Ocean Planets

An ocean planet is a class of planet whose entire surface is covered by an ocean of liquid water that is much deeper than the oceans on Earth. Up to half or more of the mass of an ocean planet can be in the form of water. In comparison, only 0.02 percent of the Earth’s mass is made up of water. This gives an ocean planet a lower bulk density than a rocky planet like the Earth, resulting in a larger diameter for a given mass in comparison to a rocky planet. A planet such as the Earth which formed close to its parent star tends to acquire much lower water content due to the scarcity of volatiles at close distances. In order to have such an enormous amount of water, an ocean planet will have to form in the cooler outer regions of the protoplanetary disk where it can acquire a water-rich cometary-like bulk composition. The planet subsequently migrates inwards into the habitable zone where it is warm enough for an ocean of liquid water to exist on the planet’s surface for it to become an ocean planet.

For a bulk composition by mass comprising of 1/2 water, 1/3 silicates and 1/6 metals, a 6 Earth-mass ocean planet will have 2 times the Earth’s diameter and 1.54 times the Earth’s surface gravity. In comparison, a rocky planet of the same mass with an Earth-like bulk composition by mass of 2/3 silicates and 1/3 metals will have 1.63 times the Earth’s diameter and 2.24 times the Earth’s surface gravity. With the given bulk composition, this 6 Earth-mass ocean planet is expected to have an interior structure which comprises of a thick water layer extending to a depth of 4800 km, followed by a silicate mantle from 4800 km to 8400 km and a metallic core from 8400 km down to the planet’s centre at 12800 km.

Only the uppermost portion of the thick water layer of this 6 Earth-mass ocean planet can exist as a liquid water ocean with thousands of kilometres of high pressure ice separating it from the silicate mantle beneath. This is due to the fact that at a certain depth beneath the surface, hydrostatic pressure becomes large enough for a high pressure phase of ice known as ice VI to exist. As a result, it is reasonable to consider how deep this liquid water ocean may be. Assuming an isothermal profile and surface temperatures of 0, 7 and 30 degrees Centigrade, the resulting ocean depths are estimated to be 40, 45 and 65 km respectively. Assuming an adiabatic profile and surface temperatures of 0, 7 and 30 degrees Centigrade, the resulting ocean depths are estimated to be 60, 72 and 133 km respectively. For both isothermal and adiabatic cases, a higher ocean surface temperature corresponds to a larger ocean depth. In reality, the ocean depth for a given surface temperature will be somewhere between the limits defined by the isothermal and adiabatic cases.

On an ocean planet, thousands of kilometres of high pressure solid ice separates the surface ocean from the silicate mantle. This inhibits interaction between liquid water and silicates which limits the availability of elements necessary for life (iron, magnesium, potassium, sodium, etc). However, such elements can still be delivered to the ocean by micrometeorites or be already present as dissolved material in the ocean. Although ocean planets have no analogues in the Solar System, searches for planets around other stars have revealed a number of planets that may turn out to be ocean planets.