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K2-18b
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.
A controversial discovery of dimethyl sulfide (DMS) was reported in 2025, a chemical that could serve as a biosignature on exoplanets. It has not been widely accepted as proof of extraterrestrial life, however, as its presence could be explained by non-life chemical processes and there are doubts that the observations actually show the presence of DMS instead of other compounds or measurement artifacts.
K2-18 is an M dwarf of the spectral class M3V in the constellation Leo, 38.025 ± 0.079 parsecs (124.02 ± 0.26 ly) distant. The star is colder and smaller than the Sun, having a temperature of 3,457 K (3,184 °C; 5,763 °F) and a radius 45% of the Sun's, and is not visible to the naked eye from the Earth. The star is 2.4 ± 0.6 billion years old and displays moderate stellar activity, but whether it has starspots, which would tend to create false signals when a planet crosses them, is unclear. K2-18 has an additional planet inside of K2-18b's orbit, K2-18c, which may interact with K2-18b through tides.
It is estimated that up to 80% of all M dwarf stars have planets in their habitable zones, including the stars LHS 1140, Proxima Centauri and TRAPPIST-1. The small mass, size and low temperatures of these stars and frequent orbits of the planets make it easier to characterize the planets. On the other hand, the low luminosity of the stars can make spectroscopic analysis of planets difficult, and the stars are frequently active with flares and inhomogeneous stellar surfaces (faculae and starspots), which can produce erroneous spectral signals when investigating a planet.
K2-18b has a radius of 2.610±0.087 R🜨, a mass of 8.63±1.35 M🜨, and orbits its star in 33 days. From Earth, it can be seen passing in front of the star. The planet is most likely tidally locked to the star, although considering its orbital eccentricity, a spin-orbit resonance like Mercury is also possible.
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. The planet may either be rocky with a thick envelope or have a Neptune-like composition. A pure water planet with a thin atmosphere is less likely. Planets with radii of about 1.5–2 R🜨 are unexpectedly rare relative to their expected occurrence rate, a phenomenon known as the radius valley. Presumably, planets with intermediary radii cannot hold their atmospheres against the tendency of their own energy output and the stellar radiation to drive atmospheric escape. (Planets with even smaller radii are known as super-Earths and those with larger radii as sub-Neptunes.)
The planet may have taken a few million years to form. Significant tidal heating is unlikely. Internal heating may increase temperatures at large depths, but is unlikely to significantly affect the surface temperature. If an ocean exists, it is probably underlain by a high-pressure ice layer on top of a rocky core, which might destabilize the planet's climate by preventing material flows between the core and the ocean. The existence of exomoons, which could affect the climate and habitability of K2-18b, has been examined. It appears that the Hill sphere where a moon can be held by the planet is too small to allow a moon lifespan to exceedin 10 million years. It is not clear whether planets like K2-18b can host exomoons; it is possible that tidal effects or orbital interactions would destroy them.
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K2-18b
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.
A controversial discovery of dimethyl sulfide (DMS) was reported in 2025, a chemical that could serve as a biosignature on exoplanets. It has not been widely accepted as proof of extraterrestrial life, however, as its presence could be explained by non-life chemical processes and there are doubts that the observations actually show the presence of DMS instead of other compounds or measurement artifacts.
K2-18 is an M dwarf of the spectral class M3V in the constellation Leo, 38.025 ± 0.079 parsecs (124.02 ± 0.26 ly) distant. The star is colder and smaller than the Sun, having a temperature of 3,457 K (3,184 °C; 5,763 °F) and a radius 45% of the Sun's, and is not visible to the naked eye from the Earth. The star is 2.4 ± 0.6 billion years old and displays moderate stellar activity, but whether it has starspots, which would tend to create false signals when a planet crosses them, is unclear. K2-18 has an additional planet inside of K2-18b's orbit, K2-18c, which may interact with K2-18b through tides.
It is estimated that up to 80% of all M dwarf stars have planets in their habitable zones, including the stars LHS 1140, Proxima Centauri and TRAPPIST-1. The small mass, size and low temperatures of these stars and frequent orbits of the planets make it easier to characterize the planets. On the other hand, the low luminosity of the stars can make spectroscopic analysis of planets difficult, and the stars are frequently active with flares and inhomogeneous stellar surfaces (faculae and starspots), which can produce erroneous spectral signals when investigating a planet.
K2-18b has a radius of 2.610±0.087 R🜨, a mass of 8.63±1.35 M🜨, and orbits its star in 33 days. From Earth, it can be seen passing in front of the star. The planet is most likely tidally locked to the star, although considering its orbital eccentricity, a spin-orbit resonance like Mercury is also possible.
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. The planet may either be rocky with a thick envelope or have a Neptune-like composition. A pure water planet with a thin atmosphere is less likely. Planets with radii of about 1.5–2 R🜨 are unexpectedly rare relative to their expected occurrence rate, a phenomenon known as the radius valley. Presumably, planets with intermediary radii cannot hold their atmospheres against the tendency of their own energy output and the stellar radiation to drive atmospheric escape. (Planets with even smaller radii are known as super-Earths and those with larger radii as sub-Neptunes.)
The planet may have taken a few million years to form. Significant tidal heating is unlikely. Internal heating may increase temperatures at large depths, but is unlikely to significantly affect the surface temperature. If an ocean exists, it is probably underlain by a high-pressure ice layer on top of a rocky core, which might destabilize the planet's climate by preventing material flows between the core and the ocean. The existence of exomoons, which could affect the climate and habitability of K2-18b, has been examined. It appears that the Hill sphere where a moon can be held by the planet is too small to allow a moon lifespan to exceedin 10 million years. It is not clear whether planets like K2-18b can host exomoons; it is possible that tidal effects or orbital interactions would destroy them.