“There’s always going to be a way to make something abiotically.” With these words, chemist Eleanor Browne of the University of Colorado Boulder captured the humility accompanying one of the most exciting scientific reports of 2025: the identification of dimethyl sulfide and dimethyl disulfide in the air of exoplanet K2-18b. These two molecules, on our planet, are almost exclusively the work of living creatures ocean phytoplankton and bacteria, to be exact. But, their existence 124 light-years away has also sparked excitement and controversy throughout the astrophysical world.
The trip to that point started with the James Webb Space Telescope (JWST), whose sensitivity has opened up new possibilities in exoplanetary science. JWST’s capability to take and analyze the faintest whispers of starlight passing through a distant planet’s atmosphere is revolutionary. When K2-18b passes in front of its red dwarf star, some of the starlight gets filtered through its atmosphere, leaving behind a spectral footprint that, when deciphered with utmost care, indicates the existence of certain molecules. The team led by Cambridge, employing JWST’s MIRI (Mid-Infrared Instrument), registered a signal for dimethyl sulfide and dimethyl disulfide that was, according to Professor Nikku Madhusudhan, “strong and clear” and, importantly, consistent across independent analysis and robustness tests.
It is not the molecules themselves that make this detection incredible, but their concentration. On Earth, dimethyl sulfide rarely exceeds one part per billion by volume in the atmosphere. On K2-18b, the team estimates levels exceeding ten parts per million thousands of times higher than on our own planet. Professor Madhusudhan, introducing the results, said, “You need thousands of times of Earth’s concentrations to be able to explain the data.” This richness, added to the planet’s location in the so-called habitable zone where liquid water might be present has prompted the scientists to suggest that K2-18b could be a “Hycean” planet: an ocean-covered world with a hydrogen-rich atmosphere, a theoretical type only recently proposed by planetary scientists.
But the path from detection to confirmation is steep. In exoplanet biosignatures, statistical proof is king. The Cambridge researchers’ result has hit the ‘three-sigma’ mark i.e., a 0.3% chance that the signal is a statistical anomaly. For a scientific find to gain general acceptance, however, five-sigma is needed, dropping the chance of error to below 0.00006%. This is not pedantry; the record of exoplanetary science illustrates that initial indications can vanish in the light of additional analysis. The much-touted case of phosphine in Venusian clouds was ultimately explained away by computational errors in the data.
The technical capabilities of the JWST have been key to this progress. The telescope’s complement of spectroscopic instruments NIRISS, NIRSpec, and now MIRI has allowed scientists to observe various ranges of wavelengths, allowing them to independently test for atmospheric components. This redundancy is crucial in an area where the signals are weak and contamination risk both instrumental and astrophysical is always present.
However, the analysis of these biosignatures is ridden with difficulty. Lab tests have proved that dimethyl sulfide can be formed abiotically under specific conditions, for example, by subjecting a model atmosphere to ultraviolet radiation. The molecule has also been found on comet 67P and in the interstellar medium, further complicating things. As Chris Lintott, an astronomer at Oxford, noted, “Looking for what’s biological on Earth isn’t a good guide to what might be biological elsewhere.” The planetary environment’s context its atmospheric chemistry, temperature, and geology must all be examined before making conclusions regarding life.
Compounding the controversy are rival models of K2-18b itself. Some scientists propose the planet might be a miniature gas giant with no surface, or a molten-rock-based world instead of a water-dominated one. The lack of ammonia in the atmosphere, for instance, would mean absorption by a massive ocean or by some other type of planetary interior altogether.
In spite of these doubts, finding the presence of dimethyl sulfide and dimethyl disulfide in such large quantities is the strongest evidence to date for possible biological activity in the universe outside our solar system. Cambridge’s team is still wary. According to Professor Madhusudhan, “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,” Madhusudhan said. “That’s how science has to work.”
With only 16 to 24 hours of extra JWST observation time, the team plans to take their result up to the five-sigma level. If verified, the consequences would resonate far beyond astronomy and change the way humanity understands its place in the universe. Meanwhile, the hunt goes on, fueled by the persistent interplay between technical creativity, statistical discipline, and scientific caution that characterizes the edge of exoplanetary science.
