UK space scientists have made a breakthrough in the search for aliens by discovering water for the first time ever on a planet orbiting another star.

They detected the vital substance as steam in the atmosphere of a giant world called HD189733b in the constellation of Vulpecula, the Fox. It is a find of a kind that Nasa predicted earlier this year.

The planet, which is 63 light years away from Earth, is a giant gas ball like Jupiter. It is more than 30 times closer to its own sun than we are to ours and is so hot that there cannot be life as we know it.

But the scientists, led by researchers from University College London, are excited to find that water exists on alien worlds. They used Nasa’s Spitzer space telescope, which has previously checked out an exoplanet’s windy weather, to detect it. Spitzer has also identified a planet as glowing like a cinder.

Professor Keith Mason, head of the UK’s Science and Technology Facilities Council, said: “This first conclusive evidence of the presence of water vapour in the atmosphere of a planet beyond our Solar System provides an exciting breakthrough in our knowledge of extra-solar planets. It represents a real step forward into establishing whether we are alone in the universe or whether there is life on other planets.”

Planet HD 189733b, which is 15 per cent bigger than Jupiter, is known as a transiting planet because it passes directly in front of its star, as viewed from the Earth. Parts of its atmosophere reach 2,000 degrees.

The international team of astronomers discovered the water vapour by analysing the star’s light as it passed through the planet’s atmosphere during the star’s “eclipses”.

Dr Giovanna Tinetti, of UCL, said: “Although HD 189733b is far from being habitable, and is actually quite a hostile environment, our discovery shows how water might be common out there and how our method can be used in the future to study more life-friendly environments.

“The ‘holy grail’ for today’s planet hunters is to find an Earth-like planet that also has water in its atmosphere. That discovery, when it happens, will provide real evidence that planets outside of our Solar System might harbour life. So the discovery that water exists on an extra—solar gas giant is a vital milestone along that road of discovery.”

Picture: An artist’s impression of the planet. Credit: ESA – C. Carreau

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