Ocean world

An ocean world, ocean planet or water world is a type of planet or natural satellite that contains a substantial amount of water in the form of oceans, as part of its hydrosphere, either beneath the surface, as subsurface oceans, or on the surface, potentially submerging all dry land. The term ocean world is also used sometimes for astronomical bodies with an ocean composed of a different fluid or thalassogen, such as lava (the case of Io), ammonia (in a eutectic mixture with water, as is likely the case of Titan's inner ocean) or hydrocarbons (like on Titan's surface, which could be the most abundant kind of exosea). The study of extraterrestrial oceans is referred to as planetary oceanography.

Earth is the only astronomical object known to presently have bodies of liquid water on its surface, although subsurface oceans are suspected to exist on Jupiter's moons Europa and Ganymede and Saturn's moons Enceladus and Titan. Several exoplanets have been found with the right conditions to support liquid water. There are also considerable amounts of subsurface water found on Earth, mostly in the form of aquifers. 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 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 is likely that exoplanets with oceans are common in the Milky Way galaxy, based on mathematical modeling studies.

Overview

An ocean world is a planet or moon that has a lot of water, either on its surface or hidden underneath. Scientists are very interested in studying these places because they might be able to support life. In our Solar System, some of the best-known ocean worlds are Callisto, Enceladus, Europa, Ganymede, and Titan. These worlds can be explored with special space machines called space probes, unlike faraway planets around other stars.

Outside our Solar System, scientists have found several planets that might also be ocean worlds, such as GJ 1214 b, Kepler-22b, and TRAPPIST-1. These distant planets could have deep oceans that might look very different from Earth's oceans due to extreme pressures and temperatures.

History

Early scientists discovered that some moons, like Europa, might contain water. They learned that heat from radioactive materials and the pull of gravity could create hidden oceans inside these icy worlds.

Later, space telescopes and probes like the Hubble Space Telescope, Pioneer, Galileo, Voyager, Cassini–Huygens, New Horizons, and the Kepler space observatory found many planets outside our solar system, some of which might have oceans. These discoveries show just how common water worlds could be in space.

Formation

Planetary objects that form in the outer Solar System start as a mixture of water and rock, with a density lower than rocky planets. Icy planets and moons forming near the frost line mostly contain water and silicates. Those forming farther out can include ammonia and methane as well as other gases.

Planets that form before the gaseous circumstellar disk disappears can move quickly into the habitable zone. Because water mixes easily with magma, much of a planet’s water gets trapped inside. As the planet cools, water released from the mantle can form a steam atmosphere that turns into an ocean. Ocean formation needs a heat source like radioactive decay or tidal heating. Planets that move from outer, water-rich areas to closer to their star are more likely to have lots of water. This movement could turn icy planets into ocean planets.

Structure

Scientists study the inside of icy moons and planets by measuring their density, gravity, and shape. These measurements help determine if the body has separated into different layers or if it behaves like a fluid over long periods. There are several methods to detect hidden oceans inside these bodies, including studying their magnetic fields, movements, and surface features.

Icy moons usually have a layer of water above a rocky core. Smaller moons, like Enceladus, have an ocean directly above the rock, covered by a solid layer of ice. Larger moons, like Ganymede, have layers of different forms of ice with an ocean in between. The amount of heat a subsurface ocean retains compared to how quickly it loses heat affects whether the ocean stays liquid. Smaller ocean worlds might keep their oceans warm through heat from radioactive decay or tidal forces. Some scientists think that cryovolcanism, where water erupts through the ice, might happen on these worlds, as seen on Enceladus and Europa in our solar system. Oceans on planets outside our solar system could be much deeper than Earth's oceans, possibly reaching hundreds of kilometers deep, depending on the planet's size and conditions.

Further information: Super-dense water

Atmospheric models

Further information: Extraterrestrial atmosphere

For a planet or moon to have liquid water on its surface for a long time, it needs to be in the right place around its star, called the habitable zone. It also needs a protective magnetic field to shield its atmosphere from harmful space winds and enough gravity to keep its atmosphere from floating away into space.

The atmosphere of a planet forms when it is young, either from gases released from inside the planet or from gases captured from nearby space. The temperature on the planet’s surface depends on the greenhouse gases in its atmosphere. These gases trap heat from the star, making the planet warmer. Some planets close to their stars can have thick, steamy atmospheres but still keep their water for billions of years. However, harmful radiation from the star can cause the atmosphere to escape into space, leading to the loss of water. Scientists study these conditions to understand where and how water might exist on other planets.

Oceanography

Some worlds far from Earth, like Enceladus, Titan, Ganymede, and Europa, might have strong ocean currents. On Enceladus, warm water may rise at the poles while colder water sinks near the equator. Europa could also have warm water rising near its equator. Scientists think Titan and Ganymede might not have regular patterns for moving heat.

Astrobiology

Further information: Astrobiology, Planetary habitability, and Circumstellar habitable zone

Ocean worlds are very interesting to scientists because they might be able to support life. Life as we know it needs liquid water, energy, and nutrients—all of which could be found on some of these planets. For example, in August 2018, scientists said that water worlds might be able to support life.

However, if an ocean world is completely covered in water, it might be hard for life like that on Earth to live there. If there is a thick layer of ice between the water and the rocky inside of the planet, it could be even harder. Some small bodies, like Europa and Enceladus, are thought to be good places for life because their oceans might touch the rocky center, which could provide heat and important chemicals.

Molecular oxygen (O₂) can be made by natural processes or by living things through photosynthesis. But having a lot of oxygen does not always mean a planet can support life, because early life might have trouble living in such conditions.