Recent discoveries have led astrobiologists to think that moons are the most promising places for alien life to exist in our Solar System. And now several major space missions are being planned over the next decade to search for hints of life there.

Unlike our neighbouring planets, some of the moons have plenty of liquid water. Jupiter’s moon, Europa, for example, is thought to contain more liquid water than all of Earth's oceans combined. This water – and any life in it – is protected from space radiation and asteroid impacts by a thick layer of kilometers-deep surface ice.

The discovery of plumes of water shooting up from Saturn’s moon Enceladus and Europa have suggested they could have warm interiors that can support liquid oceans, heated not by the Sun, but by an internal dynamo powered by radioactive decay in their cores or by tidal heating generated by the gravitational attraction of the planets that they orbit.

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There is now evidence for water oceans on several moons, including Europa, Enceladus, Callisto and Ganymede. One study published this June estimates that the Enceladus ocean is around one billion years old. Others have suggested it may be billions of years old – plenty of time for life to evolve.

These oceans are thought to be salty, containing sodium chloride, like Earth’s oceans, which is another boost for the prospects of Earth-like life.

Also, there is likely to be an interface between the liquid water and the rocky mantle below the oceans – key ingredients for interesting chemistry that scientists think led to the origins of life on Earth. Nasa’s Cassini mission, for example, detected molecules in Enceladus’ water plumes that hint at the existence of hydrothermal vents on the moon’s ocean floor.