The star GJ 436, a diminutive star (red dwarf) 30 light-years from the Sun, was known since 2004 to harbour a 22-Earth mass planet, orbiting 4 million kilometers from the star (0.03 Astronomical Units).
Measurements show that the planet has a diameter of about 50,000 km, four times that of the Earth. From the size and mass of the planet, the astronomers could infer that it is mainly composed of water. If the planet contained mostly hydrogen and helium – like Jupiter or Saturn – it would be much larger, and if it was made up of rock and iron like Earth, Mars and Venus, it would be much smaller.
This water world can either be surrounded by a light envelope of hydrogen and helium, like Neptune and Uranus, or be entirely surrounded by water, like most of Jupiter’s satellites. As the planet is close to its host star, its surface temperature is expected to be at least 300 C (600 F). The water in its atmosphere would therefore be in the form of steam. Inside, the water is crushed under intense pressure and adopts states unknown on Earth, except in physicist’s laboratories. Says Frédéric Pont: “water has more than a dozen solid states, only one of which is our familiar ice. 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. Physicists call these exotic forms of water ‘Ice VII’ and ‘Ice X’.