Are We Finally Not Alone? Webb Detects Life’s Signature on Distant Ocean Planet
Astronomers may have found the strongest hint of life beyond Earth thanks to new data from the James Webb Space Telescope.
They’ve detected sulfur-based molecules in the atmosphere of exoplanet K2-18b that, on Earth, are only produced by living organisms. While the evidence isn’t conclusive yet, it matches predictions for a “Hycean” world — one potentially covered in oceans and capable of hosting microbial life. If further observations confirm these findings, it could mark a turning point in our search for extraterrestrial life.
Possible Biosignature Found on Distant Exoplanet
Astronomers have found the most compelling potential signs of life yet beyond our solar system — but they’re interpreting the results with caution.
Using data from the James Webb Space Telescope (JWST), a team led by the University of Cambridge has identified possible traces of the molecules dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS) in the atmosphere of exoplanet K2-18b. This planet orbits within the habitable zone of its star, where conditions might allow liquid water to exist.
Astronomers have detected the most promising signs yet of a possible biosignature outside the solar system, although they remain cautious. Using data from the James Webb Space Telescope (JWST), the astronomers, led by the University of Cambridge, have detected the chemical fingerprints of dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS), in the atmosphere of the exoplanet K2-18b, which orbits its star in the habitable zone. Credit: A. Smith, N. Madhusudhan (University of Cambridge)
Earth-Like Clues from Alien Atmosphere
On Earth, both DMS and DMDS are produced exclusively by living organisms — mainly microbes like marine phytoplankton. Although it’s possible these molecules on K2-18b could come from unknown non-biological processes, their presence offers the strongest indication so far that life might exist on a planet outside our solar system.
The findings have reached what’s known as a “three-sigma” level of statistical confidence. This means there’s a 0.3% chance the signal is a fluke. For a scientific discovery to be considered confirmed, however, it must reach “five-sigma” significance—less than a 0.00006% chance of being due to random variation.
More Telescope Time Could Seal the Deal
The researchers say between 16 and 24 hours of follow-up observation time with JWST may help them reach the all-important five-sigma significance. Their results are reported in The Astrophysical Journal Letters.
Earlier observations of K2-18b — which is 8.6 times as massive and 2.6 times as large as Earth, and lies 124 light years away in the constellation of Leo — identified methane and carbon dioxide in its atmosphere. This was the first time that carbon-based molecules were discovered in the atmosphere of an exoplanet in the habitable zone. Those results were consistent with predictions for a ‘Hycean’ planet: a habitable ocean-covered world underneath a hydrogen-rich atmosphere.
The graph shows the observed transmission spectrum of the habitable zone exoplanet K2-18 b using the JWST MIRI spectrograph. The vertical shows the fraction of starlight absorbed in the planet’s atmosphere due to molecules in the planet’s atmosphere. The data are shown in the yellow circles with the 1-sigma uncertainties. The curves show the model fits to the data, with the black curve showing the median fit and the cyan curves outlining the 1-sigma intervals of the model fits. The absorption features attributed to dimethyl sulphide and dimethyl disulphide are indicated by the horizontal lines and text. The image behind the graph is an illustration of a hycean planet orbiting a red dwarf star. Credit: A. Smith, N. Madhusudhan (University of Cambridge)
A Hint That Sparked a Closer Look
However, another, weaker signal hinted at the possibility of something else happening on K2-18b. “We didn’t know for sure whether the signal we saw last time was due to DMS, but just the hint of it was exciting enough for us to have another look with JWST using a different instrument,” said Professor Nikku Madhusudhan from Cambridge’s Institute of Astronomy, who led the research.
To determine the chemical composition of the atmospheres of faraway planets, astronomers analyse the light from its parent star as the planet transits, or passes in front of the star as seen from the Earth. As K2-18b transits, JWST can detect a drop in stellar brightness, and a tiny fraction of starlight passes through the planet’s atmosphere before reaching Earth. The absorption of some of the starlight in the planet’s atmosphere leaves imprints in the stellar spectrum that astronomers can piece together to determine the constituent gases of the exoplanet’s atmosphere.
New Instruments, Stronger Evidence
The earlier, tentative, inference of DMS was made using JWST’s NIRISS (Near-Infrared Imager and Slitless Spectrograph) and NIRSpec (Near-Infrared Spectrograph) instruments, which together cover the near-infrared (0.8-5 micron) range of wavelengths. The new, independent observation used JWST’s MIRI (Mid-Infrared Instrument) in the mid-infrared (6-12 micron) range.
“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,” said Madhusudhan. “The signal came through strong and clear.”
“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 in Baltimore, USA.
Possible Biosignatures: DMS and DMDS Explained
DMS and DMDS are molecules from the same chemical family, and both are predicted to be biosignatures. Both molecules have overlapping spectral features in the observed wavelength range, although further observations will help differentiate between the two molecules.
However, the concentrations of DMS and DMDS in K2-18b’s atmosphere are very different than on Earth, where they are generally below one part per billion by volume. On K2-18b, they are estimated to be thousands of times stronger – over ten parts per million.
Hycean Worlds Could Host Life After All
“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.”
Madhusudhan says that while the results are exciting, it’s vital to obtain more data before claiming that life has been found on another world. He says that while he is cautiously optimistic, there could be previously unknown chemical processes at work on K2-18b that may account for the observations. Working with colleagues, he is hoping to conduct further theoretical and experimental work to determine whether DMS and DMDS can be produced non-biologically at the level currently inferred.
A Profound Scientific Puzzle Unfolds
“The inference of these biosignature molecules poses profound questions concerning the processes that might be producing them” said co-author Subhajit Sarkar of Cardiff University.
“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, also from Cambridge’s Institute of Astronomy.
“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.”
A Historic Step Toward Answering: Are We Alone?
While he is not yet claiming a definitive discovery, Madhusudhan says that with powerful tools like JWST and future planned telescopes, humanity is taking new steps toward answering that most essential of questions: are we alone?
“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.”
Reference: “New Constraints on DMS and DMDS in the Atmosphere of K2-18 b from JWST MIRI” by Nikku Madhusudhan, Savvas Constantinou, Måns Holmberg, Subhajit Sarkar, Anjali A. A. Piette and Julianne I. Moses, 17 April 2025, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/adc1c8
The James Webb Space Telescope is a collaboration between NASA, ESA and the Canadian Space Agency (CSA). The research is supported by a UK Research and Innovation (UKRI) Frontier Research Grant.
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