The vast majority of stars in our Milky Way galaxy have at least one planet orbiting them, astronomers have concluded - and lots of these could be comparatively Earth-like.
Over the past 16 years, more than 700 confirmed exoplanets have been ideitified. Most were found by detecting the planet's effect on the gravitational pull or luminance of the host star - techniques which are both much more sensitive to planets that are either massive or close to their stars. Many planets will be missed.
However, gravitational microlensing can detect planets over a wide range of mass, as well as those that lie much further from their stars. It involves examining the way that the gravitational field of a host star, combined with that of possible planets, acts like a lens, magnifying the light of a background star.
If the star that acts as a lens has a planet in orbit around it, the planet can make a detectable contribution to the brightening effect on the background star.
"We have searched for evidence for exoplanets in six years of microlensing observations. Remarkably, these data show that planets are more common than stars in our galaxy," says Arnaud Cassan of the Institut dʼAstrophysique de Paris.
"We also found that lighter planets, such as super-Earths or cool Neptunes, must be more common than heavier ones."
It's not the easiest of techniques.It takes a very rare chance alignment of a background and lensing star for a microlensing event to be seen at all. And, to spot a planet during an event, an additional chance alignment of the planet's orbit is also needed.
Despite this, however, in the last six years, three exoplanets have been detected in this way: a super-Earth, and planets with masses comparable to Neptune and Jupiter. Either the astronomers were incredibly lucky, or planets are so abundant in the Milky Way that it was almost inevitable, says the team.
Combining information about the three positive exoplanet detections with data on seven additional detections from earlier work - as well as the huge numbers of non-detections in the six years' worth of data — indicates that the second theory is the right one.
The resuts suggest that one in six of the stars studied hosts a planet of similar mass to Jupiter, half have Neptune-mass planets and two thirds have super-Earths.
"We used to think that the Earth might be unique in our galaxy. But now it seems that there are literally billions of planets with masses similar to Earth orbiting stars in the Milky Way," says Daniel Kubas, co-lead author of the paper.