properties are still
poorly understood.
From a scientific perspective, the work is a technical tour de force. Using two independent pipelines, JExoRES and JexoPipe, helped confirm the consistency of the spectral features. The team tested various models and priors, including the effects of bin width, correlated noise, and different light-curve trends. The robustness of the DMS or DMDS signals under these different treatments adds weight to the interpretation, though not enough to eliminate alternative explanations.
The observational frontier will soon extend even further with future instruments like Ariel and the Extremely Large Telescope on the horizon. For now, K2-18 b continues to orbit its quiet M-dwarf star as a mystery—possibly a chemical echo of life, or simply a planet teaching us humility in our cosmic explorations. Either way, it represents a crucial stepping stone in answering one of humanity’s oldest questions. Are we alone in the universe? K2-18 b has given us a glimpse of how difficult that answer may be to find, but also how worthwhile the search remains.
