The planet is therefore too compact to be made mostly of hydrogen gas, like Jupiter, the researchers say, but not compact enough to be a rocky 'super Earth', as some had speculated. Instead, they believe it must be made mostly of an exotic form of water.

Although the parent star is much cooler than the Sun, the planet orbits 13 times closer to the star than Mercury's orbit around the Sun. That means the surface must be a blazing hot 300° C or more, keeping water in its atmosphere in vapour form.

But the high pressures in the planet's interior would compress the water so much that it would stay solid even at hundreds of degrees Celsius -- the expected temperatures inside the planet. There are a variety of exotic 'hot ice' states possible in such conditions, with names like 'Ice VII' and 'Ice X'.

"Water has more than a dozen solid states, only one of which is our familiar ice," says team member Frederic Pont of Geneva University. "Under very high pressure, water turns into other solid states denser than both ice and liquid water, just as carbon transforms into diamond under extreme pressures."

Saturn's gravity causes ice on its moon Enceladus to grind together, generating the plumes of ice crystals and water vapour seen in recent years by the Cassini spacecraft, new calculations suggest.

The findings suggest that any liquid ocean on the moon may be buried beneath an icy shell several kilometres thick, making it difficult to ever retrieve a liquid sample that could be tested for life.