section_path: Formation
section_name: Formation
paragraph_text: Formation
  
  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 H O and silicates. Those that form farther out can acquire ammonia (NH) and methane (CH) as hydrates, together with CO, N, and CO.
  
  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 in 2003.