Back in 2015, the Kepler Space Telescope spotted a strange planet orbiting a red dwarf in the constellation of Leo, some 100 light years from Earth. K2-18b, as it was designated, is about eight times more massive than Earth with twice the radius, making it a type of planet astronomers call a mini-Neptune. It orbits its parent star in just 33 days, placing it firmly within the habitable zone, the region around a star where liquid water can exist.
Precise conditions on such a planet are impossible to determine. But astronomers can place limits on what is possible and what isn’t. And that’s when they realized that K2-18b was far from ordinary.
Until then, astronomers believed all planets fell into the category of gas giant, like Jupiter, Saturn and Neptune, or rocky body, like Earth, Mars and Venus. But K2-18b allowed for another possibility — that it was entirely covered by an ocean of water hundreds of kilometers deep.
