Hycean planet

A hycean planet (/ˈhaɪʃən/ HY-shən) is a hypothetical type of planet with liquid water oceans underneath a hydrogen-rich atmosphere. They are an abundant type of exoplanet and are thought to offer conditions that are favorable to life.^[1]

Artist depiction of a hycean planet

Definition

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.^[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]

History

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.^[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 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, and in 2024 found even stronger evidence (see the K2-18b candidate section below).

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_🜨 (5 M_🜨).^[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 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.^[2] Another limiting factor is that X-ray and UV radiation from the star (especially active stars) can destroy the water molecules.^[11]

Another factor limiting the habitable zone of hycean planets is their orbital eccentricity. Planets with moderately eccentric orbits have their surface temperatures increase from tidal heating from their parent stars. 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.^[14] 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”.^[15]

Features

• They are regarded to be covered in oceans and seas.^[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.^[16]
• 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.^[17]

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.^[18]

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.^[19][20] 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,^[21] but strong evidence of dimethyl sulfide as well as dimethyl disulfide was found in 2025.^[22] Another possibility is that K2-18b is a lava world with a hydrogen atmosphere.^[23]

Other candidates

• K2-3b, a potential Dark hycean planet^[10] but may be too hot.^[24]
• K2-3c^[10] but may be too hot.^[24]
• Kepler-138d^[25]
• LTT 1445 A b^[10] but may be too hot^[24] and too water-poor.^[26]
• TOI-732 c^[10] but may be too hot.^[24]
• TOI-1266 c^[10] but may be too hot.^[24]
• TOI-175 d^[10] but may be too hot.^[24]
• TOI-2136 b^[27]
• TOI-270 c, a potential Dark hycean planet^[10] but may be too hot.^[24]
• TOI-270 d^[10] but may be too hot.^[24]
• TOI-776 b, a potential Dark hycean planet^[10] but may be too hot.^[24]
• TOI-776 c^[10] but may be too hot.^[24]

See also

• Hot Neptune
• Exoplanet
• Ocean world