Hycean planet

The term hycean is a portmanteau of hydrogen and ocean. Due to the presence of extraterrestrial liquid water, hycean planets are regarded as promising candidates for planetary habitability.{{harvnb|Madhusudhan et al.|2021|p=3}}{{cite web|access-date=2021-08-30|first=Nicola|last=Davis|date=2021-08-30|language=en|title='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|url=https://www.theguardian.com/science/2021/aug/26/mini-neptune-beyond-solar-system-may-soon-yield-sign-life-hycean-exoplanet-cambridge-astronomer|website=The Guardian}} They are usually considered to be larger and more massive than Earth.{{harvnb|Madhusudhan et al.|2021|p=4}} As of 2023, there are no confirmed hycean planets, but the Kepler mission detected many candidates.{{cite web |url= https://www.space.com/hycean-exoplanets-may-not-support-life|title= Hycean exoplanets may not be able to support life after all|first= Paul|last= Sutter|date= May 2, 2023|publisher= Space.com|accessdate=May 5, 2023}}

The term "hycean planet" was coined in 2021 by a team of planet researchers led by Nikku Madhusudhan 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.{{harvnb|Madhusudhan et al.|2021|p=12}} Their water-rich compositions imply that they can have larger sizes than comparable non-hycean planets, thus making their observation and the detection of biosignatures easier.{{harvnb|Madhusudhan et al.|2023|p=1}} Hycean worlds could be investigated for biosignatures by terrestrial telescopes and space telescopes like the James Webb Space Telescope (JWST).{{Cite web |date=2021-08-27 |title=Alien life could be living on big 'Hycean' exoplanets |url=https://www.bbc.co.uk/newsround/58308604 |access-date=2021-08-31 |website=BBC News}} In 2023, the JWST investigated K2-18b and found evidence for both a hycean atmosphere and the presence of dimethyl sulfide ─ a potential biosignature, and in 2024 found even stronger evidence (see the K2-18b candidate section below).

Hycean planets could be considerably larger than previous estimates for habitable planets, with radii reaching {{Earth radius|2.6|link=y}} ({{earth radius|2.3}}) and masses of {{Earth mass|sym=y|10|link=y}} ({{Earth mass|sym=y|5}}). Moreover, the habitable zone of such planets could be considerably larger than that of Earth-like planets. The planetary equilibrium temperature can reach {{convert|430|K|C F}} for planets orbiting late M-dwarfs.{{harvnb|Madhusudhan et al.|2021|p=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.{{harvnb|Madhusudhan et al.|2021|p=6}}

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){{harvnb|Madhusudhan et al.|2021|p=5}} 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,{{harvnb|Madhusudhan et al.|2021|p=10}} thus the night side has temperate temperatures while the day side is too hot for life.{{harvnb|Madhusudhan et al.|2021|p=11}} 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.

• They are regarded to be covered in oceans and seas.
• 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.{{harvnb|Gargaud et al.|2011|at=Red Dwarf}}
• 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.{{Cite journal |last=Petraccone |first=Luigi |date=27 November 2023 |title=Planetary entropy production as a thermodynamic constraint for exoplanet habitability |journal=Monthly Notices of the Royal Astronomical Society |volume=527 |issue=3 |page=5550 |doi=10.1093/mnras/stad3526 |doi-access=free}}

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.{{Cite web |last=Darling |first=David |title=Hycean planet |url=http://www.daviddarling.info/encyclopedia/H/Hycean_planet.html |access-date=2023-05-24 |website=www.daviddarling.info}}

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.{{Cite web |title=Hycean Planets {{!}} StarDate Online |url=https://stardate.org/radio/program/2022-08-30 |access-date=2023-05-24 |website=stardate.org}}{{Cite journal |last=Piaulet |first=Caroline |last2=Benneke |first2=Björn |last3=Almenara |first3=Jose M. |last4=Dragomir |first4=Diana |last5=Knutson |first5=Heather A. |last6=Thorngren |first6=Daniel |last7=Peterson |first7=Merrin S. |last8=Crossfield |first8=Ian J. M. |last9=M. -R. Kempton |first9=Eliza |last10=Kubyshkina |first10=Daria |last11=Howard |first11=Andrew W. |last12=Angus |first12=Ruth |last13=Isaacson |first13=Howard |last14=Weiss |first14=Lauren M. |last15=Beichman |first15=Charles A. |date=February 2023 |title=Evidence for the volatile-rich composition of a 1.5-Earth-radius planet |journal=Nature Astronomy |language=en |volume=7 |issue=2 |pages=206–222 |arxiv=2212.08477 |bibcode=2023NatAs...7..206P |doi=10.1038/s41550-022-01835-4 |issn=2397-3366 |s2cid=254764810 |last16=Fortney |first16=Jonathan J. |last17=Fossati |first17=Luca |last18=Lammer |first18=Helmut |last19=McCullough |first19=P. R. |last20=Morley |first20=Caroline V. |last21=Wong |first21=Ian}}

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. The presence of this compound was yet to be confirmed at the time,{{cite web |last=Yan |first=Isabelle |title=Webb Discovers Methane, Carbon Dioxide in Atmosphere of K2-18 b |website=NASA |date=2023-09-08 |url=https://www.nasa.gov/goddard/2023/webb-discovers-methane-carbon-dioxide-in-atmosphere-of-k2-18b/ |access-date=2023-09-12}} but strong evidence of dimethyl sulfide as well as dimethyl disulfide was found in 2025.{{Cite web |title=Strongest hints yet of biological activity outside the solar system |url=https://www.eurekalert.org/news-releases/1080558 |access-date=2025-04-19 |website=EurekAlert! |language=en}} Another possibility is that K2-18b is a lava world with a hydrogen atmosphere.{{Cite journal |last=Shorttle |first=Oliver |last2=Jordan |first2=Sean |last3=Nicholls |first3=Harrison |last4=Lichtenberg |first4=Tim |last5=Bower |first5=Dan J. |date=February 2024 |title=Distinguishing Oceans of Water from Magma on Mini-Neptune K2-18b |journal=The Astrophysical Journal Letters |volume=962 |issue=1 |pages=L8 |arxiv=2401.05864 |bibcode=2024ApJ...962L...8S |doi=10.3847/2041-8213/ad206e |issn=2041-8205 |doi-access=free}}

• K2-3b, a potential Dark hycean planet but may be too hot.{{Cite journal |last=Pierrehumbert |first=Raymond T. |date=1 February 2023 |title=The Runaway Greenhouse on Sub-Neptune Waterworlds |journal=The Astrophysical Journal |volume=944 |issue=1 |pages=20 |arxiv=2212.02644 |bibcode=2023ApJ...944...20P |doi=10.3847/1538-4357/acafdf |doi-access=free}}
• K2-3c but may be too hot.
• Kepler-138d{{cite journal |last1=Piaulet |first1=Caroline |last2=Benneke |first2=Björn |last3=Almenara |first3=Jose M. |last4=Dragomir |first4=Diana |last5=Knutson |first5=Heather A. |last6=Thorngren |first6=Daniel |last7=Peterson |first7=Merrin S. |last8=Crossfield |first8=Ian J. M. |last9=M.-R. Kempton |first9=Eliza |last10=Kubyshkina |first10=Daria |last11=Howard |first11=Andrew W. |last12=Angus |first12=Ruth |last13=Isaacson |first13=Howard |last14=Weiss |first14=Lauren M. |last15=Beichman |first15=Charles A. |last16=Fortney |first16=Jonathan J. |last17=Fossati |first17=Luca |last18=Lammer |first18=Helmut |last19=McCullough |first19=P. R. |last20=Morley |first20=Caroline V. |last21=Wong |first21=Ian |title=Evidence for the volatile-rich composition of a 1.5-Earth-radius planet |journal=Nature Astronomy |date=15 December 2022 |volume=7 |issue=2 |pages=206–222 |doi=10.1038/s41550-022-01835-4 |arxiv=2212.08477 |bibcode=2023NatAs...7..206P |s2cid=254764810 |url=https://www.nature.com/articles/s41550-022-01835-4 |language=en}}
• LTT 1445 A b but may be too hot and too water-poor.{{cite journal |last1=Phillips |first1=Caprice L |last2=Wang |first2=Ji |last3=Edwards |first3=Billy |last4=Martínez |first4=Romy Rodríguez |last5=Asnodkar |first5=Anusha Pai |last6=Gaudi |first6=B Scott |title=Exploring the potential of Twinkle to unveil the nature of LTT 1445 Ab |journal=Monthly Notices of the Royal Astronomical Society |date=2023 |volume=526 |issue=2 |pages=2251–2264 |doi=10.1093/mnras/stad2822 |doi-access=free |url=https://academic.oup.com/mnras/article/526/2/2251/7276613 |language=en|url-access=subscription }}
• 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{{cite journal |last1=Kawauchi |first1=K. |last2=Murgas |first2=F. |last3=Palle |first3=E. |last4=Narita |first4=N. |last5=Fukui |first5=A. |last6=Hirano |first6=T. |last7=Parviainen |first7=H. |last8=Ishikawa |first8=H. T. |last9=Watanabe |first9=N. |last10=Esparaza-Borges |first10=E. |last11=Kuzuhara |first11=M. |last12=Orell-Miquel |first12=J. |last13=Krishnamurthy |first13=V. |last14=Mori |first14=M. |last15=Kagetani |first15=T. |last16=Zou |first16=Y. |last17=Isogai |first17=K. |last18=Livingston |first18=J. H. |last19=Howell |first19=S. B. |last20=Crouzet |first20=N. |last21=Leon |first21=J. P. de |last22=Kimura |first22=T. |last23=Kodama |first23=T. |last24=Korth |first24=J. |last25=Kurita |first25=S. |last26=Laza-Ramos |first26=A. |last27=Luque |first27=R. |last28=Madrigal-Aguado |first28=A. |last29=Miyakawa |first29=K. |last30=Morello |first30=G. |last31=Nishiumi |first31=T. |last32=Rodríguez |first32=G. E. F. |last33=Sánchez-Benavente |first33=M. |last34=Stangret |first34=M. |last35=Teng |first35=H. |last36=Terada |first36=Y. |last37=Gnilka |first37=C. L. |last38=Guerrero |first38=N. |last39=Harakawa |first39=H. |last40=Hodapp |first40=K. |last41=Hori |first41=Y. |last42=Ikoma |first42=M. |last43=Jacobson |first43=S. |last44=Konishi |first44=M. |last45=Kotani |first45=T. |last46=Kudo |first46=T. |last47=Kurokowa |first47=T. |last48=Kusakabe |first48=N. |last49=Nishikawa |first49=J. |last50=Omiya |first50=M. |last51=Serizawa |first51=T. |last52=Tamura |first52=M. |last53=Ueda |first53=A. |last54=Vievard |first54=S. |title=Validation and atmospheric exploration of the sub-Neptune TOI-2136b around a nearby M3 dwarf |journal=Astronomy & Astrophysics |date=1 October 2022 |volume=666 |pages=A4 |doi=10.1051/0004-6361/202243381 |arxiv=2202.10182 |bibcode=2022A&A...666A...4K |s2cid=247011479 |url=https://www.aanda.org/articles/aa/abs/2022/10/aa43381-22/aa43381-22.html |language=en |issn=0004-6361}}
• 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.

• Hot Neptune
• Exoplanet
• Ocean world

{{reflist|2}}

{{refbegin}}

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{{refend}}

• {{cite web|access-date=2021-08-30|author=University of Cambridge|date=2021-08-30|language=en|title=New class of habitable exoplanets represent a big step forward in the search for life|url=https://phys.org/news/2021-08-class-habitable-exoplanets-big-life.html|website=Phys.org}}
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• {{cite web|access-date=2021-08-30|date=2021-08-26|language=en|title=Astronomers Identify New Class of Exoplanets: Hycean Worlds|url=http://www.sci-news.com/astronomy/hycean-worlds-10004.html|website=Science News}}
• {{cite web|access-date=2021-08-30|first=Liz|last=Cohen|date=2021-08-27|language=en|title=Scientists may find life on Earth-like planets covered in oceans within the next few years|url=https://www.cbsnews.com/news/hycean-exoplanets-extraterrestrial-life-earth-like-planets-oceans/|website=CBS News}}

{{Exoplanet}}

{{Water}}

Category:Types of planet

Category:Hypothetical astronomical objects

Category:Extraterrestrial water

Category:2021 neologisms

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