Scientists detect potential biosignatures on distant exoplanet K2-18b

Signs of alien life?

In what may be a historic breakthrough in the search for extraterrestrial life, astronomers using the James Webb Space Telescope (JWST) have detected compelling chemical signatures on a distant exoplanet that suggest the possible presence of life. The planet, known as K2-18b, orbits within its star’s habitable zone and is located 124 light years away in the constellation Leo.

Led by researchers at the University of Cambridge, the international team has identified chemical traces in K2-18b’s atmosphere that match those of dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS). These molecules are considered strong potential biosignatures because, on Earth, they are exclusively produced by biological processes, primarily marine phytoplankton.

Although the scientists remain cautious in interpreting the findings, they describe this as the strongest indication that biological activity may occur on a planet beyond our solar system.

Significant milestone

The findings, published in The Astrophysical Journal Letters, mark a significant milestone. The detection has reached a ‘three-sigma’ confidence level, meaning there is only a 0.3 per cent chance that the results are due to random noise. However, to confirm a scientific discovery, Researchers require a five-sigma level of certainty, minimising the likelihood of a false positive to less than 0.00006 per cent.

Astronomers estimate that between 16 and 24 more hours of observation time with the JWST will be necessary to reach this gold-standard level of statistical significance. If confirmed, the implications could fundamentally alter our understanding of life in the universe.

K2-18b has intrigued scientists for years. It is a sub-Neptune planet, roughly 8.6 times the mass of Earth and 2.6 times its size. Earlier observations with the JWST revealed the presence of carbon dioxide and methane in its hydrogen-rich atmosphere—elements essential for life as we know it and indicators of a potentially habitable world.

These earlier discoveries were consistent with models of what is known as a “Hycean” planet — a type of exoplanet hypothesised to have a global ocean beneath a hydrogen-rich atmosphere. Habitable worlds are considered promising candidates for hosting life, especially microbial life similar to that found in Earth’s oceans.

New observational technique

In the latest study, astronomers used a new observational technique with JWST’s Mid-Infrared Instrument (MIRI), which can detect a different range of wavelengths than the telescope’s previous tools. The MIRI observations revealed a strong and independent signal of what appears to be DMS or DMDS.

“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 lead researcher Professor Nikku Madhusudhan of the University of Cambridge’s Institute of Astronomy.

“Seeing the signal emerge so clearly and remain consistent throughout rigorous analysis was an incredible realisation,” added Måns Holmberg, co-author and researcher at the Space Telescope Science Institute in Baltimore, United States.

DMS and DMDS belong to a chemical group with biological origins on Earth. While it is still unclear whether both are present on K2-18b, they share overlapping spectral features that make them difficult to differentiate without further observations.

Interestingly, the estimated concentrations of these molecules in K2-18b’s atmosphere are orders of magnitude higher than what is typically found on Earth. While Earth’s atmosphere contains DMS in concentrations below one part per billion, K2-18b is thought to have levels exceeding ten parts per million, thousands of times stronger.

This elevated presence is consistent with earlier theoretical predictions. “Earlier theoretical work had predicted that high levels of sulphur-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.”

Caution urged

Despite the excitement, the researchers caution that unknown chemical processes may produce DMS or DMDS without the need for life. Further theoretical and experimental work will be needed to determine whether these gases can be generated through abiotic means at the observed levels.

“The inference of these biosignature molecules poses profound questions concerning the processes that might produce them,” said co-author Subhajit Sarkar from Cardiff University.

“This work is the starting point for all the investigations that are now needed to confirm and understand the implications of these exciting findings,” added Savvas Constantinou, another co-author from Cambridge’s Institute of Astronomy.

The data were gathered by analysing the light from K2-18b’s parent star as the planet passed in front of it, a technique known as transit spectroscopy. During such transits, a small portion of the star’s light filters through the exoplanet’s atmosphere, leaving distinct absorption patterns that astronomers can decode to determine its chemical composition.

The new findings were made possible by the JWST’s advanced instrumentation. Previous observations were made with JWST’s Near-Infrared Imager and Slitless Spectrograph (NIRISS) and Near-Infrared Spectrograph (NIRSpec), which cover wavelengths from 0.8 to 5 microns. The latest MIRI data extend the analysis into the mid-infrared range (6-12 microns), offering an entirely new perspective on the planet’s atmospheric chemistry.

While the team has made one of the most compelling cases for potential life beyond Earth, Madhusudhan remains grounded in scientific rigour. “It’s important that we’re deeply sceptical of our 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,” he said. “That’s how science has to work.”

Right tools

He added that humanity is now equipped with the tools to investigate the possibility of life elsewhere in the cosmos seriously. “Decades from now, we may look back at this point and recognise it was when the living universe came within reach,” Madhusudhan said. “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, the European Space Agency, and the Canadian Space Agency. The research is supported by a UK Research and Innovation (UKRI) Frontier Research Grant.

If confirmed, this discovery would mark one of the most significant scientific milestones of the 21st century. With additional observations planned and the prospect of even more advanced telescopes on the horizon, the coming years may finally bring definitive answers to one of the oldest questions in human history.

Image: Artist’s impression of the exoplanet K2-18b. Credit: A. Smith/N. Mandhusudhan