The Milky Way galaxy may contain billions of habitable planets, astronomers have said.

The discovery raises the tantalising possibility that life could exist on billions of worlds in our galaxy alone.

The team at the University of Copenhagen came to the conclusion after a six-year star survey, which suggests planets are commonplace in our galaxy.

Scientists estimate as many as 10 billion stars in the Milky Way may host planets in the "habitable", or "Goldilocks", zone.

This is the orbital band within which conditions are not too cold and not too hot but "just right" to allow surface liquid water and, potentially, life.

However, scientists stress that just because a planet has conditions suitable for life it does not follow that life has evolved there.

Over the past 16 years, astronomers have made more than 700 confirmed detections of "exoplanets" orbiting distant stars.

The vast majority have been Jupiter-like gas giants or scalding hot planets hugging close to their stars. Both offer little hope of finding life.

In those cases astronomers relied on spotting tiny "wobbles" in the host star caused by a planet's gravitational pull, or the minute dimming of starlight as a planet crossed in front of its star.

Both techniques are not suited to finding small rocky planets like the Earth in the habitable zone.

The new survey employed a radically different method called "gravitational microlensing". This involves a foreground star's gravity acting like a "magnifying glass" to bend and amplify light from a background star.

If there is a planet orbiting the foreground star, a small extra "bump" might be seen in the light signal.

The technique just happens to be most sensitive to planets a mid-distance away from the star - in other words, those in the "habitable zone".

However, very special conditions are needed to detect planets by gravitational microlensing. The background and foreground stars have to be lined up, and an additional chance alignment of the planet's orbit is also needed.

Despite these obstacles, analysis of six years' worth of microlensing data from telescopes around the world uncovered an unexpected number of exoplanets.

"In a six-year period from 2002 to 2007 we observed 500 stars at high resolution," said Danish astronomer Dr Uffe Grae Jorgensen, head of Astrophysics and Planetary Science at the University of Copenhagen.

"In 10 of the stars we directly see the lens effect of a planet, and for the others we could use statistical arguments to determine how many planets the stars had on average."

The results are published today in the journal Nature.

Combined with exoplanet findings using different detection methods, they suggest around 10 billion stars out of the 100 billion that fill the Milky Way have habitable zone planets.

The findings showed that planets orbiting stars were "more the rule than the exception" and billions of them may be habitable, said Dr Jorgensen.

However it was quite another thing to jump to the conclusion that life had arisen on large numbers of these worlds as it had on Earth.

Life as we know it on Earth had developed as a result of "many unique events", Dr Jorgensen pointed out. But he added: "Perhaps other coincidences in other solar systems have led to entirely different and exciting new forms of life."

Dr Martin Dominik, who led a British team from the University of St Andrews involved in the research, said: "We do not know yet where all the planets are, how big or small, dense or fluffy they are, or whether they are home to life or not, but our latest results tell us that while we may not see all the planets, wherever in the sky we look, they are there."