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.
On Earth, DMS and DMDS are only produced by life, primarily microbial life such as marine phytoplankton. While an unknown chemical process may be the source of these molecules in K2-18b's atmosphere, the results are the strongest evidence yet that life may exist on a planet outside our solar system.
The observations have reached the 'three-sigma' level of statistical significance – meaning there is a 0.3% probability that they occurred by chance. To reach the accepted classification for scientific discovery, the observations would have to cross the five-sigma threshold, meaning there would be below a 0.00006% probability they occurred by chance.
The international team, which also draws on expertise from Cardiff University, the University of Birmingham, the Space Telescope Science Institute and the Space Science Institute, 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.
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.
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. 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.
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.
"Earlier theoretical work had predicted that high levels of sulfur-based gases like DMS and DMDS are possible on Hycean worlds," said Professor 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."
While the results are exciting the team says it's vital to obtain more data before claiming that life has been found on another world.
"The inference of these biosignature molecules poses profound questions concerning the processes that might be producing them," said co-author Dr Subhajit Sarkar from Cardiff University's School of Physics and Astronomy.
On Earth these molecules are produced exclusively by life, which raises the potential for life on K2-18b, although we must remain open to non-biological causes in that environment, which is very different from Earth. So, further observations with JWST are needed to reach the desired 5-sigma detection level for these molecules. Also, additional theoretical and experimental work may be needed to exclude non-biological causes.
"Our work is the starting point for all the investigations that are now needed to confirm and understand the implications of these exciting findings," added co-author Savvas Constantinou, also from Cambridge's Institute of Astronomy.
While the team is not yet claiming a definitive discovery, they say 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 Professor 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."
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.
The paper, 'New Constraints on DMS and DMDS in the Atmosphere of K2-18b from JWST MIRI', is published in The Astrophysical Journal Letters .
