Billions of potentially habitable planets may exist in our galaxy, the Milky Way, according to a study.

Around 100 "super-Earths" may be found on our galactic doorstep, at distances less than 30 light years from the sun.

Astronomers came up with the prediction after conducting a survey of red dwarf stars, which account for 80% of stars in the Milky Way.

They calculate that around 40% of red dwarfs have a rocky planet not much bigger than Earth orbiting the "habitable zone" where liquid surface water can exist.

Where there is water, there could be life - although being in the habitable zone is no guarantee that life has evolved on a planet.
Dr Xavier Bonfils, from Grenoble University in France, who led the international team, said: "Because red dwarfs are so common - there are about 160 billion of them in the Milky Way - this leads us to the astonishing result that there are tens of billions of these planets in our galaxy alone."

The astronomers surveyed a carefully chosen sample of 102 red dwarfs using the European Southern Observatory's 3.6-metre telescope at La Silla, Chile.

A total of nine super-Earths - planets with masses between one and 10 times that of Earth - were found. Two were located within the habitable zones of the stars Gliese 581 and Gliese 667 C.

These data were combined with other observations, including those of stars which did not have planets.

The astronomers worked out that habitable zone super-Earths orbiting red dwarfs occurred with a frequency of around 41%.
On the other hand massive planets similar to Jupiter and Saturn were rare around red dwarfs. Less than 12% of the stars were expected to have such "gas giants".

Because red dwarfs are common near the sun, many "super-Earths" may not be far away in astronomical terms. The scientists estimate there could be around 100 habitable zone planets within 30 light years.

Red dwarfs are cooler than the sun, which means planets must orbit close to their parent stars to be warm enough to be habitable.

This might not be good news for life.

"The habitable zone around a red dwarf, where the temperature is suitable for liquid water to exist on the surface, is much closer to the star than the Earth is to the sun," said team member Dr Stephane Udry, from Geneva Observatory.
"But red dwarfs are known to be subject to stellar eruptions or flares, which may bathe the planet in X-rays or ultraviolet radiation, and which may make life there less likely."