A team of European astronomers led by Michaël Gillon, a researcher from Liege University, has measured the transit of a Neptune-sized planet around another star. For the first time, the size and density of such a small extra-solar planet has been measured, showing that this planet is made up mainly of water.
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). Observations from the OFXB observatory in St-Luc, Switzerland, showed a periodic dimming of the star due to the passage of the planet in front of it. This event, called a transit, was subsequently confirmed with telescopes at the Wise Observatory in Israel, then precisely measured with the Euler telescope of Geneva University Observatory in Chile.
These 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. Michaël Gillon says: "This discovery is an important step towards the detection and study of Earth-like planets."
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'. If Earth’s oceans were much deeper, there would be such exotic forms of solid water at the bottom." Inside GJ 436’s planet, this strange ice is moreover heated to many hundred degrees.
The detection of such a « hot ice world » has important consequences. It shows for the first time that planets similar to the "ice giants" Uranus and Neptune in our own Solar System exist at close distances from their star (the planet of GJ436 orbits every 2.6 days). Many of the planets of similar mass detected around other stars by the astronomers may therefore also be composed mainly of water. Some of them will have cooler temperatures, allowing the water on the surface to be liquid. Such planets covered by a single huge ocean have been dubbed "ocean planets" by the specialists.
"The Corot satellite, which started operating at the beginning of this year, has among its main objectives to measure the size of planets like the one we just measured around GJ436, and even smaller. The Corot mission, to which astronomers from Geneva University are active participants, will allow a closer study of ocean planets and telluric planets like our own," says Didier Queloz.
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