Water on planet 276 trillion miles away is sign that we may not be alone

The breakthrough by British astronomers also makes it more likely that our descendants will one day be able to find new homes among the stars.

Although past studies have hinted at the presence of water on planets orbiting far distant suns, this is the first conclusive proof.

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It was found in the atmosphere of a planet known as HD 189733b, which orbits a star 64 light years - or 376 trillion miles - away in the constellation of Vulpecula the Fox.

The planet is a huge gas giant like Jupiter or Saturn, rather than a small rocky planet like the Earth. It orbits a star very like our Sun, although a little cooler.

Because its orbit lies just three million miles from its star, it is incredibly hot - with temperatures reaching around 920C (1700F). The Earth, in contrast, lies 93 million miles from the Sun.

The discovery was made using Nasa's Spitzer telescope, which is in orbit around the Earth.

Dr Giovanna Tinetti, of University College London, found that as the planet passed in front of its sun, it absorbed starlight in a way that could only be explained by the presence of steam in its atmosphere.

"Although HD 189733b is far from habitable, and actually provides a rather hostile environment, our discovery shows that water might be more common out there than previously thought," said Dr Tinetti, who reports the discovery in the journal Nature.

Ten years ago, scientists could only speculate about the existence of planets outside the solar system - the collection of planets, asteroids and comets that orbit our Sun.

But thanks to recent advances in astronomy, they have since identified more than 200 planets orbiting stars many light years from the Earth.

Earlier this year, astronomers found the first planet capable of supporting human life, lying in the so-called "Goldilocks zone" around a star, where the temperature for life is not too hot, and not too cold.

"The Holy Grail for today's planet hunters is to find an Earth-like planet that also has water in its atmosphere.

"When it happens, that discovery will provide real evidence that planets outside the solar system might harbour life," said Dr Tinetti.

"Finding the existence of water on an extra-solar gas giant is a vital milestone along that road of discovery."