New evidence from the James Webb Space Telescope suggests astronomers may have found the strongest hints yet of life beyond our solar system.
Scientists using Webb’s Mid-Infrared Instrument (MIRI) have detected the chemical signatures of dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) in the atmosphere of K2-18b, a potentially habitable exoplanet located 124 light-years away. On Earth, these sulfur compounds are produced almost exclusively by living organisms, primarily marine microbes.
“This is an independent line of evidence, using a different instrument than we did before and a different wavelength range of light, where there is no overlap with the previous observations,” explained Professor Nikku Madhusudhan from Cambridge’s Institute of Astronomy, who led the research. “The signal came through strong and clear.”
The discovery builds on previous Webb observations that found carbon-based molecules methane and carbon dioxide in K2-18b’s atmosphere, consistent with predictions for a “Hycean” world – a habitable ocean-covered planet with a hydrogen-rich atmosphere. Those earlier measurements also provided a tentative hint of DMS, but the evidence was weak.
This time, the detection reaches a “three-sigma” level of statistical significance – meaning there’s only a 0.3% probability the signal occurred by chance. While impressive, the gold standard for claiming a definitive discovery is five-sigma, which would require additional observation time with Webb.
What makes the finding particularly intriguing is the concentration of these potential biosignature gases in K2-18b’s atmosphere. The researchers estimate levels of at least 10 parts per million, thousands of times higher than found on Earth.
“It was an incredible realisation seeing the results emerge and remain consistent throughout the extensive independent analyses and robustness tests,” said co-author Måns Holmberg, a researcher at the Space Telescope Science Institute.
The research team urges caution, however. While the findings align with predictions for a life-bearing ocean world, unknown chemical processes could theoretically produce these compounds without biology.
“Earlier theoretical work had predicted that high levels of sulfur-based gases like DMS and DMDS are possible on Hycean worlds,” said Madhusudhan. “And now we’ve observed it, in line with what was predicted. 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.”
K2-18b itself is larger than Earth – about 8.6 times as massive and 2.6 times wider – orbiting within its star’s habitable zone, where temperatures could allow liquid water to exist on its surface.
The team plans follow-up observations with Webb that could reach the five-sigma threshold for scientific discovery with just 16-24 additional hours of telescope time.
“Our work is the starting point for all the investigations that are now needed to confirm and understand the implications of these exciting findings,” said co-author Savvas Constantinou from Cambridge’s Institute of Astronomy.
While stopping short of claiming definitive evidence of alien life, Madhusudhan believes we’re entering a new era in the search for extraterrestrial biology.
“Decades from now, we may look back at this point in time and recognise it was when the living universe came within reach,” said Madhusudhan. “This could be the tipping point, where suddenly the fundamental question of whether we’re alone in the universe is one we’re capable of answering.”
The research was published April 17 in The Astrophysical Journal Letters.
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