Astronomers using the NASA/ESA/CSA James Webb Space Telescope have discovered carbon dioxide and methane in the atmosphere of the habitable zone exoplanet K2-18b.

These detections are consistent with an exoplanet that may contain ocean-covered surface underneath a hydrogen-rich atmosphere.

K2-18 is a red dwarf star located approximately 111 light-years away in the constellation Leo.

Also known as EPIC 201912552, the star hosts two super-Earth exoplanets: K2-18b and K2-18c.

K2-18c has a mass about 7.5 times that of Earth, orbits the host star one every 9 days, and is probably too hot to be in the habitable zone.

K2-18b has a radius of 2.2 times that of Earth and is about 8 times as massive.

The planet orbits its star every 33 days at a distance of approximately 0.15 AU and has an Earth Similarity Index of 0.73.

It receives 1.28 times the light intensity of Earth, and its equilibrium temperature is 28 degrees Fahrenheit (minus 2 degrees Celsius).

Webb’s detection of methane and carbon dioxide — along with the non-detection of ammonia — in the atmosphere of K2-18b add to recent studies suggesting that this exoplanet could be a Hycean world, one which has the potential to possess a hydrogen-rich atmosphere and a water ocean-covered surface.

“Our findings underscore the importance of considering diverse habitable environments in the search for life elsewhere,” said Professor Nikku Madhusudhan, an astronomer at the University of Cambridge.

“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 K2-18b spectra, obtained with Webb’s Near-Infrared Imager and Slitless Spectrograph (NIRISS) and Near-Infrared Spectrograph (NIRSpec), display an abundance of methane and carbon dioxide in the exoplanet’s atmosphere, as well as a possible detection of a molecule called dimethyl sulfide. Image credit: NASA / ESA / CSA / Ralf Crawford, STScI / Joseph Olmsted, STScI.

The astronomers also detected another, weaker, signal of a molecule called dimethyl sulfide.

On Earth, dimethyl sulfide is only produced by life, primarily microbial life such as marine phytoplankton, suggesting the possibility of biological activity on K2-18b.

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

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