Water found on extra-solar planet
Updated on 12 April 2007
Water has been identified for the first time in the atmosphere of a planet outside the Solar System, it has been revealed.
The discovery increases the chances of life being found among the stars - but probably not on this world. The planet, nicknamed Osiris by astronomers, orbits so close to its parent star that its surface reaches a scorching 1,100C.
Osiris, which is 150 light years from Earth and has the official title HD 209458b, was the first extra-solar world known to have an atmosphere, mainly composed of hydrogen. Scientists have also learned that its atmosphere is boiling away into space at an extraordinary rate.
It was this that earned the planet its unofficial name. Osiris, the Egyptian god of life and death, lost part of his body having been killed and cut into pieces by his brother to prevent him returning to life.
In 2004 astronomers revealed that the planet's outer atmosphere contained carbon and oxygen, thought to be swept up from the deeper levels by the flow of escaping hydrogen. The new discovery of water vapour emerged from measurements taken as Osiris passed directly in front of its parent star.
Because it orbits just 4.3 million miles (seven million kilometres) from the star, Osiris makes one of these "transits" every 3.5 days. Water absorption in the atmosphere affects light reflected by the planet in a way that can be measured from Earth.
By comparing the observations with the predictions made by new theoretical models, the scientists were able to confirm the presence of water.
Dr Travis Barman, who led the US team from the Lovell Observatory in Arizona, said: "We now know that water vapour exists in the atmosphere of one extra-solar planet, and there is good reason to believe that other extra-solar planets contain water vapour."
Osiris, discovered in 1999, is a type of low-orbit extra-solar planet known as a "hot Jupiter". It has almost the same mass as Jupiter, and like that planet is a "gas giant" without a solid surface.
Its parent star lies in the constellation of Pegasus and is bright enough to be seen with binoculars.
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