In a discovery that may one day be recognized as one of the most important milestones in the search for extraterrestrial life, astronomers using the James Webb Space Telescope (JWST) have detected the strongest hint yet of a biosignature outside our solar system. The exoplanet in question, K2-18b—a super-Earth orbiting within the habitable zone of its star—has revealed the presence of complex molecules that, on Earth, are known to be produced exclusively by life.
Although researchers remain appropriately cautious, the latest findings could represent a historic turning point in humanity’s quest to determine whether we are alone in the universe.
A Promising Signal From a Distant World
K2-18b lies 124 light-years away in the constellation Leo. This intriguing world is about 8.6 times more massive and 2.6 times larger than Earth. It’s located within its star’s habitable zone—a region where temperatures could permit the presence of liquid water, a key ingredient for life as we know it.
Previous observations had already flagged K2-18b as a candidate for further study. Data from earlier instruments suggested the presence of methane and carbon dioxide—carbon-based molecules—within its hydrogen-rich atmosphere. These are the kinds of gases that could indicate a planet similar in structure to a “Hycean world”—a hypothetical class of exoplanets featuring vast oceans beneath thick, hydrogen-dominated skies. Such conditions, researchers argue, could be ideal for hosting microbial life.
But it was a weaker signal, hinting at something far more exciting, that truly piqued astronomers’ curiosity.
Detecting the Chemical Fingerprints of Life
Using the extraordinary capabilities of the James Webb Space Telescope, a team led by Professor Nikku Madhusudhan from the University of Cambridge focused on that ambiguous signal. The team directed JWST to re-examine K2-18b with its Mid-Infrared Instrument (MIRI), which scans a different part of the light spectrum compared to previous observations.
And there it was—clear and strong: the spectral signatures of dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS), complex sulfur-bearing molecules that, on Earth, are only produced by life—specifically, by marine microorganisms such as phytoplankton.
To be clear, this isn’t definitive proof of alien life. But it is, as scientists would put it, “a statistically significant detection.” The data currently meets a three-sigma threshold—meaning there’s only a 0.3% chance the signal occurred randomly. With about 16–24 more hours of telescope time, the team hopes to push that significance to the all-important five-sigma level—a 1-in-3.5 million chance that the result is a fluke. That level of certainty is widely accepted as the standard for scientific discovery.
Why DMS and DMDS Matter
Dimethyl sulfide and dimethyl disulfide belong to the same family of molecules. Both contain sulfur atoms and both are recognized as potential biosignatures—compounds that, if found in a planet’s atmosphere, could indicate the presence of life.
On Earth, DMS is created almost exclusively by living organisms. It’s emitted in huge quantities by oceanic plankton and is actually responsible for that faint “smell of the sea.” DMDS is similarly produced during biological processes, although it can also form under certain rare chemical conditions. The presence of either in significant quantities is noteworthy, especially in the context of an exoplanet.
Here’s the jaw-dropping part: on K2-18b, the concentration of these molecules is estimated to be thousands of times higher than it is on Earth—more than ten parts per million, compared to less than one part per billion in Earth’s atmosphere.
What could possibly explain such high levels?
The Case for a Hycean World Teeming with Life
Professor Madhusudhan, who has long championed the idea of Hycean worlds as potentially life-friendly environments, believes that these findings perfectly align with prior predictions.
“Earlier theoretical work had predicted that high levels of sulfur-based gases like DMS and DMDS are possible on Hycean worlds,” he explained. “And now we’ve observed it, in line with what was predicted.”
K2-18b, with its dense hydrogen-rich atmosphere, carbon-bearing molecules, and now sulfur-based potential biosignatures, may be the most promising candidate for alien life ever discovered. It presents a compelling picture: an enormous ocean-covered planet with a warm, hydrogen-dense sky and an active biosphere below.
If that sounds like the setting of a science fiction novel, it’s because for decades, this kind of world was the stuff of speculation. Now, it’s part of our observational reality.
A Journey Through the Starlight
How exactly are astronomers able to detect these molecules on such a distant world?
The process is both simple in concept and dazzlingly complex in execution. As K2-18b passes in front of its star—a phenomenon known as a transit—a tiny fraction of starlight filters through its atmosphere before it reaches Earth. Different gases in the planet’s atmosphere absorb light at different wavelengths, creating a kind of spectral “barcode” that reveals their identity.
JWST, with its cutting-edge instruments like NIRISS, NIRSpec, and MIRI, can detect these barcodes across a wide swath of the infrared spectrum. The latest observations relied on MIRI’s mid-infrared capabilities to offer an entirely new window into the planet’s atmospheric chemistry—without overlapping with earlier datasets.
“The signal came through strong and clear,” said Madhusudhan. “It was an incredible realization seeing the results emerge and remain consistent throughout the extensive independent analyses and robustness tests.”
Tempering Optimism with Scientific Rigor
Despite the excitement, the researchers remain firmly grounded.
They stress that while DMS and DMDS are produced by life on Earth, it is possible that non-biological processes unfamiliar to us could produce similar molecules on other planets. Indeed, the universe is vast and varied—chemical processes might operate in ways we’ve never encountered.
To account for this, the team is now working to explore whether there are any abiotic pathways—that is, non-living chemical reactions—that could produce DMS or DMDS in such high concentrations under the conditions found on K2-18b.
Co-author Subhajit Sarkar of Cardiff University emphasized this need for further analysis: “The inference of these biosignature molecules poses profound questions concerning the processes that might be producing them.”
That’s why skepticism, they say, is vital. Not because they doubt their work, but because that’s the foundation of good science.
“It’s important that we’re deeply skeptical of our own results,” Madhusudhan said. “Because it’s only by testing and testing again that we will be able to reach the point where we’re confident in them. That’s how science has to work.”
A New Era in the Search for Life
Even as the researchers stress caution, they can’t hide their sense of wonder.
“This could be the tipping point,” Madhusudhan mused, “where suddenly the fundamental question of whether we’re alone in the universe is one we’re capable of answering.”
The discovery highlights the transformative power of the James Webb Space Telescope, a joint venture between NASA, the European Space Agency (ESA), and the Canadian Space Agency (CSA). With its unprecedented sensitivity and infrared vision, JWST is not only reshaping our understanding of the universe—it’s giving us tools to directly address one of humanity’s oldest and most profound questions.
Savvas Constantinou, a co-author from Cambridge, put it this way: “Our work is the starting point for all the investigations that are now needed to confirm and understand the implications of these exciting findings.”
Looking Forward: What Comes Next?
The road ahead is both thrilling and filled with hard work. The researchers plan to schedule additional observing time with JWST to increase the statistical certainty of their results. Simultaneously, teams around the world will work to replicate the findings and to build theoretical models that might explain how such molecules could be produced without life.
If the signals are confirmed—and no plausible abiotic explanation emerges—then humanity may be closer than ever to the first true detection of life beyond Earth.
But even if that confirmation remains elusive for now, the broader message is clear: we are entering a golden age of planetary discovery.
With every new world we observe, with every strange molecule detected, and with every improved telescope sent into the heavens, we take one step closer to answering the question that has haunted philosophers, poets, and scientists alike for centuries.
Are we alone?
The answer may soon lie not in the stars, but in the spectral signature of a far-off ocean world—drifting in the quiet shadows of a distant sun.
Reference: Nikku Madhusudhan et al, New Constraints on DMS and DMDS in the Atmosphere of K2-18 b from JWST MIRI, The Astrophysical Journal Letters (2025). DOI: 10.3847/2041-8213/adc1c8
To learn more about Hycean worlds, visit hycean.group.cam.ac.uk.