A group of astronomers has identified what may be the clearest signs yet of biological activity beyond our solar system. In a new study published in The Astrophysical Journal Letters, researchers say they have detected chemical compounds on the exoplanet K2-18b that, on Earth, are only produced by living organisms.
The findings, made using the James Webb Space Telescope, suggest the presence of dimethyl sulfide (DMS) and potentially dimethyl disulfide (DMDS) in the planet’s atmosphere, both of which are byproducts of marine microbial life on Earth. The discovery is not a confirmation of alien life, but it marks a major step forward in the search for habitable worlds.
What is K2-18b?
K2-18b is an exoplanet located 124 light-years from Earth. It lies in the habitable zone of its host star, meaning its surface could support liquid water. Scientists believe it may be a “Hycean world”—a planet with a hydrogen-rich atmosphere and a vast ocean covering its entire surface.
Lead study author Nikku Madhusudhan, an astrophysicist at the University of Cambridge, said that earlier theoretical models had predicted sulfur-based gases like DMS could exist on Hycean worlds. “Now we’ve observed it,” he said in a statement. “Given everything we know about this planet, a Hycean world with an ocean teeming with life best fits the data.”
A cautious breakthrough
The molecules were detected using the telescope’s Mid-Infrared Instrument, which provides a new line of evidence independent from earlier observations made with other instruments. Previous Webb data had already revealed methane and carbon dioxide in K2-18b’s atmosphere, hinting at complex chemical processes.
However, Madhusudhan and his team are cautious. The presence of DMS and DMDS could also be explained by unknown non-biological chemical processes. “It’s important we remain skeptical of our own results,” he said. “That’s how science has to work.”
Experts urge patience
Not all astronomers agree on the nature of K2-18b. MIT astrophysicist Sara Seager, who was not involved in the study, noted that some researchers believe the planet could instead be a mini-Neptune or even have a molten magma surface beneath its atmosphere—an environment far too hostile for life as we know it.
Seager warned against jumping to conclusions. “Enthusiasm is outpacing evidence,” she said, adding that claims of biosignatures will likely remain in the “candidate” category for a long time.
A high bar for confirmation
The current data indicates the molecules could be present in concentrations thousands of times stronger than what’s typically found on Earth. But proving that they are actually there—and produced by life—requires more observation and verification.
Astrobiologist Eddie Schwieterman of UC Riverside, who was not part of the study, called the findings “exciting” but said more evidence is needed. His team’s research shows that reproducing the detected levels of DMS would require biological activity roughly 20 times higher than Earth’s average.
He also noted the absence of ethane, a molecule that would typically be present if DMS were breaking down under ultraviolet light. “It makes me think we’ve missed something,” he said.
Not yet proof, but a step closer
The detection currently meets a “three-sigma” level of statistical significance—meaning there’s a 0.3% chance the result is due to random noise. For a scientific discovery to be confirmed, it must reach the more stringent five-sigma threshold.
Still, researchers say this marks a major advancement. “We’ve shown that it is possible to detect potential biosignatures,” Madhusudhan said. “Whether or not life exists on K2-18b, we’ve proven we have the tools to search for it.”
He added, “The bigger question now is whether we as a species are ready to find life as we don’t know it—and to define what it even means to be alive elsewhere.”
