Astronomers claim they have detected the most promising "hints" of potential for life on a planet beyond our solar system — but other scientists are sceptical.
It follows vigorous debate in scientific circles about whether the planet K2-18b could be an ocean world capable of hosting microbial life.
Using the James Webb Space Telescope, a British-US team of researchers detected signs of gases that on Earth are produced only by biological processes.
The two gases — dimethyl sulfide (DMS) and dimethyl disulfide — are generated on Earth by living organisms, primarily microbial life such as marine algae called phytoplankton.
However, the scientists stressed more observations were needed to confirm these findings, adding they were not announcing a definitive discovery.
But Cambridge University astrophysicist and lead author Nikku Madhusudhan said the implications could be significant.
"Frankly, I think this is the closest we have come to seeing a feature that we can attribute to life," he said.
What do we know about K2-18b?
A rare exoplanet, K2-18b is more than eight times the mass of Earth and 2.5 times as big.
It orbits in the habitable "goldilocks" zone — meaning it's neither too hot nor too cold to have liquid water.
Located about 124 light-years from Earth in the constellation Leo, it's long been considered the prime candidate for a "hycean planet".
That's an ocean world bigger than Earth with a hydrogen-rich atmosphere.
Telescopes observe these far-off exoplanets when they cross in front of their star, allowing astronomers to analyse how molecules block the light streaming through their atmosphere.
In 2023, the Webb telescope detected methane and carbon dioxide in K2-18b's atmosphere.
It was the first time such carbon-based molecules were detected on an exoplanet in a habitable zone.
It also detected weak signals of the chemical DMS.
This led astronomers to turn Webb towards the planet again a year ago, this time using its mid-infrared instrument to detect different wavelengths of light.
They found much stronger signs of the chemicals, although still well below the "five sigma" threshold of statistical significance scientists seek for such discoveries.
What do other scientists say?
Other experts are urging caution.
Previous research by Oxford University planetary physics professor Raymond Pierrehumbert indicated K2-18b is too hot for life.
He told AFP if the planet did have water, it would be "hellishly hot" and uninhabitable, adding oceans of lava were more plausible.
Sara Seager, a professor of planetary science at MIT, pointed to previous claims of water vapour in K2-18b's atmosphere that turned out to be a different gas.
She said Mars, Venus and moons such as Saturn's Enceladus all have "more chance to be realised as life-hosting".
Christopher Glein, principal scientist at the Space Science Division of the Southwest Research Institute, was cautiously optimistic.
"Yet we must be very careful to test the data as thoroughly as possible," he said.
What kind of life could be on K2-18b?
Hycean planets are not expected to host intelligent alien life.
Rather, scientists posit they may host tiny microbes similar to those in Earth's oceans billions of years ago.
The presence of DMS also doesn’t mean the planet is home to life – last year, scientists found traces on a comet, suggesting it can be produced in non-organic ways.
Mr Madhusudhan estimated it would take just 16 to 24 more hours of Webb's time to confirm these latest findings — a process he said could happen in the next few years.
"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," he said.
AFP/Reuters
