Another detection of water on an extrasolar planet

For the second time, astronomers claim to have found water vapour in the atmosphere of a giant planet (a "hot Jupiter") outside our Solar System. The detection in the extrasolar planet HD 189733b was made using NASA's powerful Spitzer Space infrared Telescope and is reported in the journal Nature.

The team looked for the signal of water absorption in starlight poking through the edges of the atmosphere when the planet passed in front of its star. The planet was discovered in 2005 as it dimmed the light of its parent star by three percent when transiting in front of it. Using Spitzer, Tinetti and the team observed the starlight dim at two infrared bands (3.6 and 5.8 micrometers). “Water is the only molecule that can explain that behaviour,” says Tinetti. Had the planet been a rocky planet with no atmosphere, both these bands and a third one (8 micrometres), recently measured by a team at Harvard, would have shown the same behavior.

Instead of a rocky world like Earth, HD 189733b is large, with a mass similar to that 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 and orbits in 365 days. Although water is a key ingredient for biology, the planet is far too hot to harbor life.