English: A hycean planet is a hypothetical type of planet, described as a hot, water-covered planet with a hydrogen atmosphere. The presence of extraterrestrial liquid water makes Hycean planets promising candidates for planetary habitability. According to researchers, density data imply that both rocky Super-Earths and Sub-Neptunes can fit this type, and it is thus expected that they will be common exoplanets. Currently there are no confirmed hycean planets, but the Kepler mission detected many candidates.Hycean planets could be considerably larger than what habitable planets were previously thought to be, with radii reaching 2.6 R⊕ (2.3 R⊕) and masses of 10 M⊕ (5 M⊕). Moreover, the habitable zone of such planets could be considerably larger than that of Earth-like planets. The planetary equilibrium temperature can reach 500 K (227 °C; 440 °F) at late M-dwarfs.There could also exist tidally locked 'Dark Hycean' planets (habitable only on the side of permanent night) or 'Cold Hycean' planets (with negligible irradiation). Hycean worlds could be soon investigated for biosignatures by terrestrial telescopes and space telescopes like the James Webb Space Telescope.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.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 the 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.Hycean planets have hydrogen-rich atmospheres. The atmospheres on Hycean planets are thought to be made up of hydrogen, helium, and water vapor.They are thought to be covered in oceans. The oceans on Hycean planets are thought to be much deeper than the oceans on Earth.They are 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 necessary for life, including liquid water, energy, and organic molecules.The discovery of Hycean planets may represent a new frontier in the search for life beyond Earth. These planets are thought to be very different from Earth, but they could still be home to forms of life. Astronomers plan to use telescopes like the James Webb Space Telescope to search for Hycean planets and to learn more about their potential for human habitability.One such candidate planet is K2-18b, which orbits a faint star with a period of about 33 days. It could have liquid water, contains a considerable high amount of hydrogen gas in its atmosphere, and is far enough from its star. It clearly resides in its star’s habitable zone. It is discovered to contain water in its atmosphere. Such candidate planets can be studied for biomarkers.