About 40 percent of red dwarf stars may have Earth-sized planets orbiting them that have the right conditions for life.

Red dwarfs – which are smaller and cooler than our sun – are extremely common, making up 80 percent of stars in the galaxy. Their ubiquity suggests that there are tens of billions of possible places to look for life beyond Earth, with at least 100 such planets located nearby.

The new estimate comes from a team of astronomers using the European Southern Observatory's HARPS planet-hunting telescope to look at a sample of 102 nearby red dwarfs over a six-year period. The telescope checked for a characteristic wobble from the star, indicating that at least one planet was tugging on it while orbiting around.

The search found nine planets with between one and 10 Earth masses, including two in the habitable zone, possibly giving them the right temperature to have liquid water. Because red dwarfs don't produce as much heat as our sun, their habitable zones occur much closer to the star.

Larger planets, about the size of Jupiter, were found around less than 12 percent of red dwarfs, suggesting they are rarer than small rocky worlds.

Until recently, astronomers could only guess at the number of stars with planets around them. Now, with the more than 700 confirmed exoplanets, researchers finally have enough data to begin homing in on the true number.

A previous team suggested that one quarter of sun-like stars have an Earth-sized planet around them, while another group estimated that one planet exists for each of the hundred billion stars in our galaxy.

Astronomers hope to someday build a telescope capable of directly imaging the light from an extrasolar planet and see if they contain the telltale chemicals of life, such as oxygen or methane.

Image: NASA/JPL-Caltech