Hycean planet

A hycean planet (/ˈhaɪʃən/ HY-shən) is a hypothetical type of exoplanet that features a liquid water ocean underneath a hydrogen-rich atmosphere. The term hycean is a portmanteau of hydrogen and ocean.

A hycean planet is a hypothetical type of planet with liquid water oceans under a hydrogen atmosphere.^[1] The presence of extraterrestrial liquid water makes hycean planets regarded as promising candidates for planetary habitability.^[2][3][4] They are usually considered to be larger and more massive than Earth.^[5] As of 2023, there are no confirmed hycean planets, but the Kepler mission detected many candidates.^[2]

The term "hycean planet" was coined in 2021 by a team of exoplanet researchers at the University 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 around red 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 in The 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 detection of biosignatures easier.^[7] Hycean worlds could be investigated for biosignatures by terrestrial telescopes and space telescopes like the James Webb Space Telescope (JWST).^[3][8] In 2023, the JWST investigated K2-18b and found evidence for both a hycean atmosphere and the presence of dimethyl sulfide ─ a potential biosignature.

Hycean planets could be considerably larger than previous estimates for habitable planets, with radii reaching 2.6 R_🜨 (2.3 R_🜨) and masses of 10 M_E (5 M_E).^[7] Moreover, the habitable zone of such planets could be considerably larger than that of Earth-like planets. The planetary equilibrium temperature can reach 430 K (157 °C; 314 °F) for planets orbiting late M-dwarfs.^[9] 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.^[10]

Such planets can have many distinct atmospheric compositions and internal structures.^[7] Also possible are tidally locked "dark hycean" planets (habitable only on the side of permanent night)^[11] or "cold hycean" planets (with negligible irradiation, being kept warm by the greenhouse effect).^[9] Dark hycean worlds can form when the atmosphere does not effectively transport heat from the permanent day side to the permanent night side,^[12] thus the night side has temperate temperatures while the day side is too hot for life.^[13] Cold hycean planets may exist even in the absence of stars, e.g. rogue planets.^[13]

Although the presence of water may help them be

• They are regarded to be covered in oceans.[6]
• They have hydrogen-rich atmospheres. The atmospheres on hycean planets are thought to be made up of hydrogen, helium, and water vapor.^[1]
• Dark hycean planets thought to be common around red dwarf stars.^[13] Red dwarf stars are the most common type of star in the Milky Way galaxy.^[14]
• They are considered to be a promising place to search for life beyond Earth. Hycean planets have the ingredients that is necessary for life, including liquid water, energy, and organic molecules.^[6]
• Their atmospheres may have less methane and ammonia than comparable non-hycean Neptune-like planets, if they have water oceans.^[5]
• They might have a much higher free energy availability for their ecosystems than Earth.^[15]

Hycean planets may be capable of supporting extraterrestrial life, despite their properties differing drastically from Earth's. Astronomers plan to use telescopes like the James Webb Space Telescope to search for hycean planets and to learn more about their potential for habitability.^[16]

One such candidate planet is K2-18b, 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's habitable zone. Such candidate planets can be studied for biomarkers.^[17][18] In 2023, the

• K2-3b, a potential Dark hycean planet^[10] but may be too hot.^[21]
• K2-3c^[10] but may be too hot.^[21]
• Kepler-138d^[22]
• LTT 1445 A b^[10] but may be too hot^[21] and too water-poor.^[23]
• TOI-732 c^[10] but may be too hot.^[21]
• TOI-1266 c^[10] but may be too hot.^[21]
• TOI-175 d^[10] but may be too hot.^[21]
• TOI-2136 b^[24]
• TOI-270 c, a potential Dark hycean planet^[10] but may be too hot.^[21]
• TOI-270 d^[10] but may be too hot.^[21]
• TOI-776 b, a potential Dark hycean planet^[10] but may be too hot.^[21]
• TOI-776 c^[10] but may be too hot.^[21]

• Hot Neptune
• Exoplanet
• Ocean world