The exoplanet K2-18b has become a subject of astronomical research and public interest. Situated 124 light-years from Earth, this world orbits a red dwarf star. Its discovery in 2015 by the Kepler space telescope led to studies of its properties, positioning it in the search for habitable environments beyond our solar system. The planet’s characteristics suggest it may harbor conditions unlike any on Earth, making it a target for advanced observation.
Physical Profile of K2-18b
K2-18b is classified as a sub-Neptune, a planet with a size between that of Earth and Neptune. It has a mass approximately 8.6 times that of Earth and a radius about 2.6 times larger. These dimensions result in a density intermediate between rocky planets like Earth and gas-rich planets like Neptune, suggesting a composition that may include a substantial atmosphere or a large volume of water.
The planet orbits its host star, K2-18, every 33 days. K2-18 is a red dwarf, a star smaller and cooler than our sun. Despite the star’s lower energy output, K2-18b is positioned within its habitable zone. This is the orbital region where conditions could allow liquid water to exist on a planet’s surface, given sufficient atmospheric pressure.
The Hycean Planet Hypothesis
The physical properties of K2-18b have made it a candidate for a theoretical class of worlds known as “Hycean” planets. This term, a blend of “hydrogen” and “ocean,” describes a planet with a liquid water ocean beneath a dense, hydrogen-rich atmosphere. The concept was introduced in 2021 by researchers at the University of Cambridge.
The Hycean model expands the search for life beyond Earth-like rocky planets. Worlds like K2-18b, which are larger than Earth, were previously thought to be too hot to maintain liquid water. The Hycean hypothesis suggests that the extensive hydrogen atmosphere could create conditions that permit a temperate surface environment, making these common types of exoplanets potential targets for habitability studies.
K2-18b’s size, density, and location in its star’s habitable zone align with the parameters of a Hycean world. While its internal structure could also be consistent with a gas-rich mini-Neptune, the possibility of a vast global ocean remains a strong interpretation of the available data.
Atmospheric Composition Findings
Observations using the James Webb Space Telescope (JWST) have provided evidence about the makeup of K2-18b’s atmosphere. Using a technique known as transmission spectroscopy, the telescope analyzed starlight filtering through the atmosphere to identify the molecules present. This work confirmed carbon-bearing molecules, specifically methane (CH4) and carbon dioxide (CO2).
The detection of these gases lends support to the Hycean planet theory. Furthermore, the analysis revealed a scarcity of ammonia. The absence of substantial ammonia is consistent with the hypothesis of a large water ocean, as it is highly soluble in water and would be depleted from the upper atmosphere if an ocean were present. This combination of gases points toward a world with a hydrogen-rich atmosphere interacting with a liquid water ocean.
A Potential Biosignature Detection
A notable result from the atmospheric study of K2-18b is the tentative detection of dimethyl sulfide (DMS). On Earth, DMS is a gas produced almost exclusively by biological processes, such as by marine phytoplankton. Finding this molecule in an exoplanet’s atmosphere is significant because it is considered a potential biosignature—a substance that could indicate the presence of life.
The initial detection of DMS was reported as tentative and requires further verification. The signal in the data was not strong enough to be definitive, prompting caution from the scientific community. Researchers emphasize the need for additional, robust evidence before firm conclusions can be drawn about its origin.
To validate this finding, scientists plan follow-up observations of K2-18b. Using different instruments on the JWST, such as the Mid-Infrared Instrument (MIRI), could provide an independent line of evidence to help confirm or refute the presence of DMS. Reaching a higher level of statistical confidence is the next step for researchers.