Clear Signs of Biological Activity Detected Beyond the Solar System

Using data from the James Webb Space Telescope (JWST), astronomers led by the University of Cambridge have detected chemical traces of dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS) in the atmosphere of exoplanet K2-18b, which orbits its star within the habitable zone.

On Earth, DMS and DMDS are produced exclusively by life — primarily microbial life such as marine phytoplankton. While an unknown chemical process could theoretically be the source of these molecules in K2-18b's atmosphere, the results represent the most compelling evidence yet that life may exist on a planet beyond our Solar System.

Statistical Significance

The observations reached a statistical significance level of "three sigma," meaning the probability of them occurring by chance is 0.3%. To reach the accepted threshold for a scientific discovery, observations would need to exceed five sigma — meaning the probability of occurring by chance would be less than 0.00006%.

The researchers say that 16 to 24 hours of follow-up observation with JWST could help them achieve the critical five-sigma significance. Their findings are published in The Astrophysical Journal Letters.

About Planet K2-18b

Earlier observations of K2-18b — which is 8.6 times more massive and 2.6 times larger than Earth, located 124 light-years away in the constellation Leo — revealed the presence of methane and carbon dioxide in its atmosphere. This was the first time carbon-based molecules had been detected in the atmosphere of an exoplanet in the habitable zone. These findings were consistent with predictions for a "Hycean" planet: a habitable world covered by an ocean beneath a hydrogen-rich atmosphere.

A weaker earlier signal hinted at the possibility of something more happening on K2-18b. "We didn't know for sure whether the signal we were seeing last time was from DMS, but even a hint of it was exciting enough for us to look again at JWST using a different instrument," said Professor Nikku Madhusudhan of the Cambridge Institute of Astronomy, who led the research.

Observational Methodology

To determine the chemical composition of distant planetary atmospheres, astronomers analyze light from the host star as the planet passes in front of it when viewed from Earth. During K2-18b's transit, JWST detects the dip in the star's brightness, and a tiny fraction of starlight passes through the planet's atmosphere before reaching Earth. Absorption of part of the starlight by the planetary atmosphere leaves imprints in the stellar spectrum, which astronomers piece together to identify the constituent gases.

The previous preliminary finding of DMS was made using JWST's NIRISS (Near-Infrared Imager and Slitless Spectrograph) and NIRSpec (Near-Infrared Spectrograph) instruments, which together cover the near-infrared (0.8–5 micron) wavelength range. The new independent observation used JWST's MIRI (Mid-Infrared Instrument) in the mid-infrared (6–12 micron) range.

"This is an independent line of evidence using a different instrument than we used before, and a different wavelength range of light with no overlap with previous observations," said Madhusudhan. "The signal came through strong and clear."

"It was an incredible realization to see the results emerge and remain consistent during extensive independent analysis and robustness checks," said co-author Måns Holmberg, a researcher at the Space Telescope Science Institute in Baltimore, USA.

Properties of the Molecules

DMS and DMDS are molecules from the same chemical family, and both are proposed biosignatures. Both molecules have overlapping spectral features in the observed wavelength range, although further observations will help differentiate the two molecules.

However, the concentrations of DMS and DMDS in K2-18b's atmosphere differ greatly from those on Earth, where they are typically below one part per billion by volume. On K2-18b, they are estimated to be thousands of times stronger — more than ten parts per million.

"Earlier theoretical work predicted that high levels of sulfur-based gases like DMS and DMDS are possible on Hycean worlds," said Madhusudhan. "And now we have observed exactly that, consistent with what was predicted. Given everything we know about this planet, a Hycean world with an ocean teeming with life is the scenario that best fits the data we have."

Researchers Urge Caution

Madhusudhan says that while the results are exciting, it is critically important to gather more data before claiming that life has been found on another world. He says that although he is cautiously optimistic, previously unknown chemical processes could be occurring on K2-18b that might explain the observations. Working with colleagues, he hopes to conduct further theoretical and experimental work to determine whether DMS and DMDS can be produced abiotically under the conditions currently proposed.

"The inference of these biosignature molecules raises profound questions about the processes that could produce them," said co-author Subhajit Sarkar of Cardiff University.

"Our work is a starting point for all the research now needed to confirm and understand the implications of these exciting discoveries," said co-author Savvas Constantinou, also from the Cambridge Institute of Astronomy.

"It is important that we be deeply skeptical of our own results, because only through repeated observations will we reach the point where we are confident in them," said Madhusudhan. "That is how science should work."

Future Prospects

Although Madhusudhan stops short of claiming a definitive discovery, he argues that with powerful instruments like JWST and future planned telescopes, humanity is taking new steps toward answering the most important of all questions: are we alone?

"Decades from now, we may look back on this moment in time and realize that this was when a living universe came within reach," said Madhusudhan. "This could be a turning point, where the fundamental question of whether we are alone in the universe suddenly becomes one we can actually answer."

Project Information

The James Webb Space Telescope is a joint project of NASA, ESA, and the Canadian Space Agency (CSA). The research is supported by a Frontier Research Grant from UK Research and Innovation (UKRI).

Citation: 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