Astronomers have reported the strongest evidence to date of potential biological activity beyond our solar system, following the detection of key molecules in the atmosphere of the distant exoplanet K2-18b, located approximately 124 light-years from Earth.

K2-18b - 8.6 times as massive and 2.6 times as large as Earth - orbits in the habitable zone of a cool red dwarf star in the constellation Leo. Previous studies had already revealed the presence of methane and carbon dioxide in its atmosphere.

The latest findings, published Thursday in The Astrophysical Journal Letters, mark the first time carbon-based molecules have been identified in the atmosphere of an exoplanet located in a habitable zone.

Using data from the James Webb Space Telescope (JWST), a team of astronomers led by the University of Cambridge detected the chemical fingerprints of dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS) in the planet's atmosphere. These sulfur-based compounds, on Earth, are produced exclusively by living organisms - primarily marine microbial life such as phytoplankton - making them promising biosignatures in the search for extraterrestrial life.

While an unknown chemical process might be responsible for producing these molecules on K2-18b, the discovery represents the most compelling evidence so far that life could exist on a planet beyond our solar system.

"Decades from now, we may look back at this point in time and recognize it was when the living universe came within reach," said Professor Nikku Madhusudhan from Cambridge's Institute of Astronomy, who led the research. "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."

Although the discovery marks a major milestone in astrobiology, scientists emphasize the importance of rigorous verification. Madhusudhan noted that while the findings are exciting, more data is needed before any claims about extraterrestrial life can be made.

Currently, the observations have reached the "three-sigma" level of statistical significance, indicating only a 0.3 percent probability that the findings are due to chance. However, to meet the standard threshold for scientific discovery, the results must achieve five-sigma significance, reducing the chance to less than 0.00006 percent.

Researchers estimate that an additional 16 to 24 hours of follow-up observation with JWST may be required to reach this benchmark.

While cautiously optimistic, Madhusudhan acknowledged that unknown chemical processes on K2-18b could also explain the presence of DMS and DMDS. He and his colleagues plan to pursue further theoretical and experimental work to investigate whether these molecules could be produced non-biologically at the levels currently observed.

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