But now, a team led by Michael Gillon of Geneva University in Switzerland have observed the planet transiting its host star using a telescope at the Observatoire Francois-Xavier Bagnoud (OFXB) in Saint-Luc, Switzerland.
They have been able to measure the planet's width, which provides clues to its composition and structure. It turns out to be about 50,000 kilometres wide, roughly four times the width of Earth and about the size of Neptune.
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."
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