Space scientists have spotted water for the first time in the atmosphere of another planet deep in the galaxy. The Hubble space telescope detected the crucial ingredient for life as steam on a planet in the constellation of Pegasus.

Regular observations were made of the giant Jupiter-like world as it passed in front of its parent star which lies 150 light-years away and resembles our own Sun.

During these transits every three and a half days, the planet appeared to swell in infrared light – a clear indication of the presence of water.

Astronomers say the discovery suggests water is a common ingredient of planets’ atmospheres throughout the Milky Way. Around 200 of the alien worlds have so far been discovered outside our own solar system.

Astronomer Travis Barman, of the Lowell Observatory in Arizona, made the discovery which was confirmed by computer studies carried out by Heather Knutson at Harvard University. He said: “We know water vapour exists in the atmospheres of one extrasolar planet and there is good reason to believe that other extrasolar planets contain water vapour.”

The planet, called HD209458b and nicknamed Osiris, is termed a hot Jupiter because it lies only four million miles from its home sun. Life as we know it could not live there but the discovery boosts the chances that smaller planets around such stars might be home to ET. The discovery will be published in the Astrophysical Journal.

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