section_path: Overview/Exoplanets
section_name: Exoplanets
paragraph_text: Exoplanets
  
  Outside the Solar System, exoplanets that have been described as candidate ocean worlds include GJ 1214 b, Kepler-22b, Kepler-62e, Kepler-62f, and the planets of Kepler-11 and TRAPPIST-1.
  
  More recently, the exoplanets TOI-1452 b, Kepler-138c, and Kepler-138d have been found to have densities consistent with large fractions of their mass being composed of water. Additionally, models of the massive rocky planet LHS 1140 b suggest its surface may be covered in a deep ocean.
  
  Although 70.8% of all Earth 's surface is covered in water, water accounts for only 0.05% of Earth's mass. An extraterrestrial ocean could be so deep and dense that even at high temperatures the pressure would turn the water into ice. The immense pressures of many thousands of bar in the lower regions of such oceans, could lead to the formation of a mantle of exotic forms of ice such as ice V. This ice would not necessarily be as cold as conventional ice. If the planet is close enough to its star that the water reaches its boiling point, the water will become supercritical and lack a well-defined surface. Even on cooler water-dominated planets, the atmosphere can be much thicker than that of Earth, and composed largely of water vapor, producing a very strong greenhouse effect. Such planets would have to be small enough not to be able to retain a thick envelope of hydrogen and helium, or be close enough to their primary star to be stripped of these light elements. Otherwise, they would form a warmer version of an ice giant instead, like Uranus and Neptune.