Ocean planet

An ocean planet, ocean world, water world, aquaplanet or panthalassic planet is a type of terrestrial planet that contains a substantial amount of water either at its surface or subsurface. The term ocean world is also used sometimes for astronomical bodies with an ocean composed of a different fluid, such as lava (the case of Io), ammonia (the case of Titan's inner ocean) or ethane (which could be the most abundant kind of exosea).Astrobiology mission concepts to water worlds in the outer Solar System: An ocean planet, ocean world, water world, aquaplanet or panthalassic planet is a type of terrestrial planet that contains a substantial amount of water either at its surface or subsurface. The term ocean world is also used sometimes for astronomical bodies with an ocean composed of a different fluid, such as lava (the case of Io), ammonia (the case of Titan's inner ocean) or ethane (which could be the most abundant kind of exosea). Earth is the only astronomical object known to have bodies of liquid water on its surface, although several exoplanets have been found with the right conditions to support liquid water. For exoplanets, current technology cannot directly observe liquid surface water, so atmospheric water vapormay be used as a proxy. The characteristics of ocean worlds—or ocean planets—provide clues to their history and the formation and evolution of the Solar System as a whole. Of additional interest is their potential to originate and host life. Water worlds are of extreme interest to astrobiologists for their potential to develop life and sustain biological activity over geological timescales. The five best established water worlds in the Solar System include Europa, Enceladus, Ganymede, and Callisto. A host of other bodies in the outer Solar System are inferred by a single type of observation or by theoretical modeling to have subsurface oceans, and these include: Dione, Pluto, Triton, and Ceres, as well as Mimas, Eris, and Oberon. Important preliminary theoretical work was carried prior to the planetary missions launched starting in 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 (NH3) was 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. 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 in March 7, 2009, has discovered thousands of exoplanets, about 50 of them of Earth-size in or near habitable zones. Planets of almost all 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 1 August 2019, there are 4,103 confirmed planets in 3,056 systems, with 665 systems having more than one planet. Planetary objects that form in the outer Solar System begin as a comet-like mixture of roughly half water and half rock by mass, displaying a density lower than that of rocky planets. Icy planets and moons that form near the frost line should contain mostly H2O and silicates. Those that form farther out can acquire ammonia (NH3) and methane (CH4) as hydrates, together with CO, N2, and CO2. Planets that form prior to the dissipation of the gaseous circumstellar disk experience strong torques that can induce rapid inward migration into the habitable zone, especially for planets in the terrestrial mass range. Since water is highly soluble in magma, a large fraction of the planet's water content will initially be trapped in the mantle. As the planet cools and the mantle begins to solidify from the bottom up, large amounts of water (between 60% and 99% of the total amount in the mantle) are exsolved to form a steam atmosphere, which may eventually condense to form an ocean. Ocean formation requires differentiation, and a heat source, either radioactive decay, tidal heating, or the early luminosity of the parent body. Unfortunately, the initial conditions following accretion are theoretically incomplete. Planets that formed in the outer, water-rich regions of a disk and migrated inward are more likely to have abundant water. Conversely, planets that formed close to their host stars are less likely to have water because the primordial disks of gas and dust are thought to have hot and dry inner regions. So if a water world is found close to a star, it would be strong evidence for migration and ex situ formation, because insufficient volatiles exist near the star for in situ formation. Simulations of Solar System formation and of extra-solar system formation have shown that planets are likely to migrate inward (i.e., toward the star) as they form. Outward migration may also occur under particular conditions. Inward migration presents the possibility that icy planets could move to orbits where their ice melts into liquid form, turning them into ocean planets. This possibility was first discussed in the astronomical literature by Marc Kuchner and Alain Léger in 2004.