The term "hycean planet" was coined in 2021 by a team of planet researchers led byNikku Madhusudhan at theUniversity of Cambridge, as a portmanteau of "hydrogen" and "ocean," used to describe planets that are thought to have large oceans and hydrogen-rich atmospheres. Hycean planets are thought to be common aroundred dwarf stars, and are considered to be a promising place to search for life beyond Earth. The term was first used in a paper published inThe Astrophysical Journal on August 31, 2021.[3]
Life on hycean planets would probably be entirely aquatic.[6] Their water-rich compositions imply that they can have larger sizes than comparable non-hycean planets, thus making their observation and the detection ofbiosignatures easier.[7] Candidate hycean worlds such asK2-18b have been investigated forbiosignatures by terrestrial telescopes andspace telescopes like theJames Webb Space Telescope (JWST).[3][8][9]
Hycean planets could be considerably larger than previous estimates for habitable planets, with radii reaching 2.6 R🜨 (2.3 R🜨) and masses of10M🜨 (5M🜨).[7] Moreover, the habitable zone of such planets could be considerably larger than that of Earth-like planets. Theplanetary equilibrium temperature can reach 430 K (157 °C; 314 °F) for planets orbitinglate M-dwarfs.[10] However, mass and radius do not by themselves inform the composition of a planet, as bodies with identical mass and radius can have distinct compositions: A given planet may thus be either a hycean planet or a super-Earth.[11]
Such planets can have many distinct atmospheric compositions and internal structures.[7] Also possible aretidally locked "dark hycean" planets (habitable only on the side of permanent night)[12] or "cold hycean" planets (with negligible irradiation, being kept warm by thegreenhouse effect).[10] Dark hycean worlds can form when the atmosphere does not effectively transport heat from the permanent day side to the permanent night side,[13] thus the night side has temperate temperatures while the day side is too hot for life.[14] Cold hycean planets may exist even in the absence of stars, e.g.rogue planets.[14]
Although the presence of water may help them behabitable planets, their habitability may be limited by a possiblerunaway greenhouse effect. Hydrogen reacts differently to starlight's wavelengths than do heavier gases like nitrogen and oxygen. If the planet orbits aSun-like star at oneAstronomical unit (AU), the temperature would be so high that the oceans would boil and water would become vapor. Current calculations locate the habitable zone where water would remain liquid at 1.6 AU, if the atmospheric pressure is similar to Earth's, or at 3.85 AU if it is the more likely tenfold to twentyfold pressure. All current hycean planet candidates are located within the area where oceans would boil, and are thus unlikely to have actual oceans of liquid water.[2] Another limiting factor is thatX-ray andUV radiation from the star (especiallyactive stars) can destroy the water molecules.[12]
Another factor limiting the habitable zone of hycean planets is their orbitaleccentricity. Planets with moderately eccentric orbits have their surface temperatures increase fromtidal heating from their parentstars. This effect significantly truncates the habitable zone of the planet at larger orbital radii. While it is thought that only a few hycean planets have been discovered where this effect significantly impacts the planet, it is important to note as more candidate hycean planets are discovered.[15] Previous models of a exoplanets orbit and where it places on the habitable zone have not taken into account the tidal forces between the planet and the star meaning many planets that are thought to be potentially habitable may not be. However, other planets on the system could stabilize the orbit of the hycean planets for billions of years in a process called “forced eccentricity”.[16]
Dark hycean planets thought to be common around red dwarf stars.[14] Red dwarf stars are the most common type of star in theMilky Way galaxy.[17]
They are considered to be a promising place to search for life beyond Earth. Hycean planets have the ingredients that are necessary for life, including liquid water, energy, andorganic molecules.[6]
Their atmospheres may have lessmethane andammonia than comparable non-hyceanNeptune-like planets, if they have water oceans.[5]
They might have a much higherfree energy availability for their ecosystems than Earth.[18]
Hycean planets may be capable of supporting extraterrestrial life, despite their properties differing drastically from Earth's. Astronomers plan to use telescopes like theJames Webb Space Telescope to search for hycean planets and to learn more about their potential for habitability.[19]
One such candidate planet isK2-18b, which orbits a faint star with a period of about 33 days. This candidate planet could have liquid water, containing a considerable high amount of hydrogen gas in its atmosphere, and is far enough from its star, such that it resides within its star'shabitable zone. Such candidate planets can be studied forbiomarkers.[20][21]In 2023, theJames Webb Space Telescope detectedcarbon dioxide andmethane in the atmosphere of K2-18b, but it did not detect large amounts ofammonia. This supports the hypothesis that K2-18b could indeed have a water ocean. The same observations also suggest that K2-18b's atmosphere might containdimethyl sulfide, a compound associated with life on Earth. The presence of this compound was yet to be confirmed at the time,[22] but evidence of dimethyl sulfide as well asdimethyl disulfide was found in 2025.[23] Another possibility is that K2-18b is alava world with a hydrogen atmosphere.[24]
^Piaulet, Caroline; Benneke, Björn; Almenara, Jose M.; Dragomir, Diana; Knutson, Heather A.; Thorngren, Daniel; Peterson, Merrin S.; Crossfield, Ian J. M.; M. -R. Kempton, Eliza; Kubyshkina, Daria; Howard, Andrew W.; Angus, Ruth; Isaacson, Howard; Weiss, Lauren M.; Beichman, Charles A.; Fortney, Jonathan J.; Fossati, Luca; Lammer, Helmut; McCullough, P. R.; Morley, Caroline V.; Wong, Ian (February 2023). "Evidence for the volatile-rich composition of a 1.5-Earth-radius planet".Nature Astronomy.7 (2):206–222.arXiv:2212.08477.Bibcode:2023NatAs...7..206P.doi:10.1038/s41550-022-01835-4.ISSN2397-3366.S2CID254764810.
