How the strongest evidence yet that life exists on another planet was found by astronomers led by University of Cambridge
The strongest evidence yet that life may exist on a planet outside our solar system has been found by a team of astronomers led by the University of Cambridge.
They have detected the chemical fingerprints of dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS) in the atmosphere of the exoplanet K2-18b, 124 light years away in the constellation of Leo.
On Earth, these substances are only produced by life - primarily microbial life such as marine phytoplankton.
It is possible an unknown chemical process is the source of these molecules in K2-18b’s atmosphere, and the astronomers remain cautious over the findings, but they represent the most promising signs yet of a biosignature beyond our own solar system.
Professor Nikku Madhusudhan, from Cambridge’s Institute of Astronomy, who led the research, said: “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.”
The findings could even suggest that the planet is “teeming with life”, he said. If so, it would effectively confirm that life is very common in the galaxy.
Prof Madhusudhan described the work as a “major leap” in demonstrating our ability to find life beyond our planet and a “technical marvel”.
The exoplanet K2-18b, which is 2.6 times the size and 8.6 times the mass of Earth, orbits its star in what is known as the ‘habitable zone’ - meaning a distance at which liquid water could exist at the surface.
Earlier observations had identified methane and carbon dioxide in its atmosphere, representing the first time that carbon-based molecules were discovered in the atmosphere of an exoplanet in the habitable zone, as reported by the Cambridge Independent in 2023.
These results were consistent with predictions for what astronomers call a ‘Hycean’ planet - meaning a habitable ocean-covered world beneath a hydrogen-rich atmosphere.
But another, weaker signal hinted at something else.
The powerful James Webb Space Telescope (JWST) was deployed again to look at it.
“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 Prof Madhusudhan.
To determine the chemical composition of a distant planet’s atmosphere, astronomers analyse the light from its parent star as the planet transits - meaning passes in front of - the star, as seen from our position.
During a transit, telescopes can detect a drop in stellar brightness. A tiny fraction of the starlight passes through the planet’s atmosphere before it reaches Earth.
The absorption of some of this starlight in the planet’s atmosphere leaves imprints in the stellar spectrum, which astronomers are able to piece together to determine the constituent gases in an exoplanet’s atmosphere by comparing it to the stellar spectrum when the planet is not in front of the star.
The earlier and more tentative suggestion of DMS came using the James Webb Space Telescope’s NIRISS (Near-Infrared Imager and Slitless Spectrograph) and NIRSpec (Near-Infrared Spectrograph) instruments. These cover the near-infrared (0.8-5 micron) range of wavelengths.
The latest independent observations were made using the telescope’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 Prof Madhusudhan. “The signal came through strong and clear.”
Co-author Måns Holmberg, a researcher at the Space Telescope Science Institute in Baltimore, USA, added: “It was an incredible realisation seeing the results emerge and remain consistent throughout the extensive independent analyses and robustness tests.”
The observations have reached the ‘three-sigma’ level of statistical significance, which means there is just a 0.3 per cent probability that they occurred by chance.
However, to reach the accepted classification for scientific discovery, the observations must cross the five-sigma threshold - meaning below 0.00006 per cent probability that they occurred by chance.
With between 16 and 24 hours of follow-up observation time from JWST, the researchers believe they could reach this five-sigma significance.
DMS and DMDS, which come from the same chemical family, are both predicted to be biosignatures. The molecules have overlapping spectral features in the observed wavelength range but further observations will help differentiate between the two.
Concentrations of DMS and DMDS in K2-18b’s atmosphere are very different from in Earth’s, where they are generally below one part per billion by volume.
On K2-18b, the concentration is estimated to be more than ten parts per million - which is thousands of times stronger.
“Earlier theoretical work had predicted that high levels of sulfur-based gases like DMS and DMDS are possible on Hycean worlds,” said Prof 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.”
But while Prof Madhusudhan is excited by the results, he stressed that it is vital to obtain more data before claiming that life has been found on another world.
He is cautiously optimistic, but unknown processes could yet be at work on K2-18b that account for the observations.
He intends to work with colleagues to conduct further theoretical and experimental work to determine whether DMS and DMDS can be produced non-biologically at the level currently inferred.
“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. That’s how science has to work,” said Prof Madhusudhan.
Co-author Savvas Constantinou, also from Cambridge’s Institute of Astronomy, added: “Our work is the starting point for all the investigations that are now needed to confirm and understand the implications of these exciting findings,”
And co-author Subhajit Sarkar, of Cardiff University, said: “The inference of these biosignature molecules poses profound questions concerning the processes that might be producing them.”
The research, supported by a UK Research and Innovation (UKRI) Frontier Research Grant, is reported in The Astrophysical Journal Letters.
To learn more about Hycean worlds, visit hycean.group.cam.ac.uk.
