An artistic impression of the exoplanet K2-18b, which orbits its star in the habitable zone, and may support alien life. Credit: A Smith, N Madhusudhan (University of Cambridge).

Astronomers have detected the most promising signs yet of a possible biosignature of alien life outside the solar system.

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 researchers say between 16 and 24 hours of follow-up observation time with JWST may help them reach the all-important five-sigma significance.

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.

Research lead Professor Nikku Madhusudhan from Cambridge’s Institute of Astronomy, said: “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.”

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 star light 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).

Tiny fraction of starlight

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.

Madhusudhan said: “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.”

Co-author Måns Holmberg, a researcher at the Space Telescope Science Institute in Baltimore, USA, said: “It was an incredible realisation seeing the results emerge and remain consistent throughout the extensive independent analyses and robustness tests.”

An alternative artistic imagining of K2-18b. Credit: Arndt Stelter, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons.

Very different

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.

Madhusudhan said: “Earlier theoretical work had predicted that high levels of sulfur-based gases like DMS and DMDS are possible on Hycean worlds 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.”

Co-author Savvas Constantinou, also from Cambridge’s Institute of Astronomy, said: “Our work is the starting point for all the investigations that are now needed to confirm and understand the implications of these exciting findings.”

Madhusudhan added: “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.”

While he is not yet claiming a definitive discovery, Madhusudhan said 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?

He stated: “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 research is published in The Astrophysical Journal Letters.