Something in the air

Scientists say they have the first conclusive evidence of the presence of water vapour in the atmosphere of a planet beyond our Solar System.

An artist’s impression of the planet HD 189733b. Scientists have reported the first conclusive discovery of the presence of water vapour in the atmosphere. (Credit: ESA - C.Carreau)

Giovanna Tinetti, ESA fellow at the Institute d’Astrophysique de Paris, and colleagues from around the world, used data from NASA’s Spitzer Space Telescope to analyse the transit of the gas giant HD 189733b across its star.

The planet was discovered in 2005 as it dimmed the light of its parent star by 3% while passing in front of it. There is nothing particularly unusual about that, except that the starlight absorbed showed a distinct pattern - the planet’s atmosphere absorbed less infrared radiation at 3.6 micrometres than would be expected for a planet devoid of atmosphere.

“Water is the only molecule that can explain that behaviour,” said Tinetti.
However, those extraterrestrial life hunters hoping to strike it lucky on the planet should think again. “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,” said Tinetti.

Instead of a rocky world like Earth - HD 189733b is large, about 1.15 times the mass of Jupiter. Located just 4.5 million km from its star, it orbits it in 2.2 days. In comparison, Earth is 150 million km from the Sun. Even Mercury, the innermost planet, is 70 million km away.

HD 189733b’s atmospheric temperature is about 1000 Kelvin (a little more than 700°C) or higher, implying that the significant amounts of water vapour in the atmosphere cannot condense to fall as rain or form clouds. The temperature would have to be about five times lower to form clouds of water vapour or rain.

That does not mean the atmosphere is sedate, however. 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. This probably generates fierce winds sweeping from the day-side to the night-side. “There are a thousand things to learn about these planets,” says Tinetti.