K2-18b Distance from Earth and Basic Properties
K2-18b is located about 124 light-years from Earth in the constellation Leo, making it one of the closest known sub-Neptune exoplanets with potential atmospheric biosignaturesNASA Exoplanet Exploration. The planet was originally identified in Kepler K2 mission data and later confirmed with transit observations from NASA and ESA facilities.
The planet orbits within the habitable zone of a cool M-dwarf star, where temperatures could allow liquid water on a possible surface or beneath a thick hydrogen-rich envelopeNASA Webb Mission Page. Its radius is roughly 2.6 times Earth's, and its mass is estimated between 8 and 10 Earth masses, classifying it as a sub-Neptune rather than a rocky super-Earth.
Latest Observations and Atmospheric Findings
James Webb Space Telescope Results
Webb transit spectroscopy data released in 2023 and refined in 2024 detected methane, carbon dioxide, and a possible dimethyl sulfide signal in K2-18b's atmosphereNASA Webb Mission Page. These measurements help constrain the planet's temperature structure, cloud properties, and potential for a water-rich interior beneath a gaseous envelope.
Implications for Habitability
While K2-18b is not confirmed as habitable, the presence of carbon-bearing molecules and a temperate orbital position makes it a top target for follow-up studiesESO MUSE Instrument Page. Researchers use these data to model whether a liquid water ocean could exist under a high-pressure ice layer or within a temperate atmospheric layer.
Comparison with Other Nearby Exoplanets and Future Missions
K2-18b in the Context of Nearby Habitable Zone Worlds
At 124 light-years, K2-18b is farther than the closest known habitable zone candidates such as those around Proxima Centauri or TRAPPIST-1, but its larger size and thick atmosphere offer unique chemistry to studyNASA Webb Science Overview. Compared to rocky worlds, sub-Neptunes like K2-18b provide a broader range of masses and radii for testing planetary formation models.
Upcoming Observations and Data Releases
Planned Webb and ground-based extremely large telescope observations aim to refine atmospheric abundances, search for additional molecules, and improve constraints on the planet's bulk compositionNASA Webb Mission Page. These datasets will help rank K2-18b among nearby exoplanets for potential biosignature follow-up in the coming decade.