Exoplanet K2 – 18b

NASA recently announced that K2-18b, also known as EPIC 201912552 b, is an exoplanet orbiting the red dwarf star K2-18. It is located approximately 120 -124 light-years (about 38 parsecs) away from Earth in the constellation Leo. It was discovered in 2015 by the Kepler space telescope using the transit method, which detects the slight dimming of a star as a planet passes in front of it. With a mass of approximately 8.6 to 8.9 times the mass of Earth, and classified as a sub-Neptune or super-Earth, planets of this size, between Earth and Neptune, are not found in our solar system. K2 -18b’s density is estimated at 2.67 g/cm³, intermediate between Earth and Neptune, suggesting a composition that includes a substantial hydrogen-rich envelope rather than being purely rocky. Its orbit around its host star K2-18 is once every 32.9 days, being within or slightly inside the star’s habitable zone, and receiving stellar radiation like Earth’s insolation. Its estimated equilibrium temperature (without atmospheric effects) is around -8 ± 5 °C.
 
The host star, K2-18, is an M-type red dwarf, located approximately 124 light-years from Earth in the constellation Leo. It is cooler and smaller than the Sun, with a temperature of about 3,184 °C and a radius 45% that of the Sun. The star is estimated to be 2.4 billion years old and shows moderate stellar activity. The K2-18 system also hosts at least one other planet, K2-18c, orbiting closer to the star.
 
Observations by the James Webb Space Telescope (JWST) have provided significant details about K2-18b’s atmosphere. Apart from hydrogen, the JWST detected the presence of carbon-bearing molecules, specifically methane and carbon dioxide in 2023. Each makes up about 1% of the atmosphere. Ammonia concentrations appear to be very low. Water vapour was initially reported based on Hubble Space Telescope data in 2019, but later JWST observations indicated low concentrations of less than 0.1%, possibly due to atmospheric layering. There is conflicting evidence regarding the presence of water clouds. If they exist, they are likely icy, though liquid water clouds are possible. Other potential cloud compositions include ammonium chloride, sodium sulfide, potassium chloride, and zinc sulfide. The planet’s nature is debated. It could be a gas-rich mini-Neptune, potentially lacking a distinct surface. Alternatively, it might be a “Hycean” world – a hypothetical type of planet characterized by a water ocean covering the surface beneath a hydrogen-rich atmosphere. If an ocean exists, it is likely underlain by a mantle of high-pressure ice, like Neptune.
 
K2-18b’s location in the habitable zone and the detection of carbon molecules and potential for liquid water make it a target of interest in the search for extraterrestrial life. Initial JWST observations reported a possible detection of dimethyl sulfide (DMS). On Earth, DMS is primarily produced by marine life, particularly phytoplankton. A 2025 report suggested DMS levels 20 times higher than on Earth, potentially indicating ongoing replenishment due to the molecule’s short lifespan. This potential detection generated excitement as DMS is considered a possible biosignature. However, the DMS inference is considered less robust and requires further validation. 
 
K2-18b’s atmosphere is fundamentally different from Earth’s, being hydrogen-rich, much thicker, and lacking oxygen. It is more akin to the atmospheres of ice giants like Neptune than to rocky planets like Earth. While both planets may have water in some form, the environmental conditions on K2-18b would be extremely challenging for life as we know it.
 
References:
(2023, September 11). Webb Discovers Methane, Carbon Dioxide in Atmosphere of K2 – 18b. NASA. https://www.nasa.gov/universe/exoplanets/webb-discovers-methane-carbon-dioxide-in-atmosphere-of-k2-18-b/

(2025, April 23). K2 -18b. In Wikipedia. https://en.wikipedia.org/wiki/K2-18b

​Faulkner, D.R. (2025, April 23). Have Astronomers Finally Found Evidence of Life on K2 – 18b? Answers in Genesis. https://answersingenesis.org/blogs/danny-faulkner/2025/04/23/have-astronomers-finally-found-evidence-life-k2-18b/