A six-year study that surveyed millions of stars using the gravitational microlensing technique has concluded that planets around stars are the rule rather than the exception, with every star in the Milky Way predicted to host a planet.

Many planet-hunting surveys detect planets by monitoring the light output from a star as a planet transits in front of it, or by detecting the subtle change in velocity of the star due to the orbiting planet's gravitational pull. Both techniques are biased towards planets that orbit relatively close to their stars. In a study reported today in the journal Nature, astronomers use the gravitational microlensing technique, in which exoplanets are detected by the way that the gravitational field of their host stars and possible planets acts like a lens, magnifying the light of a background star. If the lensing star hosts a planet, it contributes to the brightening effect on the background star. This method allows the detection of planets much further from their stars, including in their habitable zones.

The team used data from PLANET (Probing Lensing Anomalies NETwork) and OGLE (Optical Gravitational Lensing Experiment). "Amongst the millions of stars monitored by the OGLE collaboration between 2002 and 2007, around 3,000 show a microlensing event," explains Arnaud Cassan of the Institut d'Astrophysique de Paris and lead author of the Nature paper. "A subset of these light curves were monitored very carefully by the PLANET collaboration."

Although microlensing is a powerful tool, it is rare that a background and lensing star will be perfectly aligned, and furthermore, that there is an additional alignment of the planet's orbit. Nonetheless, the team identified ten stars displaying the lens effect of a planet, over a six year period from 2002 to 2007.

"In ten 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," says co-author Uffe Gråe Jørgensen of the Niels Bohr Institute at the University of Copenhagen. "To be exact, we found that the zone that corresponds to the area between Venus and Saturn in our Solar System had an average of 1.6 planets the size of five Earth masses or more."

Statistical analysis of all three methods of planet detection – transits, radial velocity and microlensing – shows that out of the Milky Way’s 100 billion stars, a fraction will likely lie in the star's habitable zone, although that doesn't necessarily mean that a planet will be habitable.

"Most planets detected by microlensing are cold planets, probably frozen," says Cassan. "Our statistical study is at the moment limited to super-Earths planets, although a planet with a mass as low as three Earths has been detected by microlensing. The stars' microlensing probes are mainly red dwarfs, that is, stars with masses of around 30 to 40 percent the mass of the Sun, so the habitable zone for these stars is much closer to the star than for systems with a Sun-like star."