European astronomers said on Friday they had devised a technique to detect water in the atmosphere of planets orbiting other stars.

Using a telescope in Chile, they teased out a tell-tale infra-red signature from water in the atmosphere of a gassy planet called HD 189733b, which orbits its star every two days and is hot enough to melt steel.

The work is another technical breakthrough in the fast-moving field of exoplanet research, they said.

A total of 723 planets have been spotted outside our own Solar System since 1995, according to the website http://exoplanets.org, while NASA (http://planetquest.jpl.nasa.gov/) puts the tally at 879.

Several thousand findings by the specialist Kepler orbital telescope await confirmation.

So far, no exoplanet spotted has the potential to be a home away from home for us humans.

It would have to be a rocky planet, rather than a gas one, orbiting in a balmy zone which would enable water to exist in liquid form and thus nurture life as we know it.

The new technique should aid the search, as it can be used by big telescopes on the ground as well as more expensive ones in orbit, said Jayne Birkby who led a team from Leiden University in the Netherlands.

"We knew our technique worked for simple molecules at shorter wavelengths, but in order to hunt for water, we had to go to longer wavelengths where the Earth's atmosphere really starts to obstruct the signals we are looking for, so we weren't sure we would find anything," she said in a press release.

"Of course we were delighted when we saw the signal jump out at us. It means we can do much more with this technique."

Birkby was to present the research on Friday at a meeting of Britain's Royal Astronomical Society in Edinburgh, Scotland, the RAS said.

The team used an instrument called the cryogenic high-resolution infra-red echelle spectrograph, or CRIRES, mounted on the European Southern Observatory's Very Large Telescope in Chile's Atacama desert.

"In the next decade our work will help astronomers refine their search for Earth-like planets - and even life - in orbit around other stars," said Birkby.

"It's incredibly exciting to think that in my lifetime we will reach a day when we can point up to a star and say with confidence that it has a world just like our own."