There is life on planet K2-18b: Strongest signs yet of biological activity detected outside the Solar System

A team of astronomers led by the University of Cambridge has detected the chemical fingerprints of possible signs of life on a planet outside the solar system. Using data from the James Webb Space Telescope (JWST), they identified the molecules dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS) in the atmosphere of the exoplanet K2-18b, which orbits within the habitable zone of its star.

On Earth, these molecules are only produced by living organisms, mainly marine microorganisms such as phytoplankton. Although it is possible that an unknown chemical process is behind their presence on K2-18b, these results represent the strongest evidence to date that life could exist on a planet beyond the solar system.

The observations have reached a statistical significance level of three sigma, which indicates a 0.3% probability that the finding is due to chance. To be considered a scientific discovery, the threshold of five sigma must be reached, equivalent to a probability of less than 0.00006% that the result is fortuitous. Researchers estimate that an additional 16 to 24 hours of observation with the JWST could be enough to reach that level.

planet life signals — Transmission spectrum of the habitable zone exoplanet K2-18 b using the JWST MIRI spectrograph. Credit: A. Smith/N. Mandhusudhan / University of Cambridge

The exoplanet K2-18b is located 124 light-years away, in the constellation Leo. It is 8.6 times more massive and 2.6 times larger than Earth. Previous observations had already detected methane and carbon dioxide in its atmosphere, marking the first time carbon-based molecules were found on a planet in a habitable zone. This supported the theory that K2-18b could be a Hycean planet: covered by an ocean beneath a hydrogen-rich atmosphere.

The DMS and DMDS signal was now detected with the JWST’s MIRI (Mid-Infrared Instrument), which analyzes light in the mid-infrared range (6–12 microns).

This finding confirms a signal previously observed with other instruments, but now through a completely different technique and light range, which lends it greater credibility.

The analysis of the transmission spectrum — starlight filtered through the planet’s atmosphere as it passes in front of its star — allowed astronomers to identify the gases present on K2-18b. The detected concentrations of DMS and DMDS are thousands of times higher than on Earth, exceeding ten parts per million.

Although the researchers are cautiously optimistic, they stress that further studies are necessary to rule out other unknown chemical processes that could be generating these molecules without the involvement of life. The team will continue with theoretical and experimental research to test this possibility.

This work marks a milestone in the search for life on other worlds. If future observations confirm these signs, we may be facing a turning point in the exploration of the living universe.

SOURCES

University of Cambridge

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