Ocean world

An ocean world, ocean planet or water world is a planet or natural satellite with a lot of water. This water can be in oceans on the surface or deep underneath, as subsurface oceans. Sometimes, the water might not even be normal water! It could be something like lava (like on Io), or a mix of water and ammonia (like on Titan). Scientists study these faraway oceans in a field called planetary oceanography.

Earth is the only place we know that has oceans of liquid water on its surface right now. But we think there might be hidden oceans inside some of Jupiter's moons, like Europa and Ganymede, and Saturn's moons, like Enceladus and Titan. Scientists have also found distant planets, called exoplanets, that might have the right conditions for liquid water. On Earth, there is a lot of water stored underground in places called aquifers. For faraway planets, we can't see the water directly, so we look for signs of water vapor in their atmospheres.

Ocean worlds help us learn about how planets form and change over time, and they might even be places where life could exist. In 2020, scientists from NASA said that planets with oceans might be very common in our Milky Way galaxy. They figured this out using mathematical modeling studies.

Overview

An ocean world is a planet or moon with a lot of water, either on its surface or hidden underneath. Scientists study these places because they might be able to support life. In our Solar System, some well-known ocean worlds are Callisto, Enceladus, Europa, Ganymede, and Titan. We can explore these worlds with special space machines called space probes.

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

History

Early scientists found that some moons, like Europa, might have water. They learned that heat from rocks and the pull of gravity could make 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 far from our solar system. Some of these might have oceans too. These discoveries show how common water worlds might be in space.

Formation

Planetary objects that form in the outer Solar System start as a mix of water and rock. They are less dense than rocky planets. Icy planets and moons that form near the frost line are mostly made of water and silicates. Those that form even farther out can also contain ammonia, methane, and 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 from the mantle can create a steam atmosphere that becomes an ocean. Ocean formation needs a heat source like radioactive decay or tidal heating. Planets that move from outer, water-rich areas closer to their star are more likely to have lots of water. This movement can change icy planets into ocean planets.

Structure

Scientists study icy moons and planets by looking at their density, gravity, and shape. These help scientists find out if the body has layers or if it acts like a liquid over time. There are many ways to find hidden oceans, like looking at their magnetic fields, how they move, and their surface.

Icy moons usually have water above a rocky core. Smaller moons, like Enceladus, have an ocean right above the rock, with ice on top. Bigger moons, like Ganymede, have layers of different kinds of ice with an ocean between them. The heat in a hidden ocean and how fast it loses heat decide if the ocean stays liquid. Smaller ocean worlds might stay warm with heat from radioactive decay or tidal forces. Some scientists think that cryovolcanism, where water comes out through the ice, might happen on these worlds, like on Enceladus and Europa in our solar system. Oceans on planets beyond our solar system could be much deeper than Earth's oceans, maybe even 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 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 atmospheres but still keep their water for billions of years. 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 exciting 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, scientists have 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.