In a potential landmark discovery, scientists using the James Webb Space Telescope have obtained what they call the strongest signs yet of possible life beyond our solar system, detecting in an alien planet's atmosphere the chemical fingerprints of gases that on Earth are produced only by biological processes.

The two gases, namely dimethyl sulfide (DMS), and dimethyl disulfide (DMDS), were identified in the observations of the planet named K2-18 b and are only generated on Earth by living organisms, primarily microbial life such as marine phytoplankton, or algae.

This suggests the planet may be teeming with microbial life, the researchers said. They stressed, however, that they are not announcing the discovery of actual living organisms but rather a possible bio-signature - an indicator of a biological process - and that the findings should be viewed cautiously, with more observations needed.

Nonetheless, they voiced excitement. These are the first hints of an alien world that is possibly inhabited, said astrophysicist Nikku Madhusudhan of the University of Cambridge's Institute of Astronomy, lead author of the study published in the Astrophysical Journal Letters.

"We have found evidence for possible biological activity on an exoplanet," Professor Madhusudhan told Reuters.

"That is a major breakthrough in our search for life beyond the solar system. Not only that there is a chance that the planet can actually be habitable, but what we are finding is that we are demonstrating that it is possible to detect bio-signatures around in atmospheres of such planets around nearby stars with existing facilities. And that's a big breakthrough,” he said.

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Madhusudhan noted that there are various efforts underway searching for signs of life in our solar system including various claims of environments that might be conducive to life in places like Mars, Venus and various icy moons.

K2-18 b is 8.6 times as massive as Earth and has a diameter about 2.6 times as large as our planet.

It orbits in the "habitable zone" - a distance where liquid water, a key ingredient for life, can exist on a planetary surface - around a red dwarf star smaller and less luminous than our sun, located about 124 light years from Earth in the constellation Leo. A light-year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km). One other planet also has been identified orbiting this star.

About 5,800 planets beyond our solar system, called exoplanets, have been discovered since the 1990s. Scientists have hypothesised the existence of exoplanets called Hycean worlds - covered by a liquid water ocean habitable by microorganisms and with a hydrogen-rich atmosphere.

Earlier observations by Webb, which was launched in 2021 and became operational in 2022, had identified methane and carbon dioxide in K2-18 b's atmosphere, the first time that carbon-based molecules were discovered in the atmosphere of an exoplanet in a star's habitable zone.

"The only scenario that explains all of this is one where it (K2-18b) is a Hycean world that is teeming with life. ," Madhusudhan said.

"However, we want to remain open to other alternatives one might come up with in the future."

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DMS and DMDS, both from the same chemical family, have been predicted as important exoplanet bio-signatures. Webb found that one or the other, or possibly both, were present in the planet's atmosphere at a 99.7% confidence level, meaning there is still a 0.3% chance of the observation being a statistical fluke.

The gases were detected at atmospheric concentrations of more than 10 parts per million by volume, thousands of times higher than their concentrations in the Earth's atmosphere, and cannot be explained without biological activity.

K2-18 b is part of the "sub-Neptune" class of planets, with a diameter greater than Earth's but less than that of Neptune, our solar system's smallest gas planet.

To ascertain the chemical composition of an exoplanet's atmosphere, astronomers analyse the light from its host star as the planet passes in front of it from the perspective of Earth, called the transit method. As the planet transits, Webb can detect a decrease in stellar brightness, and a small fraction of starlight passes through the planetary atmosphere before being detected by the telescope. This lets scientists determine the constituent gases of the planet's atmosphere.

Webb's previous observations of this planet provided a tentative hint of DMS. Its new observations used a different instrument and a different wavelength range of light.

The ultimate goal of exoplanet science is to find evidence of life on an Earth-like planet beyond our solar system. Madhusudhan said that our species for thousands of years has wondered "are we alone" in the universe, and now might be within just a few years of detecting possible alien life.

But Madhusudhan still urged caution.

"This is a monumental discovery. It is very important but we also have to be extremely cautious. The reason is that this is one of the biggest questions we have asked as a species in all of science, and we have to deal with it very carefully in that we want it to be really, really robust in establishing whether this is actually life that we have seen," Madhusudhan said.

"I think it is our responsibility as scientists and as part of society that we want to answer this very deep question in a very careful and robust manner. But the excitement is still there, nevertheless, that we have got the capability to do it now," he said.

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