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Water is vital to life on Earth and probably to life elsewhere.
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This has led astronomers to become interested in oceans.
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And to the discovery that oceans can be very different to those beneath the open skies of Earth.
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On three of the moons of Jupiter,
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and two of the moons of Saturn,
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there was good evidence of liquid oceans.
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The first of these oceans to be discovered was within Jupiter's moon Europa.
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Though Europa is small, it may boast more liquid water than the Earth.
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The ocean could be over 100 kilometers deep.
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Heat generated by Jupiter's powerful tides keeps it warm underneath the icy shell.
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Europe's Jupiter Icy Moons Explorer mission, or JUICE, will launch in 2022 to study Jupiter and three of its largest moons.
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Perhaps one of the most promising oceans for study is on Enceladus, a tiny moon of Saturn.
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This is because the ocean does not stay under the ice cap.
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Geysers at the moon's south pole shoot ocean water hundreds of kilometers out into space.
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Studies by NASA's Cassini probe showed that
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the water in these jets contain organic molecules,
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the building blocks of life.
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There is also evidence of hydrothermal vents at the bottom of the ocean.
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This is significant because it is thought that life first arose at hydrothermal vents on Earth.
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A spacecraft flying through the jets of Enceladus
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could sample the water for traces of complex
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biological molecules such as DNA or protein.
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There is no guarantee of life in the waters of Enceladus.
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Or in any of the solar system's other hidden oceans.
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But the possibility that something might be there will drive ambitious explorations for decades to come.
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Coming up, how to discover new planets.
