Key Takeaways:
- A NASA-led reanalysis of JWST data from exoplanet K2-18 b, incorporating multiple independent analyses, found no conclusive evidence of dimethyl sulfide (DMS), a potential biosignature, despite a previously reported tentative detection.
- The new study, utilizing Bayesian analysis, confirms K2-18 b's water-rich atmosphere and presence of methane and carbon dioxide, but the DMS signal, while present, remains below the 5-sigma threshold required for definitive confirmation.
- The research highlights the complexity of interpreting atmospheric data and the need for further investigation, emphasizing that abiotic processes could produce DMS in hydrogen-rich atmospheres such as K2-18 b's.
- The K2-18 b case underscores the ongoing challenges and iterative nature of exoplanet research, illustrating the need for robust methodologies, multiple lines of evidence, and advanced instrumentation, such as the upcoming Habitable Worlds Observatory, for conclusive biosignature detection.
Astronomers have long sought signs of life beyond Earth, with exoplanets like K2-18 b — a world about 124 light-years away — offering tantalizing possibilities.
In 2023 and again in April 2025, a team using the James Webb Space Telescope (JWST) detected molecules in K2-18 b’s atmosphere they claimed could be a biosignature: dimethyl sulfide (DMS), a gas produced on Earth exclusively by marine life.
Related: K2-18 b could have dimethyl sulfide in its air. But is it a sign of life?
Controversy followed, and now a new NASA-led analysis uploaded to the arXiv preprint server July 16 tempers that excitement. While confirming K2-18 b’s water-rich nature and revealing a complex atmosphere, the study finds no conclusive evidence of DMS and underscores the challenges of interpreting such signals.
Revisiting the DMS claim
The April 2025 paper, which was led by Nikku Madhusudhan at the University of Cambridge and has now been accepted for publication in The Astrophysical Journal Letters, made headlines by reporting a 3-sigma confidence that the atmosphere of K2-18 b contained DMS. This means a likelihood of just 0.3 percent that their observations of K2-18 b fit a model atmosphere that contained DMS by chance.
The team concluded they were either seeing signs of life on another planet for the first time or a new chemical process for the abiotic (not requiring life) production of DMS. Calling the moment “profound” in a Cambridge University video released in April, Madhusudhan stated, “However you put it, we are seeing new chemical processes on a planet that could be habitable.”
As Astronomy reported at the time, many astronomers expressed skepticism over the paper’s methodology for testing for the presence of DMS without fully accounting for other molecules or instrumental noise. A Phys.org report summarizing the debate noted concerns that the signal might have been overstated because the analysis did not consider other possible gases.
Unlike earlier JWST announcements, NASA did not issue a press release for the April study, reflecting the uncertainty and debate in the scientific community. However, the agency did release a statement April 18 reiterating the high bar any claims of life must pass: “We would need follow-up studies and multiple converging lines of evidence to confirm true biosignatures and rule out false positives, possibly including independent data from multiple missions and extensive atmospheric modeling.”
Collaboration in action
The latest study, led by Renyu Hu at NASA’s Jet Propulsion Laboratory and involving Madhusudhan as well as other independent teams, adopts a more measured and collaborative approach. By combining four new JWST observations of K2-18 b transiting its star with the original data, the researchers applied a method called Bayesian analysis that allowed each of three teams to develop their own interpretation of the data based on a range of possible atmospheric compositions, rather than a single model.
“We always strive to test the robustness of our findings, and one of the best ways to do that is through independent analyses by multiple teams,” Hu tells Astronomy. “By comparing results across these teams, we could identify which conclusions are robust — consistent regardless of modeling choices — and which are more sensitive to specific model assumptions.”
The result is a nuanced portrait of K2-18b: a world with an atmosphere rich in water — perhaps comprising as much as half its mass — and the presence of methane and carbon dioxide.
When it comes to DMS, the new analysis finds a tentative signal at about 2.7 sigma confidence, which, while close to the previous confidence 3 sigma, is still well below the 5-sigma level required by the scientific community for a conclusive detection. (A 5-sigma confidence represents a mere 0.00006 percent probability the data fits the model by chance.)
So, while hints of DMS persist, the evidence is still far from strong enough to claim its definitive presence.
Rather than rejecting the signal outright, the team emphasizes the path forward. “Continued characterization of K2‑18 b and similar worlds will benefit from deeper observational campaigns that establish and refine upper limits, confirm tentative molecular detections and nondetections, and employ high-resolution spectroscopy to better constrain key trace gases,” they write in the July paper’s conclusion.
The science is still exciting and points toward a future discovery of life on an exoplanet. As the authors put it: “With K2‑18 b as a trailblazer, the discovery of a potentially habitable world is no longer a distant aspiration but an emerging scientific reality.”
Life or chemistry?
One of the biggest shifts since April is the recognition that DMS might not be the smoking gun signal for life it was once thought to be — at least not on K2-18 b.
On Earth, DMS is produced exclusively by marine microorganisms, making it a prime biosignature. But the new study, supported by modeling, shows that abiotic processes can generate DMS and related molecules in hydrogen-rich atmospheres like K2-18 b’s.
“Our analysis showed that atmospheric processes on K2‑18 b that don’t involve life — particularly photochemistry [chemical reactions that occur when molecules interact with light] — can also produce detectable levels of DMS and other organosulfur molecules,” Hu says. “This means that DMS, on its own, may not be a definitive sign of life on this planet.”
When asked the same question, Madhusudhan explains, “As for whether DMS is a reliable biomarker depends on the atmospheric extent. … If the planet has a deep atmosphere then there may be a way to make DMS without biology. On the other hand, if it is a thin atmosphere with a liquid water ocean underneath, i.e., a hycean world, then biology is still the only explanation known currently.”
He optimistically continues: “The current observations are slightly more indicative of a hycean world, but we should remain open to the abiotic possibility as well.”
Supporting this, laboratory experiments in 2024 have demonstrated abiotic DMS formation under simulated exoplanetary conditions. DMS has also been detected in cometary and interstellar environments, far removed from life as we know it.
Overall, the new work broadens the context for interpreting potential biosignatures, making it clear that “context matters,” Hu emphasizes.
Lessons from history
The K2-18 b saga is not unique.
Astrobiology has previously seen high-profile claims followed by vigorous debate and reinterpretation. The 2020 announcement of phosphine on Venus, once hailed as a possible biosignature, was later questioned as researchers found alternative explanations for the spectral features. Similarly, the 1996 claim of fossilized bacteria in a Martian meteorite was dealt a critical blow when further research revealed that the key evidence, magnetite crystals, could have been formed through non-biological processes, and that the initial analysis was based on a potentially selective set of data
The search continues
While the latest study does not confirm or disprove life on K2-18 b, it advances the field by clarifying what the data do — and do not — reveal. And the planet remains an exciting laboratory for studying water-rich worlds beyond our solar system, with JWST observations pushing the limits of what we can detect.
“We are really fortunate to even be at a position where we are having these conversations,” Madhusudhan says. “We have entered a new era in the search for life beyond Earth where it is possible to detect and debate about potential biosignatures. This is unprecedented and we should make the best use of this opportunity.”
While JWST has opened a new frontier in probing exoplanet atmospheres, its instruments were not designed specifically to detect definitive signs of life. The data it gathers are groundbreaking, but often requires careful interpretation and remains limited in sensitivity. The upcoming Habitable Worlds Observatory (HWO), slated for launch in the 2040s, will be the first space telescope optimized to search for biosignatures on Earth-sized planets in habitable zones. By directly imaging exoplanets and capturing higher-resolution spectra, HWO will help resolve questions like those surrounding K2-18 b with far greater clarity.
