The quest to find habitable worlds beyond our solar system has taken a significant leap forward with the study of K2-18 b, an exoplanet located approximately 124 light-years away in the constellation Leo. Recent observations suggest that this distant world may harbour a vast ocean beneath a hydrogen-rich atmosphere, positioning it as a prime candidate in the search for extraterrestrial life.
Discovered in 2015 by NASA’s Kepler mission, K2-18 b is classified as a “super-Earth,” with a mass about 8.6 times that of our planet and a radius approximately 2.6 times larger. It orbits within the habitable zone of its host star, K2-18, a cool M-dwarf, receiving a similar amount of stellar energy as Earth does from the Sun. This positioning allows for the possibility of liquid water on its surface—a key ingredient for life as we know it.
In 2023, the James Webb Space Telescope (JWST) provided groundbreaking insights into K2-18 b’s atmosphere. Using its Near-Infrared Imager and Slitless Spectrograph (NIRISS) and Near-Infrared Spectrograph (NIRSpec), JWST detected significant amounts of methane (CH₄) and carbon dioxide (CO₂), along with a notable scarcity of ammonia (NH₃). These findings suggest the presence of a hydrogen-rich atmosphere overlaying a potential water ocean—a combination characteristic of a hypothesised class of exoplanets known as “Hycean worlds.”
One of the most intriguing aspects of JWST’s observations is the tentative detection of dimethyl sulfide (DMS) in K2-18 b’s atmosphere. On Earth, DMS is predominantly produced by marine phytoplankton, making it a potential biosignature. While the presence of DMS is not yet confirmed, its possible detection has sparked excitement about the planet’s habitability.
Despite these promising findings, scientists caution against overinterpretation. The detection of DMS is preliminary, and alternative non-biological processes could account for its presence. Further observations and analyses are necessary to confirm these results and rule out false positives. Additionally, the planet’s large size and potential high-pressure environment may pose challenges for habitability.
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