This has led astronomers to become interested in oceans.

And to the discovery that oceans can be very different to those beneath the open skies of Earth.

On three of the moons of Jupiter,

and two of the moons of Saturn,

there was good evidence of liquid oceans.

The first of these oceans to be discovered was within Jupiter's moon Europa.

Though Europa is small, it may boast more liquid water than the Earth.

The ocean could be over 100 kilometers deep.

Heat generated by Jupiter's powerful tides keeps it warm underneath the icy shell.

Europe's Jupiter Icy Moons Explorer mission, or JUICE, will launch in 2022 to study Jupiter and three of its largest moons.

Perhaps one of the most promising oceans for study is on Enceladus, a tiny moon of Saturn.

This is because the ocean does not stay under the ice cap.

Geysers at the moon's south pole shoot ocean water hundreds of kilometers out into space.

Studies by NASA's Cassini probe showed that

the water in these jets contain organic molecules,

the building blocks of life.

There is also evidence of hydrothermal vents at the bottom of the ocean.

This is significant because it is thought that life first arose at hydrothermal vents on Earth.

A spacecraft flying through the jets of Enceladus

could sample the water for traces of complex

biological molecules such as DNA or protein.

There is no guarantee of life in the waters of Enceladus.

Or in any of the solar system's other hidden oceans.

But the possibility that something might be there will drive ambitious explorations for decades to come.

Coming up, how to discover new planets.