Using data from the James Webb Space Telescope (JWST), researchers led by the University of Cambridge have found chemical markers that may point to biological activity on the exoplanet K2-18b. These markers, dimethyl sulfide (DMS) and dimethyl disulfide (DMDS), were detected in the atmosphere of the planet, which orbits within the habitable zone of its star.
On Earth, DMS and DMDS are primarily produced by marine phytoplankton. This detection in K2-18b's atmosphere presents the strongest indication yet of potential life beyond our solar system. However, the presence of these molecules might also arise from unknown chemical processes.
The findings have achieved a 'three-sigma' level of statistical significance, indicating a 0.3% chance that the results are coincidental. A discovery level would require a 'five-sigma' significance, reducing the probability to 0.00006%. Further JWST observations, ranging from 16 to 24 hours, might help scientists achieve this threshold.
The new results were published in The Astrophysical Journal Letters. A previous signal, while weaker, suggested the presence of these molecules. “We didn’t know for sure whether the signal we saw last time was due to DMS, but just the hint of it was exciting enough for us to have another look with JWST using a different instrument,” explained Professor Nikku Madhusudhan from Cambridge’s Institute of Astronomy.
The research team analyzes the atmospheric composition of distant planets by observing the light from their parent stars during planetary transits. The absorption of part of the starlight in a planet's atmosphere provides clues about its gaseous constituents. Earlier evidence using JWST's instruments involved different wavelength ranges. The new observations employed JWST's Mid-Infrared Instrument (MIRI), providing independent confirmation.
“These DMS and DMDS detections represent biosignatures, indicating biological activity as their source. The detected concentrations are much higher than typically found on Earth," Madhusudhan noted. "Given everything we know about this planet, a Hycean world with an ocean that is teeming with life is the scenario that best fits the data we have.”
While cautiously optimistic, Madhusudhan emphasizes the need for further data before claiming life exists on K2-18b. Unknown chemical processes might explain the current observations. Collaborations will continue to explore non-biological means by which these molecules could be produced at these levels.
Co-author Subhajit Sarkar of Cardiff University remarked, “The inference of these biosignature molecules poses profound questions concerning the processes that might be producing them.”
“Our work is the starting point for all the investigations that are now needed to confirm and understand the implications of these exciting findings,” added co-author Savvas Constantinou from Cambridge’s Institute of Astronomy.
Madhusudhan stressed the need for rigorous scientific validation, saying, “It’s important that we’re deeply sceptical of our own results, because it’s only by testing and testing again that we will be able to reach the point where we’re confident in them.”
Although a definitive discovery is yet to be claimed, the powerful capabilities of JWST and future telescopes could significantly advance the search for extraterrestrial life. “Decades from now, we may look back at this point in time and recognise it was when the living universe came within reach,” Madhusudhan concluded.
The James Webb Space Telescope is a collaboration between NASA, ESA, and the Canadian Space Agency. The research was financed by a UKRI Frontier Research Grant.
Reference:
Nikku Madhusudhan et al. ‘New Constraints on DMS and DMDS in the Atmosphere of K2-18b from JWST MIRI.’ The Astrophysical Journal Letters (2025). DOI: 10.3847/2041-8213/adc1c8