The James Webb Space Telescope (JWST) provides detailed atmospheric analyses of distant exoplanets. K2-18b orbits a small, cool star 124 light-years away in the constellation Leo. The planet resides within its star’s habitable zone, the orbital region where liquid water can be maintained on the surface. Recent JWST observations revealed a complex atmosphere containing molecules that suggest a water-rich environment. This discovery has shifted astrobiology’s focus toward this class of worlds in the quest to determine if life exists beyond our solar system.
Defining the Hycean World K2-18b
K2-18b is classified as a sub-Neptune, intermediate in size between Earth and Neptune. It has a radius 2.6 times that of Earth and a mass 8.6 times greater. This size suggests the planet is not rocky, but possesses a thick atmospheric envelope surrounding a dense interior. K2-18b orbits its M-dwarf host star, K2-18, every 33 days, placing it in the zone where temperatures allow for liquid surface water. This combination—a hydrogen-rich atmosphere and a likely subsurface ocean—places K2-18b into the theoretical “Hycean” category. Hycean describes a planet thought to have a massive ocean beneath a hydrogen-heavy atmosphere, making these worlds viable candidates for hosting life.
Bulk Atmospheric Composition Findings
JWST utilized infrared instruments to analyze light passing through K2-18b’s atmosphere as the planet transited its star. This technique, called transit spectroscopy, allowed astronomers to detect chemical signatures. The JWST data confirmed carbon-bearing molecules, specifically identifying methane (\(CH_4\)) and carbon dioxide (\(CO_2\)). This was the first time these molecules were confirmed in the atmosphere of a habitable zone exoplanet. The observations also showed a shortage of ammonia. This supports the Hycean hypothesis because ammonia is highly soluble and would dissolve into a deep, liquid water ocean, preventing it from remaining in the upper atmosphere. The resulting atmospheric profile—hydrogen-rich with methane, carbon dioxide, and low ammonia—matches models for an ocean world enveloped by a thick, gassy layer.
The Significance of Dimethyl Sulfide (DMS)
JWST observations suggest the tentative detection of Dimethyl Sulfide (DMS). A biosignature is any substance whose presence is best explained by biological activity. On Earth, DMS is a biosignature because it is almost exclusively produced by living organisms, primarily microscopic marine life like phytoplankton, as a metabolic byproduct. The inference of DMS in K2-18b’s atmosphere is not yet a definitive discovery, as the signal sits at the three-sigma level of statistical confidence. This means there is a small chance the detection is a random fluke, but scientists require a five-sigma threshold for a robust discovery. Models suggest that if DMS is present, its concentration would be thousands of times stronger than on Earth, potentially exceeding ten parts per million. While non-biological (abiotic) chemical processes in the planet’s hydrogen-rich environment could produce DMS, its connection to life on Earth makes it a significant clue.
Implications for the Search for Life Beyond Earth
The atmospheric profile of K2-18b, especially the potential DMS signal, changes the search for extraterrestrial life. For decades, the search focused on small, rocky exoplanets that are Earth-like. K2-18b validates the idea that life could thrive on a wider variety of environments, making the Hycean world concept a legitimate target for future study. These larger, hydrogen-enveloped worlds are also easier to observe with current technology, accelerating the rate at which atmospheric data can be gathered. Scientists are planning further JWST observations to confirm DMS and gather more data on the planet’s thermal structure. Achieving five-sigma certainty for DMS would force a deeper examination of the planet’s potential habitability. K2-18b has demonstrated JWST’s capability to probe the complex atmospheric chemistry of distant worlds, providing concrete chemical steps toward understanding habitability.