section_path: History
section_name: History
paragraph_text: History
  
  Gravitational calculations suggested by the start of 20th century that Europa's compsition was water rich, and Earth ground based observations by Gerard Kuiper revealed 1957 the water ice composition.
  
  Important preliminary theoretical work was carried out prior to the planetary missions of the 1970s. In particular, Lewis showed in 1971 that radioactive decay alone was likely sufficient to produce subsurface oceans in large moons, especially if ammonia (NH) were present. Peale and Cassen figured out in 1979 the important role of tidal heating (aka: tidal flexing) on satellite evolution and structure. The first confirmed detection of an exoplanet was in 1992. Marc Kuchner in 2003 and Alain Léger et al figured in 2004 that a small number of icy planets that form in the region beyond the snow line can migrate inward to ~1 AU, where the outer layers subsequently melt.
  
  The cumulative evidence collected by the Hubble Space Telescope, as well as Pioneer, Galileo, Voyager, Cassini–Huygens, and New Horizons missions, strongly indicate that several outer Solar System bodies harbour internal liquid water oceans under an insulating ice shell. Meanwhile, the Kepler space observatory, launched on March 7, 2009, has discovered thousands of exoplanets, about 50 of them of Earth-size in or near habitable zones.
  
  Planets of many masses, sizes, and orbits have been detected, illustrating not only the variable nature of planet formation but also a subsequent migration through the circumstellar disc from the planet's place of origin. As of 17 April 2025, there are 5,943 confirmed exoplanets in 4,461 planetary systems, with 976 systems having more than one planet.
  
  In June 2020, NASA scientists reported that it is likely that exoplanets with oceans may be common in the Milky Way galaxy, based on mathematical modeling studies.
  
  In August 2022, TOI-1452 b, a super-Earth exoplanet with potential deep oceans that is 99 light-years from Earth, was discovered by the Transiting Exoplanet Survey Satellite.