Hycean planet

A hycean planet (portmanteau of hydrogen and ocean)[1] is a hypothetical type of planet, described as a hot, water-covered planet with a hydrogen atmosphere. The presence of extraterrestrial liquid water makes them promising candidates for planetary habitability.[2][3][4] According to researchers, density data imply that both rocky Super-Earths and Sub-Neptunes (like K2-18b and TOI-1231 b)[1][4] can fit this type, and it is thus expected that they will be common exoplanets.[3] Currently there are no confirmed hycean planets, but the Kepler mission detected many candidates.[2]

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⊕)”.[3] 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 .[3]

Additionally, there could be tidally locked 'Dark Hycean' planets (habitable only on the side of permanent night) or 'Cold Hycean' planets (with negligible irradiation).[3] Hycean worlds could be soon investigated for biosignatures by terrestrial telescopes and space telescopes like the James Webb Space Telescope, which was launched at the end of 2021.[3][5]

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 heavier elements 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.[2]