Our Milky Way galaxy is swarming with tens of billions of habitable rocky planets not much bigger than Earth, European space scientists have discovered.  

Research with a giant telescope in Chile has found that there are probably around 100 of the so-called super-Earths in the Sun’s own immediate neighbourhood.

The cosmic census was made using a planet-finding spectrograph called HARPS which is attached to a giant telescope with a 3.6-meter wide light-gathering “eye” at the European Southern Observatory’s La Silla mountaintop site in Chile.

An international team of astronomers observed the most common type of star in the Milky Way, known as red dwarfs or M dwarfs, to hunt down alien planets. Their trawl over a six-year period of 102 of the stars, which are fainter and cooler than our own Sun, netted a total of nine super-Earths, which are worlds between one and ten times the size of Earth.

Two of the stars, known as Gliese 581 and Gliese 667 C, each had two super-Earths in orbit.

By estimating how heavy the planets were and their distance from their parent stars, the astronomers showed that rocky planets are very common in their habitable zones where water could exist as a liquid.

Team leader Xavier Bonfils, of Grenoble, France, said: “Our new observations with HARPS show that about 40 per cent of all red dwarf stars have a super-Earth orbiting in the habitable zone where liquid water can exist on the surface of the planet. 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.”

At the same time, the research revealed that more massive planets, similar to Jupiter and Saturn, are rarely found around red dwarfs. Less than 12 per cent of such stars are expected to have giant planets.

The astronomers now plan to look for signs of Earth-like planets passing in front of their parent stars as they orbit them – an event called a transit. Team member Xavier Delfosse said: “This will open up the exciting possibility of studying the planet’s atmosphere and searching for signs of life.”

However, there was a word of caution for anyone hoping that the findings might boost the chances of us having alien neighbours. The otherwise habitable planets might be getting heavy doses of radiation because they orbit much closer to their parent stars.

Stéphane Udry, of Geneva Observatory, said: “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.”

A separate NASA study in 2010 concluded that planets like Earth are common throughout our Milky Way galaxy.