Scientists have reported the first "conclusive discovery" of water vapour in the atmosphere of a planet beyond our solar system.

The discovery was made by analysing the transit of the gas giant HD 189733b across its star in the infrared spectrum. HD 189733b is 63 light-years away from Earth in the Vulpecula constellation.

Giovanna Tinetti, European Space Agency fellow at the Institute d'Astrophysique de Paris, and colleagues from around the world used data from Nasa's Spitzer Space Telescope.

HD 189733b was discovered in 2005 as it dimmed the light of its parent star by some three per cent when transiting in front of it.

Using Spitzer, Tinetti and the team observed the star, which is slightly fainter than the Sun. They watched 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 planet's tenuous outer atmosphere slipped across the face of the star, the absorbed starlight showed a different, distinctive pattern.

The atmosphere absorbed less infrared radiation at 3.6 micrometres than at the other two wavelengths. "Water is the only molecule that can explain that behaviour," said Tinetti.

However, the presence of water vapour does not necessarily make HD 189733b a good candidate in the search for life. "This is a far from habitable world," she said.

HD 189733b is about 1.15 times the mass of Jupiter. Located 4.5 million kilometres from its star, the planet completes an orbit in 2.2 days.

Earth is 150 million kilometres from the Sun, and Mercury, the innermost planet, is 70 million kilometres away.