Astronomers have reported the most compelling potential evidence to date of a biosignature beyond our solar system, although they urge restraint pending further verification. Using data from the James Webb Space Telescope (JWST), a research team led by the University of Cambridge identified chemical signatures of dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS) in the atmosphere of exoplanet K2-18b, which orbits within the habitable zone of its star.
On Earth, these sulfur-containing compounds are solely associated with biological processes, primarily produced by marine microorganisms. While it remains possible that an unknown abiotic mechanism is responsible for the presence of these molecules on K2-18b, their detection marks a significant step forward in the search for extraterrestrial life.
The statistical significance of the findings has reached the “three-sigma” level, suggesting a 0.3% likelihood that the results are due to random chance. However, for the detection to meet the rigorous threshold typically required to confirm a scientific discovery, it would need to reach five-sigma significance, reducing the probability of chance to less than 0.00006%.
Researchers estimate that between 16 and 24 additional hours of observation using JWST could potentially achieve the necessary level of confidence. The findings are documented in The Astrophysical Journal Letters.
K2-18b, which is approximately 124 light-years from Earth in the constellation Leo, is about 8.6 times the mass of Earth and 2.6 times its size. Earlier observations had already identified methane and carbon dioxide in its atmosphere—marking the first time carbon-bearing molecules had been found in the atmosphere of a potentially habitable exoplanet. These results aligned with predictions for a “Hycean” world—planets characterized by vast oceans beneath hydrogen-rich atmospheres that may support life.
A prior observation suggested a faint signal that could have indicated the presence of DMS, prompting researchers to re-examine the planet using a different JWST instrument to confirm the signal. “The possibility of DMS was speculative at first, but compelling enough to merit further investigation,” explained Professor Nikku Madhusudhan, who led the study.
Astronomers analyze exoplanetary atmospheres by observing starlight filtered through them as the planet transits its host star. This method allows them to identify atmospheric constituents based on how different gases absorb light at specific wavelengths.
The initial detection of DMS was made using JWST’s NIRISS and NIRSpec instruments, which observe in the near-infrared range. The new analysis employed JWST’s MIRI instrument, which observes in the mid-infrared range—providing an independent line of evidence.
“The signal was remarkably clear and consistent across independent analyses and extensive robustness testing,” noted co-author Måns Holmberg from the Space Telescope Science Institute.
While both DMS and DMDS produce similar spectral features, the distinction between the two may become clearer with future observations. On Earth, these gases are typically present at extremely low concentrations—below one part per billion. On K2-18b, however, concentrations appear to be thousands of times higher, estimated at over ten parts per million.
“This aligns with theoretical models that predicted high levels of sulfur-based compounds in Hycean planets,” said Madhusudhan. “The scenario that best fits our data is one in which a Hycean world with a potentially life-supporting ocean is responsible for these chemical signatures.”
Despite the promising findings, the team is proceeding with caution. Madhusudhan emphasized the importance of further research to determine whether such molecules could arise through non-biological processes under the conditions present on K2-18b.
Co-author Subhajit Sarkar of Cardiff University reflected on the broader implications: “These findings raise fundamental questions about the processes that could be producing these molecules.”
Fellow co-author Savvas Constantinou, also of the University of Cambridge, added, “This work is the beginning of a deeper scientific journey—one that aims to test, confirm, and understand the potential significance of these detections.”
The researchers underscored the necessity of skepticism and repetition in scientific inquiry. “Science advances through rigorous validation. Only by thoroughly testing these results can we draw confident conclusions,” said Madhusudhan.
While not yet conclusive, these findings represent a potential turning point in the search for life beyond Earth. As observational technology continues to advance, the prospect of answering one of humanity’s oldest questions—are we alone?—draws ever closer.
“Looking back years from now, this moment may be recognized as the one when the possibility of a living universe first moved within our reach,” said Madhusudhan.
Madhusudhan, N. et al. “New Constraints on DMS and DMDS in the Atmosphere of K2-18b from JWST MIRI,” published in The Astrophysical Journal Letters (2025).
DOI: 10.3847/2041-8213/adc1c8
The research was led by the University of Cambridge and conducted using data from the James Webb Space Telescope (JWST), a collaboration between NASA, ESA, and the Canadian Space Agency (CSA), with support from UK Research and Innovation (UKRI).
Madhusudhan, N. et al. “New Constraints on DMS and DMDS in the Atmosphere of K2-18b from JWST MIRI,” published in The Astrophysical Journal Letters (2025).
DOI: 10.3847/2041-8213/adc1c8
The research was led by the University of Cambridge and conducted using data from the James Webb Space Telescope (JWST), a collaboration between NASA, ESA, and the Canadian Space Agency (CSA), with support from UK Research and Innovation (UKRI).
