Scientists find water on extra-solar planet

Scientists have, for the first time, conclusively discovered the presence of water vapour in the atmosphere of a planet beyond our Solar System, according to an article appearing in Nature. They made the discovery by analysing the transit of the gas giant HD 189733b across its star, in the Infrared using data from NASA’s Spitzer Space Telescope.Giovanna Tinetti, ESA fellow at the Institute d’Astrophysique de Paris, and colleagues from around the world, targeted planet HD 189733b, 63 light-years away, in the constellation Vulpecula.The planet was discovered in 2005 as it dimmed the light of its parent star by some three percent when transiting in front of it. Using Spitzer, Tinetti and the team observed the star, which is slightly fainter than the Sun, as its starlight dim at two infrared bands (3.6 and 5.8 micrometres).Had the planet been a rocky body devoid of atmosphere, both these bands and a third one (8 micrometres), recently measured by a team at Harvard, would have shown the same behaviour. Instead, as the atmosphere absorbed less infrared radiation at 3.6 micrometres than at the other two wavelengths indicating the presence of water vapour.“Water is the only molecule that can explain that behaviour,” says Tinetti.The presence of water vapour does not necessarily make it a good candidate in the search for planets that harbour life. “This is a far from habitable world,” she adds.

HD 189733b is a 'hot jupiter', a gas giant that is roughly the size and mass of Jupiter but orbits very close to the star. These planets tend to have extensive atmospheres because heat from the nearby star gives them energy to expand. HD 189733b’s atmospheric temperature is about 1000 Kelvin or higher, implying that the significant amounts of water vapour in the atmosphere cannot condense to fall as rain or form clouds but the temperature is far too hot to produce clouds and rain.That does not mean the atmosphere is sedate as the planet is gripped so tightly by the gravity of its star that one hemisphere constantly faces the star, heating the planet only on one side generating fierce winds sweeping from the day-side to the night-side. “There are a thousand things to learn about these planets,” says Tinetti.