Using the James Webb Space Telescope (JWST), a research team led by the University of Cambridge identified the spectral presence of dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS) in the atmosphere of exoplanet K2-18b. Both molecules are produced exclusively by life on Earth, mainly by ocean-dwelling microorganisms. While the researchers remain cautious, the detection marks a significant milestone in the ongoing search for extraterrestrial life.

K2-18b lies within the habitable zone of its star and is located 124 light-years from Earth in the constellation Leo. It is roughly 2.6 times the size of Earth and eight times its mass. Previously, methane and carbon dioxide were detected in its atmosphere, supporting the theory that K2-18b could be a "Hycean" world - a planet with a water-covered surface beneath a hydrogen-rich atmosphere.

The latest findings, published in The Astrophysical Journal Letters, show that the potential DMS/DMDS signal achieved a statistical significance of three sigma, corresponding to a 0.3% chance that the observation is a fluke. A five-sigma detection is required for formal scientific confirmation, which would bring that likelihood down to less than 0.00006%.

To reach that threshold, the team estimates 16 to 24 additional hours of JWST observation time will be needed.

The earlier, tentative signal of DMS was gathered using JWST's NIRISS and NIRSpec instruments, operating in the near-infrared spectrum. This new signal was obtained using a completely different method: JWST's MIRI (Mid-Infrared Instrument), which observes mid-infrared wavelengths from 6 to 12 microns. The consistency across both sets of data strengthens confidence in the detection.

"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 Professor Nikku Madhusudhan, the project lead at Cambridge's Institute of Astronomy. "The signal came through strong and clear."

K2-18b's atmosphere appears to contain DMS and DMDS at concentrations far exceeding those found on Earth. Here, these molecules rarely exceed one part per billion; on K2-18b, the estimate is more than ten parts per million.

Madhusudhan said this matches earlier theoretical models that predicted Hycean planets could harbor high levels of sulfur-based gases. "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," he said.

Despite the promising findings, researchers stress the need for caution. Unknown non-biological mechanisms might be responsible for the observed molecules. Future theoretical and experimental studies are planned to explore whether such chemicals could be generated abiotically under exotic planetary conditions.

Co-author Subhajit Sarkar of Cardiff University noted, "The inference of these biosignature molecules poses profound questions concerning the processes that might be producing them."

The team emphasizes that rigorous scrutiny must continue. "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," Madhusudhan said.

With the capabilities of JWST and future telescopes on the horizon, astronomers believe we may be closer than ever to addressing a fundamental question: Are we alone in the universe?

"Decades from now, we may look back at this point in time and recognise it was when the living universe came within reach," Madhusudhan said. "This could be the tipping point."

The James Webb Space Telescope is a joint mission of NASA, ESA, and the Canadian Space Agency. This research was funded by a UK Research and Innovation (UKRI) Frontier Research Grant.

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