Astronomers have long searched for habitable exoplanets orbiting red dwarf stars (M dwarfs), which are considered the most common type of star in the Milky Way galaxy.
According to Xavier Bonfils of the Observatoire des Sciences de l'Univers de Grenoble in France, new observations indicate that approximately 40% 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," he explained.
Using the HARPS spectrograph on a 3.6-meter telescope at ESO's La Silla Observatory in Chile, Bonfils and his team surveyed a carefully chosen sample of 102 red dwarf stars in the southern skies over a six-year period.
A total of nine super-Earths (planets with masses between one and ten times that of Earth) were discovered, including two inside the habitable zones of Gliese 581 and Gliese 667 C, respectively.
Further research led the team to conclude that the occurrence of super-Earths in the habitable zone is 41% - with a range varying from 28% to 95%.
However, says Bonfils, more massive planets, similar to Jupiter and Saturn in our Solar System, are found to be rare around red dwarfs. Indeed, less than 12% of red dwarfs are expected to have giant planets - with masses between 100 and 1,000 times that of the Earth.
Essentially, this means there are probably about one hundred super-Earth planets in the habitable zones around stars in the neighborhood of the Sun, at distances of less than about 30 light-years
"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 Stephane Udry of the 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."
One of the planets discovered in the HARPS survey of red dwarfs is Gliese 667 Cc. This is the second planet in this triple star system and appears to be situated close to the center of the habitable zone. Although this planet is more than four times heavier than the Earth, it is the closest twin to our plant found so far and almost certainly has the right conditions for the existence of liquid water on its surface.
"Now that we know that there are many super-Earths around nearby red dwarfs we need to identify more of them using both HARPS and future instruments," noted team member Xavier Delfosse.
"Some of these planets are expected to pass in front of their parent star as they orbit - this will open up the exciting possibility of studying the planet's atmosphere and searching for signs of life."