Carbon dioxide and methane has been detected in the atmosphere of exoplanet K2-18 b - a potentially habitable world more than eight times the size of Earth.
The ground-breaking discovery means K2-18 b may belong to a unique class of exoplanets known as "Hycean" planets, which possess hydrogen-rich atmospheres and potentially water-covered surfaces, making them candidates for life.
The initial insights by NASA's James Webb Telescope were made possible by observations from NASA's Hubble Space Telescope.
K2-18 b orbits a cool dwarf star called K2-18 about 120 light years away from Earth - within the constellation Leo - and sits within the habitable zone.
These exoplanets, with sizes between Earth and Neptune, are not found in our solar system, making their characteristics a subject of active debate among scientists.
The idea that K2-18 b could be a Hycean exoplanet is particularly fascinating to scientists, with some experts believing that such planets may offer favourable conditions for life to develop.
Nikku Madhusudhan, an astronomer at the University of Cambridge and lead author of the study, said: "Our findings underscore the importance of considering diverse habitable environments in the search for life elsewhere.
"Traditionally, the search for life on exoplanets has focused primarily on smaller rocky planets, but the larger Hycean worlds are significantly more conducive to atmospheric observations."
The abundance of methane and carbon dioxide, coupled with the absence of ammonia, suggests K2-18 b features a hydrogen-rich atmosphere above a potential water ocean, scientists said.
Astronomers said the telescope's initial observations also hinted at the presence of dimethyl sulfide (DMS), a molecule primarily associated with microbial life such as marine phytoplankton on Earth, suggesting the possibility of biological activity on K2-18 b.
But Mr Madhusudhan noted "more observations are needed to determine whether it is in fact DMS that we're seeing".
Analysing exoplanet atmospheres presents a challenge due to the intense glare of parent stars, which obscures smaller celestial bodies.
To overcome this hurdle, the team examined the light passing through K2-18 b's atmosphere as it transited its host star.
The research is soon to be published in The Astrophysical Journal Letters, with the team intending to conduct further work.
"Our ultimate goal is the identification of life on a habitable exoplanet, which would transform our understanding of our place in the universe," Mr Madhusudhan concluded.
"Our findings are a promising step towards a deeper understanding of Hycean worlds in this quest."