Astronomers analyzing mid-infrared light from a planet 120 light-years away have reported the strongest signal yet for dimethyl sulfide or dimethyl disulfide in its atmosphere, two gases that on Earth are produced almost exclusively by living organisms. The finding, based on data from the James Webb Space Telescope’s MIRI instrument observing the sub-Neptune K2-18 b, shows the planet’s 6-to-12 micrometer transmission spectrum deviates from a featureless baseline at roughly 3.4 sigma and favors DMS or DMDS at about 3 sigma. But independent teams reprocessing the same data argue the signal may be an artifact of how the analysis was set up, and a separate study has found DMS in non-biological material from comets, complicating the claim that the gas can only come from microbes.
Why the K2-18 b DMS signal has scientists divided
The tension behind this result sits at the intersection of two problems: statistical significance and model dependence. A 3-sigma result means there is roughly a 1-in-740 chance the signal is random noise. In particle physics, that would be treated as an interesting hint but far short of a discovery. In exoplanet science, where data are scarce and expensive, it is among the strongest atmospheric detections ever claimed for a molecule linked to biology. The peer-reviewed analysis in The Astrophysical Journal Letters frames the result carefully, noting that the data favor DMS and/or DMDS but that additional observations are needed to distinguish between the two molecules.
The deeper worry is that the detection significance depends on choices made during the retrieval process, the computational step where researchers fit atmospheric models to the observed spectrum. An independent reanalysis using publicly available JWST observations and a separate reduction pipeline concluded that the DMS/DMDS detection is not robustly supported, finding that retrieval outcomes shift substantially depending on how the spectrum is binned and on degeneracies with other molecules that absorb at similar wavelengths. If an expanded set of high-temperature hydrocarbon opacities were included in the model grid, the detection significance could plausibly drop below 2 sigma, turning a tantalizing hint into statistical noise.
This matters for anyone following the search for extraterrestrial life because every future biosignature claim from JWST or its successors will face the same analytical vulnerabilities. If the community cannot agree on whether a 3-sigma feature in the best-studied candidate planet is real, the bar for a convincing detection anywhere else becomes much harder to clear. The debate over K2-18 b has therefore become a test case for how rigorously exoplanet scientists will challenge one another’s claims in the JWST era.
How JWST built the case from near-infrared to mid-infrared
The K2-18 b story began with JWST’s NIRISS and NIRSpec instruments, which captured a transmission spectrum spanning 0.9 to 5.2 micrometers. That earlier analysis reported methane and carbon dioxide in the planet’s atmosphere and described a possible hint of DMS, a finding that NASA highlighted in an official summary of the 2023 observations. The methane and carbon dioxide detections were relatively strong, but the DMS signal at those shorter wavelengths was marginal enough that it could not stand on its own.
The April 2025 MIRI LRS study was designed to test that hint by looking at longer wavelengths where DMS and DMDS have distinct absorption features. The mid-infrared spectrum, covering 6 to 12 micrometers, showed the 3.4-sigma deviation from a flat line that forms the basis of the current claim. The research team’s atmospheric retrieval models found DMS and/or DMDS provided the best fit at approximately 3 sigma, though the two molecules could not be cleanly separated given the data quality. In practical terms, that means the spectrum prefers some kind of sulfur-bearing molecule with the right mid-infrared fingerprints, but cannot yet say which one or in what abundance.
A separate comprehensive reanalysis of the original near-infrared JWST spectrum, using NIRISS SOSS and NIRSpec G395H data with updated handling of second-order spectral contamination, also questioned whether the earlier DMS hint was stable under alternative data-reduction assumptions. That work underscored how small choices-such as how to correct for instrument systematics or how to treat overlapping spectral orders-can subtly reshape the final transmission spectrum. Taken together, these reanalyses suggest the DMS signal is sensitive to technical decisions at every stage of the pipeline, from raw data extraction to the molecular line lists used in modeling.
Those concerns echo broader discussions in the exoplanet community about how to interpret JWST’s most provocative results. Commentaries in the scientific press have noted that early claims of potential biosignatures or exotic atmospheres are likely to face intense scrutiny as more teams apply independent pipelines to the same datasets. In that sense, K2-18 b is not an outlier but an early example of how JWST’s precision is forcing astronomers to confront the limits of their modeling assumptions.
Abiotic DMS and the limits of a biosignature
Even if the atmospheric signal is real, the claim that DMS is “made only by microbes” faces a direct challenge. A peer-reviewed study published in The Astrophysical Journal presented evidence for abiotic dimethyl sulfide in cometary matter, showing the molecule can form or persist in extraterrestrial environments without any biological input. That finding does not rule out a biological origin for DMS on K2-18 b, but it removes the logical shortcut that detection of DMS automatically equals detection of life. Instead, DMS must now be treated like many other proposed biosignatures: potentially informative, but ambiguous without a broader environmental context.
On Earth, most atmospheric DMS originates from marine life, especially phytoplankton that produce precursor compounds later broken down in the ocean and released to the air. Because of that tight association with biology, astrobiologists had long viewed DMS as a relatively “clean” biosignature candidate. The cometary result shows that under the right conditions, simple carbon- and sulfur-bearing molecules can assemble into DMS without any cells involved, likely through reactions on icy grains or in irradiated ices. If such pathways operate in protoplanetary disks or planetary atmospheres, they could seed DMS on worlds that have never hosted life.
For K2-18 b, that ambiguity is compounded by the planet’s nature as a sub-Neptune: a world larger than Earth with a significant hydrogen-rich envelope. Its surface, if it has one, is likely buried beneath high-pressure layers of gas and possibly supercritical fluids, far from the temperate oceans imagined in classic habitable-zone scenarios. Astrobiologists therefore have to ask not only whether DMS is present, but whether any plausible biosphere could exist in the planet’s atmospheric or interior environments to produce it in the first place.
What would it take to settle the debate?
Resolving the DMS controversy will require both better data and more robust models. On the observational side, additional JWST transits with MIRI and the near-infrared instruments could improve the signal-to-noise ratio and test whether the 3.4-sigma feature repeats consistently. Phase-curve or eclipse measurements might also help disentangle atmospheric temperature structures that can mimic or mask molecular signatures. Longer term, future missions designed for high-contrast imaging and high-resolution spectroscopy could probe K2-18 b’s atmosphere with far greater sensitivity.
On the modeling side, researchers are already expanding molecular opacity databases to include a wider range of hydrocarbons and sulfur species at the high temperatures relevant to sub-Neptunes. Retrieval frameworks are incorporating more flexible parameterizations of clouds, hazes, and vertical mixing, as well as more systematic ways to compare models with different underlying assumptions. Independent teams are also advocating for standardized benchmarks and community challenges, so that competing pipelines can be tested on the same synthetic datasets before being applied to real worlds.
Equally important is the cultural shift toward treating potential biosignatures as hypotheses to be stress-tested rather than trophies to be claimed. A widely discussed preprint on exoplanet atmospheres emphasized that multi-molecule, multi-wavelength evidence will be essential before drawing any conclusions about life beyond Earth. That perspective has been echoed in recent news coverage of the K2-18 b debate, which portrays the current DMS claim less as a near-miss discovery and more as a rehearsal for the kinds of arguments that will unfold when an even stronger signal appears.
For now, K2-18 b sits in an ambiguous middle ground. Its atmosphere almost certainly contains methane and carbon dioxide, and it may host sulfur-bearing molecules whose precise identity and origin remain unclear. The putative DMS signal is intriguing but fragile, vulnerable both to improved data and to more exhaustive models. Rather than a disappointment, many astronomers see that fragility as a sign of a healthy field: one that is learning, in real time, how to distinguish genuine whispers of biology from the far louder background noise of physics and chemistry.
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*This article was researched with the help of AI, with human editors creating the final content.
