Definition
An ocean planet is an exoplanet whose entire surface is covered by deep ocean, with no land showing at all. Earth's oceans are like a thin film of water on the surface, but on an ocean planet a large share of the planet's mass is water, and the ocean can be tens to hundreds of kilometers deep.
A Giant Water World With Nowhere to Stand
Imagine a globe with every continent erased and the whole thing painted blue. On an ocean planet, you look in every direction and see no island at all, just an endless horizon.
Earth's oceans average about 4 km deep, which is like the thin skin of an apple compared to the size of the planet. An ocean planet's water can run tens to hundreds of kilometers deep. Water makes up only 0.02% of Earth's weight, but on an ocean planet it could make up anywhere from 10% to as much as 50% of the weight.
The leading idea is that such a planet formed in the cold outer reaches of space, packed with ice, then drifted inward toward its star, where the ice melted into ocean. From space it looks like a clear, beautiful blue marble. Inside, though, it is a completely different world from Earth.
A Bit More Precisely: Hot Ice at the Bottom of the Sea
To be more precise, if you dive deep enough you never reach the rock or sandy seafloor we know. Hundreds of kilometers of water stack up to produce a pressure well over 10,000 atmospheres, tens to hundreds of times higher than at the bottom of Earth's deepest ocean.
Under such crushing pressure, water molecules get squeezed into an unusual crystal called high-pressure ice. This is not the ordinary ice that freezes because it is cold. Even at temperatures of hundreds of degrees, the intense pressure keeps it from melting, so it stays solid.
So a cross-section of an ocean planet shows a liquid ocean on top, a thick mantle of hot ice wrapped beneath it, and a rocky core only at the very center. Water and rock may be almost cut off from each other. Even so, some studies suggest the slowly churning ice layer might carry small amounts of rock material upward.
A Place Where Life Might Actually Struggle?
With all that water, it might seem like a perfect paradise for alien life. But scientists point out that an ocean planet could actually be a harsh place for life.
One hypothesis says life on Earth began by relying on minerals and nutrients streaming out of rock and hot springs on the seafloor. On an ocean planet, a firm barrier of high-pressure ice covers the bottom and blocks contact with the rock. As a result, the minerals life needs may not reach the water, and the ocean could become a nutrient-starved wasteland.
On top of that, there is no rain wearing down rock and no cycle regulating carbon in the atmosphere, so the planet's climate may be prone to an extreme greenhouse effect or sudden cooling. It looks like a peaceful water paradise, but underneath, the conditions are tough on life.
Common misconceptions
Because an ocean planet has so much water, alien life should thrive there far more easily than on Earth.
The ocean is so deep that high-pressure ice forms on the bottom and blocks contact with the rock. The essential minerals life needs can't get into the water, so the planet may actually be a harsh place for life.
Where you meet it
Jupiter's moon Europa has a huge ocean hidden beneath its icy shell, which makes it an important clue for studying ocean planets.
The exoplanet Gliese 1214 b has long drawn attention as a water-rich candidate, but even James Webb Space Telescope observations are blocked by thick haze, so its atmosphere and interior are still not settled.
In one sentence
An ocean planet is an exoplanet whose whole surface is covered by an ocean hundreds of kilometers deep, with a layer of high-pressure ice forming on the bottom under enormous pressure.