Webb finds methane and carbon dioxide in a distant world

Astronomers have put forth a hypothesis suggesting that K2-18b might belong to the category of Hycean exoplanets, a theoretical type of exoplanet known for having atmospheres rich in hydrogen and large water bodies. 

This is precisely why exoplanets, such as K2-18b, are viewed as potential candidates for exploring signs of life beyond our planet.

"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," said Nikku Madhusudhan, an astronomer at the University of Cambridge and lead author, in an official release. 

Detection of molecules 

However, determining the atmospheric molecules of this distant world was not easy. It is particularly difficult since these planets are sometimes obscured by the bright glare of their much bigger host stars.

However, thanks to Webb's expanded wavelength coverage and unparalleled sensitivity, the research team was able to obtain the most comprehensive spectrum ever recorded for a sub-Neptune in the habitable zone.

Surprisingly, the Webb findings support the theory that the K2-18 b may host a water ocean beneath its hydrogen-rich atmosphere. This is due to evidence showing an excess of methane and carbon dioxide and a lack of ammonia.

Not only that, but the Webb telescope found clues of a molecule called dimethyl sulfide (DMS). DMS is exclusively produced by living organisms on Earth, with phytoplankton emitting the vast majority of it in marine habitats. Although the team asserts that further data is required to confirm the existence of these molecules. 

“Upcoming Webb observations should be able to confirm if DMS is indeed present in the atmosphere of K2-18 b at significant levels,” added Madhusudhan.

More follow-up data required

Despite the fact that the planet orbits within its star's habitable zone and with this fresh evidence of carbon-containing molecules. It still does not prove that this alien world can sustain life.

Additionally, the characteristics of these presumed oceans remain unknown, raising the possibility that the water could be excessively hot to support life or might not even exist in liquid form. The planet's huge size, with a radius 2.6 times that of Earth, supports the possibility of a massive mantle of high-pressure ice within its core.

The findings are based on two K2-18 b observations made with Webb. 

Up next, the authors plan to conduct more research with Webb's sophisticated MIRI (Mid-Infrared Instrument) spectrograph to get fresh insights into the environmental conditions on K2-18 b.

"Our ultimate goal is the identification of life on a habitable exoplanet, which would transform our understanding of our place in the universe. Our findings are a promising step towards a deeper understanding of Hycean worlds in this quest," concluded Madhusudhan. 

The study is accepted for publication in The Astrophysical Journal Letters.