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. 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 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. 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. Candidate hycean worlds such as K2-18b have been investigated for biosignatures by terrestrial telescopes and space telescopes like the James Webb Space Telescope (JWST). Whether Hycean planets can actually form with the required quantities of water is controversial. ==Properties==

Hycean planets could be considerably larger than previous estimates for habitable planets, with radii reaching () and masses of (). 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. or "cold hycean" planets (with negligible irradiation, being kept warm by the greenhouse effect). 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. Previous models of an exoplanet's 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”. == Atmosphere ==

The atmospheric conditions are an important factor when determining the habitability of a planet. Hycean planets mainly contain H2 in their atmospheres and any remaining concentration of molecules is largely determined by various factors such as temperature, solar insolation and more. Atmospheric conditions can vary depending on the specific amount of clouds and haze for each planet as that changes the albedo and can affect the planet's ability to retain liquid water. Having too much albedo can make the planet too hot for water and having too little can make it too cold. The main additional compounds that could be found are: H2O, due to the potential ocean, CH4, NH4, CO, CO2, although H2 and H2O have the highest abundance by definition. The main way that Hycean worlds are being detected is through the transit method. Hycean worlds are favorable for this method because they are typically super Earths or Sub Neptunes which have a larger diameter and are therefore easier to detect compared to smaller planets. The transit method can also be used to detect elements or even biosignatures in the atmospheres of the planets. Specifically, James Webb Space Telescope (JWST) can be used to gather spectral data and use spectroscopy to see abundances of elements in a planet's atmosphere. == Features ==

• They are regarded to be covered in oceans and seas. • 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, and organic molecules. Hycean planets may be capable of supporting extraterrestrial life, despite their properties differing drastically from Earth's. Astronomers 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. The presence of this compound was yet to be confirmed at the time, but evidence of dimethyl sulfide as well as dimethyl disulfide was found in 2025. Another possibility is that K2-18b is a lava world with a hydrogen atmosphere. K2-3b Exoplanet K2-3B is a potential candidate for a dark Hycean World. It was discovered by the Kepler space telescope using the transit method in 2015. It is orbiting an M class along with a 10.1 day period with three other exoplanets. K2-3B is classified as a Super Earth with a mass of 5.11 Earth Masses. It is believed that K2-3B is too close to its star to be within the habitable zone, although that property may not be detrimental to its overall habitability due its atmospheric retention combined with a greenhouse effect from the hydrogen concentration in its atmosphere. It is estimated that K2-3B has a density anywhere from 3.11 to 3.70 g/cm3 making it less dense than the Earth which signifies less rock and possibly water on its surface or water vapor in the atmosphere. Minimal information is known about this candidate planet and more is needed to make any concrete conclusion. Kepler-138d Candidate planet Kepler-138d is very similar to K2-3B. It is also orbiting an M type star but has a longer orbital period of around 23.1 Earth days. The Kepler-138 system consists of three other exoplanets, none of which, including Kepler-138d, are within the habitable zone of their star. Similar to exoplanet K2-3b, its atmosphere may be retained enough to keep heat and allow liquid water to exist Despite the disagreement, these numbers still make Kepler-138d less dense than earth with 5.5g/cm3, making it a potential Hycean world. LTT 1445 A b The potential hycean world LTT 1445 A b was discovered and confirmed by Transiting Exoplanet Survey Satellite using the transit method and the radial velocity method. It is orbiting a M type star and is not believed to be within the habitable zone because of its orbital period of 5.4 Earth days. Similar to other hycean planets outside of the habitable zone, LTT 1445 A b’s atmosphere may regulate its heat through the greenhouse effect and may still be considered habitable. Earth’s density is 5.5g/cm3 and a rocky planet, meaning that LTT 1445 A b may be more dense than Earth meaning it may not have as much water as previously thought. Other candidates • K2-3c • TOI-732 c • 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==