The quest to find another world that sustains life has been boosted by a technique that should let less expensive ground-based telescopes join the search, a study said on Wednesday.

So far, more than 400 so-called exoplanets -- planets that orbit stars other than the Sun -- have been spotted since 1995, although none has been a rocky, watery world like our own.

The key to finding the habitability of these worlds lies especially with spectroscopy, or the analysis of the spectrum of light reflected by the planet, which gives telltales about its atmosphere.

Until now, such work has been the preserve of orbital telescopes, where viewing opportunities are strictly rationed because of the huge cost of the gadget.

But astronomers in British and Germany say they have developed a technique in data analysis that should enable relatively small ground-based telescopes to take part in the hunt.

In a paper published on Wednesday in the British journal Nature, the team were able to identify a "fluorescent" form of methane in the upper atmosphere of an exoplanet 63 light years away.

The target is HD 189733b, a gas giant closely orbiting a star in the Vulpecula, or Fox, constellation.

They analysed data from observations made in 2007 by a three-metre (10-feet)ground-based infrared telescope in Hawaii, which explored part of the spectrum that is not observable with the current generation of space telescopes.

HD 189733b swings around its sun in a planetary eclipse -- so the trick was to measure the system's light before and after the star masks the planet.

The planet's spectrum was extracted by deducting the "after" from the "before" but then had to be filtered, removing potential distortions as the light passed through Earth's atmosphere.

The study was lead-authored by Mark Swain, a NASA guest scientist at Germany's Max Planck Institute for Astronomy.

"The fact that we have have used a relatively small, ground-based telescope is exciting because it implies that the largest telescopes in the ground, using this technique, may be able to characterise terrestrial exoplanet targets," Swain said.

Such a feat may lie in the distant future.

The exoplanets observed so far have mainly been huge, gassy worlds orbiting close to their star, rather than harder-to-spot, small solid planets in the "Goldilocks zone" where the temperature is just right to let water exist in liquid form.