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JWST discovers carbon-bearing molecules, methane and carbon dioxide, in the atmosphere of the K2-18b exoplanet.

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NASA’s James Webb Space Telescope has conducted a new investigation into K2-18 b, an exoplanet that’s 8.6 times larger than Earth. The telescope discovered carbon-bearing molecules like methane and carbon dioxide, adding to recent studies that suggest K2-18 b could be a Hycean exoplanet. This type of exoplanet has the potential to have a hydrogen-rich atmosphere and a water ocean-covered surface.

The atmospheric properties of this habitable-zone exoplanet were first discovered using NASA’s Hubble Space Telescope. This initial discovery prompted further studies that have since changed our understanding of the system.

K2-18 b orbits the cool dwarf star K2-18 in the habitable zone and is located 120 light-years from Earth in the constellation Leo. Exoplanets that have sizes between those of Earth and Neptune, such as K2-18 b, are unlike anything in our solar system. These ‘sub-Neptunes’ are poorly understood, and the nature of their atmospheres is a matter of active debate among astronomers.


The idea that K2-18 b could be a Hycean exoplanet is intriguing to some astronomers who believe that these worlds are promising environments to search for evidence of life on exoplanets.

“Our findings underscore the importance of considering diverse habitable environments in the search for life elsewhere,” explained Nikku Madhusudhan, an astronomer at the University of Cambridge and lead author of the paper announcing these results. “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 high amount of methane and carbon dioxide and low amount of ammonia suggest the possibility of a water ocean beneath a hydrogen-rich atmosphere in K2-18 b. The initial Webb observations also showed signs of a molecule called dimethyl sulfide (DMS), which is only produced by living organisms on Earth. Most of the DMS in our planet’s atmosphere comes from phytoplankton found in marine environments.

The inference of DMS is less robust and requires further validation.

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

K2-18 b is located in the habitable zone and has carbon-bearing molecules, but its ability to support life is not guaranteed. Due to its size, which is 2.6 times the size of Earth, the planet is likely to have a mantle of high-pressure ice, similar to Neptune. It also has a thin hydrogen-rich atmosphere and an ocean surface. While Hycean worlds like K2-18 b are predicted to have water oceans, it is uncertain if the ocean is habitable or even liquid due to the possibility of extreme heat.

“Although this kind of planet does not exist in our solar system, sub-Neptunes are the most common type of planet known so far in the galaxy,” explained team member Subhajit Sarkar of Cardiff University. “We have obtained the most detailed spectrum of a habitable-zone sub-Neptune to date, and this allowed us to work out the molecules that exist in its atmosphere.”

Studying the atmospheres of exoplanets, like K2-18 b, is a very active area of research in astronomy. However, it can be challenging to explore these atmospheres because the exoplanets are often overshadowed by their much larger parent stars.

To overcome this challenge, the team analyzed light from K2-18 b’s parent star as it passed through the exoplanet’s atmosphere. K2-18 b is a transiting exoplanet, which means that we can detect a drop in brightness as it passes across the face of its host star. This is how the exoplanet was first discovered in 2015 with NASA’s K2 mission. During transits, a small amount of starlight passes through the exoplanet’s atmosphere before reaching telescopes like Webb. Astronomers can use the starlight’s passage through the exoplanet atmosphere to determine the gases present in the exoplanet’s atmosphere.

“This result was only possible because of the extended wavelength range and unprecedented sensitivity of Webb, which enabled robust detection of spectral features with just two transits,” said Madhusudhan. “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.”

“These results are the product of just two observations of K2-18 b, with many more on the way,” explained team member Savvas Constantinou of the University of Cambridge. “This means our work here is but an early demonstration of what Webb can observe in habitable-zone exoplanets.”

The team’s results were accepted for publication in The Astrophysical Journal Letters.

The team now intends to conduct follow-up research with the telescope’s MIRI (Mid-Infrared Instrument) spectrograph that they hope will further validate their findings and provide new 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,” concluded Madhusudhan. “Our findings are a promising step towards a deeper understanding of Hycean worlds in this quest.”

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