page_title: Ocean world
section_title: Formation
hierarchical_section_title: Ocean world / Formation
caption_attribution_description: English: ALMA image of the protoplanetary disc around HL Tauri - This is the sharpest image ever taken by ALMA — sharper than is routinely achieved in visible light with the NASA/ESA Hubble Space Telescope. It shows the protoplanetary disc surrounding the young star HL Tauri. These new ALMA observations reveal substructures within the disc that have never been seen before and even show the possible positions of planets forming in the dark patches within the system. About the Object Name: HL Tauri Type: • Milky Way&#160;: Star&#160;: Circumstellar Material&#160;: Disk&#160;: Protoplanetary • X - Stars Distance: 450 light years Constellation: Taurus Coordinates Position (RA): 4 31 38.37 Position (Dec): 18° 14' 0.99" Field of view: 0.03 x 0.03 arcminutes Orientation: North is 0.0° left of vertical .
context_page_description: An ocean world, ocean planet, water world, aquaplanet or panthalassic planet is a type of terrestrial planet that contains a substantial amount of water either at its surface or within a subsurface ocean. The term ocean world is also used sometimes for astronomical bodies with an ocean composed of a different fluid, such as lava, ammonia or hydrocarbons like on Titan's surface.
  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 vapor
  may 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.
  In June 2020, NASA scientists reported that it's likely that exoplanets with oceans may be common in the Milky Way galaxy, based on mathematical modeling studies.
context_section_description: 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
  ₂O and silicates. Those that form farther out can acquire ammonia (NH
  ₃) and methane (CH
  ₄) as hydrates, together with CO, N
  ₂, and CO
  ₂.
  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.
caption: Atacama Large Millimeter Array image of HL Tauri, a protoplanetary disk
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