Astronomers say they’ve found “the most promising signs yet” of chemicals on a planet beyond our Solar System that could indicate the presence of life on its surface.
Using the James Webb Space Telescope, the team found a possible ‘biosignature’ – the potential fingerprint of life – within its atmosphere, although they say they’re remaining “cautious”, and that this isn’t a confirmed detection.
The chemicals detected are the same as those produced by marine-dwelling organisms on Earth.
The team, led by the University of Cambridge in the UK, detected signs of dimethyl sulfide and dimethyl disulfide in the atmosphere of exoplanet K2-18b.
This planet orbits its star in the habitable zone (sometimes called the Goldilocks Zone), which is the region around a star in which an orbiting planet might have conditions suitable for the emergence of life, such as the ability for liquid water to exist on its surface.
K2-18b is 8.6 times as massive and 2.6 times as large as Earth and lies 124 lightyears away from our planet.
Building a bigger picture
This isn’t the first study of exoplanet K2-18b.
A 2023 study of K2-18b by the same team identified methane and carbon dioxide in the planet’s atmosphere.
This in itself was a huge discovery: the first time carbon-based molecules had been found in the atmosphere of an exoplanet – a planet beyond our Solar System – in the habitable zone.
Astronomers say the 2023 results showed K2-18b could be a ‘Hycean’ planet, meaning a habitable world with a liquid ocean and a hydrogen-rich atmosphere.
That earlier study found a tantalising hint of dimethyl sulfide and dimethyl disulfide, but this latest study has made a more promising detection.
“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,” says Professor Nikku Madhusudhan from Cambridge’s Institute of Astronomy, who led the research.
The team say that on Earth, dimethyl sulfide and dimethyl disulfide are only produced by life, mainly microbial life like phytoplankton we see in our oceans.
However, there could be another explanation for the detection of the chemical.
Another unknown chemical process could be the source of the molecules detected in K2-18b’s atmosphere.
Nevertheless, the team say “the results are the “strongest evidence yet” that life may exist on a planet outside our Solar System.
They say their observations have reached the ‘three-sigma’ level of statistical significance.
This means there’s a 0.3% probability the detection occurred by chance.
And to reach the accepted level that would mean scientific discovery, observations would have to meet the five-sigma threshold.
In other words, there would need to be below a 0.00006% probability they occurred by chance.
Detecting life on faraway worlds
How can scientists know what chemicals exist on a planet orbiting a star beyond our Solar System?
Key to analysing exoplanets’ atmospheres is analysing the light from their host stars.
As a planet passes in front of its host star from our perspective on Earth – known as a transit – light from that star passes through the planet’s atmosphere.
That starlight picks up chemical fingerprints as it passes through the atmosphere, so astronomers can analyse the light to learn more about the atmosphere.
The tentative detection of dimethyl sulfide in 2023 was made using the James Webb Space Telescope’s NIRISS (Near-Infrared Imager and Slitless Spectrograph) and NIRSpec (Near-Infrared Spectrograph) instruments.
This 2025 study used the Webb Telescope’s MIRI (Mid-Infrared Instrument), which observes in a different wavelength of light, offering the team a new look at this intriguing world.
“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,” says 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,” says co-author Måns Holmberg, a researcher at the Space Telescope Science Institute in Baltimore, USA.
Does K2-18b have life?
The team say dimethyl sulfide and dimethyl disulfide are molecules from the same chemical family, and could be ‘biosignatures’.
This is a term used to describe chemicals that, when detected around a distant planet, could indicate the presence of biological processes, i.e. life.
Yet the concentrations of dimethyl sulfide and dimethyl disulfide in K2-18b’s atmosphere are different from those on Earth.
On Earth, dimethyl sulfide and dimethyl disulfide are below one part per billion by volume. On K2-18b, they’re thought to be thousands of times stronger, over ten parts per million.
“Earlier theoretical work had predicted that high levels of sulfur-based gases like dimethyl sulfide and dimethyl disulfide are possible on Hycean worlds,” says 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.”
The team now hope to carry out more research into whether dimethyl sulfide and dimethyl disulfide can be produced non-biologically at the level they’re currently seeing.
“The inference of these biosignature molecules poses profound questions concerning the processes that might be producing them” says study 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,” says 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,” says Madhusudhan. “That’s how science has to work.
“Decades from now, we may look back at this point in time and recognise it was when the living universe came within reach.
“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.”
