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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.

Definition

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

History

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.

Life on hycean planets would probably be entirely aquatic. Their water-rich compositions imply that they can have larger sizes than comparable non-hycean planets, thus making detection of biosignatures easier. Hycean worlds could be investigated for biosignatures by terrestrial telescopes and space telescopes like the James Webb Space Telescope (JWST). In 2023, the JWST investigated K2-18b and found evidence for both a hycean atmosphere and the presence of dimethyl sulfide ─ a potential biosignature.

Properties

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). 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. 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.

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

Although the presence of water may help them be habitable planets, their habitability may be limited by a possible runaway greenhouse effect. Hydrogen reacts differently to starlight's wavelengths than do heavier gases like nitrogen and oxygen. If the planet orbits a sun-like star at one Astronomical 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. Another limiting factor is that X-ray and UV radiation from the star (especially active stars) can destroy the water molecules.

Features

• They are regarded to be covered in oceans.
• They have hydrogen-rich atmospheres. The atmospheres on hycean planets are thought to be made up of hydrogen, helium, and water vapor.
• Dark hycean planets thought to be common around red dwarf stars. Red dwarf stars are the most common type of star in the Milky Way galaxy.
• 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.
• Their atmospheres may have less methane and ammonia than comparable non-hycean Neptune-like planets, if they have water oceans.
• They might have a much higher free energy availability for their ecosystems than Earth.

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.

Candidates

K2-18b

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. In 2023, the James Webb Space Telescope detected carbon dioxide and methane in the atmosphere of K2-18b, but it did not detect large amounts of ammonia. This supports the hypothesis that K2-18b could indeed have a water ocean. The same observations also suggest that K2-18b's atmosphere might contain dimethyl sulfide, a compound associated with life on Earth, although this has yet to be confirmed. Another possibility is that K2-18b is a lava world with a hydrogen atmosphere.

Other candidates

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

See also

• Hot Neptune
• Exoplanet
• Ocean world

References

1. ^ Madhusudhan et al. 2021, p. 3
2. ^ Sutter, Paul (2 May 2023). "Hycean exoplanets may not be able to support life after all". Space.com. Retrieved 5 May 2023.
3. ^ Madhusudhan et al. 2021, p. 3
4. Davis, Nicola (30 August 2021). "'Mini-Neptunes' beyond solar system may soon yield signs of life – Cambridge astronomers identify new hycean class of habitable exoplanets, which could accelerate search for life". The Guardian. Retrieved 30 August 2021.
5. ^ Madhusudhan et al. 2021, p. 4
6. ^ Madhusudhan et al. 2021, p. 12
7. ^ Madhusudhan et al. 2023, p. 1
8. "Alien life could be living on big 'Hycean' exoplanets". BBC News. 27 August 2021. Retrieved 31 August 2021.
9. ^ Madhusudhan et al. 2021, p. 9
10. ^ Madhusudhan et al. 2021, p. 6
11. ^ Madhusudhan et al. 2021, p. 5
12. Madhusudhan et al. 2021, p. 10
13. ^ Madhusudhan et al. 2021, p. 11
14. Gargaud et al. 2011, Red Dwarf
15. Petraccone, Luigi (27 November 2023). "Planetary entropy production as a thermodynamic constraint for exoplanet habitability". Monthly Notices of the Royal Astronomical Society. 527 (3): 5550. doi:10.1093/mnras/stad3526.
16. Darling, David. "Hycean planet". www.daviddarling.info. Retrieved 24 May 2023.
17. "Hycean Planets | StarDate Online". stardate.org. Retrieved 24 May 2023.
18. 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. ISSN 2397-3366. S2CID 254764810.
19. Yan, Isabelle (8 September 2023). "Webb Discovers Methane, Carbon Dioxide in Atmosphere of K2-18 b". NASA. Retrieved 12 September 2023.
20. Shorttle, Oliver; Jordan, Sean; Nicholls, Harrison; Lichtenberg, Tim; Bower, Dan J. (February 2024). "Distinguishing Oceans of Water from Magma on Mini-Neptune K2-18b". The Astrophysical Journal Letters. 962 (1): L8. arXiv:2401.05864. Bibcode:2024ApJ...962L...8S. doi:10.3847/2041-8213/ad206e. ISSN 2041-8205.
21. ^ Pierrehumbert, Raymond T. (1 February 2023). "The Runaway Greenhouse on Sub-Neptune Waterworlds". The Astrophysical Journal. 944 (1): 20. arXiv:2212.02644. Bibcode:2023ApJ...944...20P. doi:10.3847/1538-4357/acafdf.
22. 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 (15 December 2022). "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. S2CID 254764810.
23. Phillips, Caprice L; Wang, Ji; Edwards, Billy; Martínez, Romy Rodríguez; Asnodkar, Anusha Pai; Gaudi, B Scott (2023). "Exploring the potential of Twinkle to unveil the nature of LTT 1445 Ab". Monthly Notices of the Royal Astronomical Society. 526 (2): 2251–2264. doi:10.1093/mnras/stad2822.
24. Kawauchi, K.; Murgas, F.; Palle, E.; Narita, N.; Fukui, A.; Hirano, T.; Parviainen, H.; Ishikawa, H. T.; Watanabe, N.; Esparaza-Borges, E.; Kuzuhara, M.; Orell-Miquel, J.; Krishnamurthy, V.; Mori, M.; Kagetani, T.; Zou, Y.; Isogai, K.; Livingston, J. H.; Howell, S. B.; Crouzet, N.; Leon, J. P. de; Kimura, T.; Kodama, T.; Korth, J.; Kurita, S.; Laza-Ramos, A.; Luque, R.; Madrigal-Aguado, A.; Miyakawa, K.; Morello, G.; Nishiumi, T.; Rodríguez, G. E. F.; Sánchez-Benavente, M.; Stangret, M.; Teng, H.; Terada, Y.; Gnilka, C. L.; Guerrero, N.; Harakawa, H.; Hodapp, K.; Hori, Y.; Ikoma, M.; Jacobson, S.; Konishi, M.; Kotani, T.; Kudo, T.; Kurokowa, T.; Kusakabe, N.; Nishikawa, J.; Omiya, M.; Serizawa, T.; Tamura, M.; Ueda, A.; Vievard, S. (1 October 2022). "Validation and atmospheric exploration of the sub-Neptune TOI-2136b around a nearby M3 dwarf". Astronomy & Astrophysics. 666: A4. arXiv:2202.10182. Bibcode:2022A&A...666A...4K. doi:10.1051/0004-6361/202243381. ISSN 0004-6361. S2CID 247011479.

Sources

• Gargaud, Muriel; Amils, Ricardo; Quintanilla, José Cernicharo; Cleaves, Henderson James; Irvine, William M.; Pinti, Daniele L.; Viso, Michel, eds. (2011). Encyclopedia of astrobiology. Springer reference. Berlin: Springer. Bibcode:2011eab..book.....G. doi:10.1007/978-3-642-11274-4. ISBN 978-3-642-11271-3. S2CID 90186988.
• Madhusudhan, Nikku; Piette, Anjali A. A.; Constantinou, Savvas (September 2021). "Habitability and Biosignatures of Hycean Worlds". The Astrophysical Journal. 918 (1): 1. arXiv:2108.10888. Bibcode:2021ApJ...918....1M. doi:10.3847/1538-4357/abfd9c. ISSN 0004-637X.
• Madhusudhan, Nikku; Sarkar, Subhajit; Constantinou, Savvas; Holmberg, Måns; Piette, Anjali A. A.; Moses, Julianne I. (October 2023). "Carbon-bearing Molecules in a Possible Hycean Atmosphere". The Astrophysical Journal Letters. 956 (1): L13. arXiv:2309.05566. Bibcode:2023ApJ...956L..13M. doi:10.3847/2041-8213/acf577. ISSN 2041-8205.

External links

• University of Cambridge (30 August 2021). "New class of habitable exoplanets represent a big step forward in the search for life". Phys.org. Retrieved 30 August 2021.
• Carter, Jamie (25 August 2021). "Is Alien Life Hiding In Plain Sight? New Class Of 'Hycean' Planet Is Where We Should Look, Say Scientists". Forbes. Retrieved 30 August 2021.
• Starr, Michelle (27 August 2021). "We Could Discover Alien Life on This New Class of 'Hycean' Exoplanets, Study Says". ScienceAlert. Retrieved 30 August 2021.
• Wall, Mike (27 August 2021). "Alien life could thrive on big 'Hycean' exoplanets - Hycean planets are up to 2.5 times bigger than Earth, with oceans and hydrogen-rich atmospheres". Space.com. Retrieved 30 August 2021.
• Strictland, Ashley (26 August 2021). "This new class of hot ocean worlds could support life". CNN News. Retrieved 30 August 2021.
• Irving, Michael (27 August 2021). "Say hi to Hycean worlds, a new class of exoplanet that could host life". NewAtlas.com. Retrieved 30 August 2021.
• "Astronomers Identify New Class of Exoplanets: Hycean Worlds". Science News. 26 August 2021. Retrieved 30 August 2021.
• Cohen, Liz (27 August 2021). "Scientists may find life on Earth-like planets covered in oceans within the next few years". CBS News. Retrieved 30 August 2021.

v t e Exoplanets
Planet Definition IAU Planetary science
Main topics	Exoplanet Exoplanet orbital and physical parameters Methods of detecting exoplanets Planetary system Planet-hosting stars
Sizes and types	Terrestrial Carbon planet Coreless planet Desert planet Dwarf planet Hycean planet Ice planet Iron planet (Super-Mercury) Lava planet Ocean world Mega-Earth Sub-Earth Super-Earth Gaseous Eccentric Jupiter Mini-Neptune (Gas dwarf) Helium planet Hot Jupiter Hot Neptune Gas giant Ice giant Super-Jupiter Super-Neptune Super-puff Ultra-hot Jupiter Ultra-hot Neptune Other types Blanet Brown dwarf Chthonian planet Circumbinary planet Circumtriple planet Disrupted planet Double planet Ecumenopolis Eyeball planet Giant planet Mesoplanet Planemo Planet/Brown dwarf boundary Planetesimal Protoplanet Pulsar planet Sub-brown dwarf Sub-Neptune Toroidal planet Ultra-cool dwarf Ultra-short period planet (USP)
Formation and evolution	Accretion Accretion disk Asteroid belt Circumplanetary disk Circumstellar disc Circumstellar envelope Cosmic dust Debris disk Detached object Disrupted planet Excretion disk Exozodiacal dust Extraterrestrial materials Extraterrestrial sample curation Giant-impact hypothesis Gravitational collapse Hills cloud Internal structure Interplanetary dust cloud Interplanetary medium Interplanetary space Interstellar cloud Interstellar dust Interstellar medium Interstellar space Kuiper belt List of interstellar and circumstellar molecules Merging stars Molecular cloud Nebular hypothesis Oort cloud Outer space Planetary migration Planetary system Planetesimal Planet formation Protoplanetary disk Ring system Rubble pile Sample-return mission Scattered disc Star formation
Systems	Exocomet Interstellar Exomoon Tidally detached Rogue planet Orbits Retrograde Trojan Mean-motion resonances Titius–Bode law
Host stars	A B Binary star Brown dwarfs F/Yellow-white dwarfs G/Yellow dwarfs Herbig Ae/Be K/Orange dwarfs M/Red dwarfs Pulsar Red giant Subdwarf B Subgiant T Tauri White dwarfs Yellow giants
Detection	Astrometry Direct imaging list Microlensing list Polarimetry Timing list Radial velocity list Transit method list Transit-timing variation
Habitability	Astrobiology Astrooceanography Circumstellar habitable zone Earth analog Extraterrestrial liquid water Galactic habitable zone Habitability of binary star systems Habitability of F-type main-sequence star systems Habitability of K-type main-sequence star systems Habitability of natural satellites Habitability of neutron star systems Habitability of red dwarf systems Habitability of yellow dwarf systems Habitable zone for complex life List of potentially habitable exoplanets Tholin Superhabitable planet
Catalogues	Nearby Habitable Systems Exoplanet Data Explorer Extrasolar Planets Encyclopaedia NASA Exoplanet Archive NASA Star and Exoplanet Database Open Exoplanet Catalogue
Lists	Exoplanetary systems Host stars Multiplanetary systems Stars with proto-planetary discs Exoplanets Discoveries Extremes Firsts Nearest Largest Heaviest Terrestrial candidates Kepler 1–500 501–1000 1001–1500 1501–2000 K2 Potentially habitable Proper names Discovered exoplanets by year before 2000 2000–2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024
Other	Carl Sagan Institute Exoplanet naming convention Exoplanet phase curves Exoplanetary Circumstellar Environments and Disk Explorer Extragalactic planet Extrasolar planets in fiction Geodynamics of terrestrial exoplanets Neptunian desert Nexus for Exoplanet System Science Planets in globular clusters Small planet radius gap Sudarsky's gas giant classification
Discoveries of exoplanets Search projects

v t e Water
Overviews	Outline Data Model Properties
States	Liquid Ice Vapor Steam superheated
Forms	Deuterium-depleted Semiheavy Heavy Tritiated Doubly labeled water Hydronium
On Earth	Cycle Distribution Hydrosphere Hydrology Hydrobiology Origin Pollution Resources management policy Supply
Extraterrestrial	Extraterrestrial liquid water Asteroidal water Planetary oceanography Ocean world Hycean planet List of Candidates Specific Europa Mars Moon Enceladus
Physical parameters	Stratification Ocean stratification Lake stratification Ocean temperature
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