New research by NASA's James Webb Space Telescope on K2-18 b has pointed to the presence of carbon-containing molecules such as methane and carbon dioxide on the exoplanet.
The discovery has raised scientists' hopes that K2-18 b may have a hydrogen-rich atmosphere and a water-covered ocean surface, conditions favorable for supporting the existence of life.
The possibility that K2-18 b is a Hycean exoplanet intrigues scientists.
K2-18 b is considered an exoplanet because it lies outside the Solar System. It orbits the cool dwarf star K2-18 in the habitable zone, 120 light-years from Earth, in the so-called constellation Leo. The planet is also classified as "sub-Neptune", as it lies between the size of Earth and Neptune.
So far, the nature of the atmospheres of sub-Neptune exoplanets has been the subject of active debate among astronomers. If the atmosphere of K2-18 b is confirmed to be hydrogen-rich and have a water-covered ocean surface, the exoplanet could be classified as Hycean, worlds considered promising by scientists for the search for evidence of life.
Discovered in 2015 with NASA's K2 mission, K2-18 b had its atmospheric properties first observed by NASA's Hubble Space Telescope. This led to further studies by James Webb, which has changed scientists' understanding of the exoplanet so far.
Nikku Madhusudhan, an astronomer at the University of Cambridge and lead author of the paper that announced the presence of carbon-containing molecules on K2-18 b, explained that the findings highlight the importance of considering diverse habitable environments when searching for life elsewhere.
In a paper published on NASA's website, the scientist stated, "Traditionally, the search for life on exoplanets has focused primarily on smaller rocky planets, but larger Hycean worlds are significantly more conducive to atmospheric observations."
Detecting dimethyl sulfide on K2-18 b is next step
The abundance of methane and carbon dioxide and the scarcity of ammonia support the hypothesis of the existence of a water ocean and a hydrogen-rich atmosphere on K2-18 b. However, these initial observations by Webb have provided something even more important for a future conclusion about the presence of life on the exoplanet: the possible detection of a molecule called dimethylsulfide (DMS).
On Earth, DMS is only produced by life. On our planet, most of the substance present in the atmosphere is emitted by phytoplankton in marine environments. However, the existence of DMS requires further validation. "Upcoming Webb observations should be able to confirm that DMS is indeed present in the atmosphere of K2-18 b at significant levels," Madhusudhan explains in the NASA paper.
Importantly, many steps remain to be taken to prove that K2-18 b can harbor life. At 2.6 times the radius of Earth, the exoplanet has a size that indicates the likely existence of a large high-pressure ice shell, like Neptune. In addition, its ocean may be too hot to be habitable. All these conditions need to be studied.
"These results are the product of only two observations of K2-18 b, with many more on the way. This means that our work here is just an initial demonstration of what Webb can observe on habitable zone exoplanets," explained Savvas Constantinou, a member of the team from the University of Cambridge.
The results of the Webb observations have been accepted for publication in The Astrophysical Journal Letters. Going forward, the team intends to use the telescope's MIRI (Mid-Infrared Instrument) spectrograph to perform follow-up studies, validate the discoveries already made and find new information about the environmental conditions of K2-18b.
"Our findings are a promising step toward a deeper understanding of the Hycean worlds in this search," Madhusudhan said. "Our ultimate goal is the identification of life on a habitable exoplanet, which would transform our understanding of our place in the universe."
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