British astronomers have detected water in the atmosphere of an enormous, fiery planet that circles a distant star far beyond our own solar system.

The discovery raises hopes that the substance considered most vital for life may be ubiquitous throughout the galaxy and wider universe.

The finding, described in Nature today, proves scientists can overcome what has long been thought one of the greatest hurdles in the search for extraterrestrial life - the ability to analyse atmospheres of distant worlds for signs of living organisms.

The planet, a Jupiter-like gas giant, circles a star identified by astronomers as HD189733, some 64 light years from our sun in the constellation of Vulpecula, or "little fox". It is slightly larger than Jupiter - itself more than 11 times wider than Earth - and passes so close to its parent star that surface temperatures soar from 700C to 1,000C when night turns to day.

Astronomers led by Giovanna Tinetti, of University College London, used Nasa's Earth-orbiting Spitzer telescope to watch the planet as it passed directly in front of its star during its 2.2-day orbit. Cameras on the telescope picked up faint changes in starlight passing through the planet's atmosphere. The atmosphere absorbed infrared light at wavelengths that could only be explained by large quantities of water vapour.

"This planet cannot be considered habitable, it's extremely hostile, but the fact that we can see water on an extra-solar planet makes us think we might be able to use the same technique to spot water on other habitable planets that are more life-friendly and more similar to Earth," Dr Tinetti said.

Scientists believe the most likely place to find a "second Earth" - a rocky planet capable of harbouring life as we know it - is in the "Goldilocks zone" of a distant solar system, where planets are neither too hot nor too cold for liquid water to form.

Keith Horne, a planet hunter at St Andrews University, said: "The fact that this gas giant has been found to have water in the atmosphere is exciting. We're very interested in finding where water exists - is it so abundant that it will be present virtually everywhere a planet forms or not?"

He added: "This planet is too hot to have our kind of life on it, but we don't know for sure that very bizarre kinds of life don't exist. Ultimately, we want to know if there's life out there or not. We want to know whether Earth is essentially unique in the galaxy, or whether there are planets close by that may harbour life. It gives us a perspective on how special we are and how special our home is."