James Webb spies distant exoplanet that could be wet, wild, and Hycean

That or it's just really gassy

The James Webb Space Telescope continues to deliver the goods with scientists publishing insights into the atmospheric properties of a habitable-zone exoplanet.

The planet in question is the snappily named K2-18 b, which orbits the dwarf star K2-18. It lies 120 light-years away in the constellation Leo and is approximately 8.6 times the size of Earth.

Data from the NASA/ESA/CSA space telescope has indicated the presence of carbon-bearing molecules in the planet's atmosphere, including methane and carbon. The findings add further weight to theories that K2-18 b could be a Hycean exoplanet.

Hycean is a portmanteau of hydrogen and ocean that describes a hypothetical type of planet. As well as K2-18 b, TOI-1231 b is also a potential candidate, but the evidence points toward the former possessing a hydrogen-rich atmosphere and a water-covered surface.

The European Space Agency (ESA) noted: "The abundance of methane and carbon dioxide, and shortage of ammonia, support the hypothesis that there may be an ocean underneath a hydrogen-rich atmosphere on K2-18 b."

Some astronomers believe that Hycean worlds could harbor life or at least be promising environments in the search for life.

Nikku Madhusudhan, an astronomer at the University of Cambridge and lead author of the paper announcing these results, said: "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."

Exoplanets such as K2-18 b present a challenge since our solar system has nothing like them. Their size puts them between the Earth and Neptune, and, according to ESA, "the nature of their atmospheres is a matter of active debate between astronomers."

As for how the team could discern the composition of the atmosphere, the sensitivity of the James Webb Space Telescope meant that astronomers could determine the gases using only the fraction of starlight that passed through the atmosphere during a transit across its host star.

Madhusudhan said: "For comparison, one transit observation with Webb provided comparable precision to eight observations with Hubble conducted over a few years and in a relatively narrow wavelength range."

The Canadian NIRISS and European NIRSpec instruments aboard the telescope were used in the observations, and additional research with the Mid-InfraRed Instrument (MIRI) spectrograph is planned.

The initial observations also showed up a possible detection of dimethyl sulphide (DMS), a molecule associated with life on Earth. Most of the DMS in Earth's atmosphere comes from phytoplankton in marine environments.

That said, researchers were at pains to note that the evidence pieced together so far does not necessarily mean the planet could support life. The ocean – if present – could, for example, simply be too hot to be habitable.

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

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