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K2-18b, also known as EPIC 201912552 b, is an exoplanet orbiting the red dwarf K2-18, located 124 light-years (38 pc) away from Earth.

The planet is a sub-Neptune about 2.6 times the radius of Earth, with a 33-day orbit within the star's habitable zone; it receives approximately a similar amount of light as the Earth receives from the Sun. Initially discovered with the Kepler space telescope, it was later observed by the James Webb Space Telescope (JWST) in order to study the planet's atmosphere. JWST discovered water vapour, carbon dioxide and methane in its atmosphere. JWST's data has been variously interpreted as indicating a water ocean planet with a hydrogen-rich atmosphere, and a gas-rich mini-Neptune. K2-18b has been studied as a potential habitable world that, temperature aside, more closely resembles an ice giant like Uranus or Neptune than Earth. It is the prototype for hycean planets, planets which have abundant water under a hydrogen envelope. It has not been widely accepted as proof of extraterrestrial life, however, as its presence could be explained by abiotic chemical processes and there are doubts that the observations actually show the presence of DMS instead of other compounds or measurement artifacts.

The density of K2-18b is about 2.67+0.52−0.47 g/cm3—intermediate between that of Earth and Neptune—implying that the planet has a hydrogen-rich envelope.

It is unclear whether observations imply that a separate liquid ocean exists on K2-18b, and detecting such an ocean is difficult from the outside; its existence cannot be inferred or ruled out solely from the mass and radius of a planet.

Observations with the Hubble Space Telescope have found that K2-18b has an atmosphere consisting of hydrogen with high metallicity. Whether there are hazes in K2-18b's atmosphere is unclear, while evidence for water clouds, the only kind of clouds likely to form at K2-18b, is conflicting. Apart from water, ammonium chloride, sodium sulfide, potassium chloride and zinc sulfide could form clouds in the atmosphere of K2-18b, depending on the planet's properties.

Climate models have been used to simulate the climate that K2-18b might have, and an intercomparison of their results for K2-18b is part of the CAMEMBERT project to simulate the climates of sub-Neptune planets. Radecka and Rimmer (2025) ran chemical models to determine whether K2-18b would be expected to have nitrogen compounds in its atmosphere.

However, a number of biosignature gases used to identify the presence of life are not reliable indicators when found in a hydrogen-rich atmosphere, thus different markers would be needed to identify biological activity at K2-18b.

The planet was discovered in 2015 by the Kepler space telescope, and its existence was later confirmed with the Spitzer Space Telescope and through Doppler velocity techniques. Early estimates of the star's radius had substantial errors, which led to incorrect planet radius estimates and the density of the planet being overestimated. The discovery of the spectroscopic signature of water vapour on K2-18b in 2019 was the first discovery of water vapour on an exoplanet that is not a Hot Jupiter and drew a lot of discussion. The properties of K2-18b have led to the definition of a hycean planet, a type of planet that has both abundant liquid water and a hydrogen envelope. Planets with such compositions were previously thought to be too hot to be habitable; findings at K2-18b instead suggest that they might be cold enough to harbour liquid water oceans conducive to life. K2-18b is probably the best known hycean planet.

Finding life outside of Earth is difficult and no such claim has as of 2025 been widely accepted by the scientific community, and neither was the claim about K2-18b. Aside from research on whether K2-18b is actually inhabited, there have been studies discussing whether life or compounds required by life like amino acids could actually form on K2-18b.

Quick Facts

• K2-18b has been studied as a potential habitable world that, temperature aside, more closely resembles an ice giant like Uranus or Neptune than Earth.
• JWST discovered water vapour, carbon dioxide and methane in its atmosphere.
• Initially discovered with the Kepler space telescope, it was later observed by the James Webb Space Telescope (JWST) in order to study the planet's atmosphere.
• It is the prototype for hycean planets, planets which have abundant water under a hydrogen envelope.
• The planet is a sub-Neptune about 2.6 times the radius of Earth, with a 33-day orbit within the star's habitable zone; it receives approximately a similar amount of light as the Earth receives from the Sun.

Source material: Wikipedia - "K2-18b". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.