Why do stars have planets

On average, all stars in the Milky Way have planets

Paris, France - Astronomers have found more than 700 planets at other stars in the past 16 years. But so far the question has remained open how often stars are orbited by planets. After six years of intensive monitoring of several million stars, an international team of researchers has now come up with the answer: On average, each star in the Milky Way has at least one planet. The scientists report on their observations in the journal "Nature".

"Our data show that there are more planets than stars in the Milky Way," explains Arnaud Cassan from the Institut d’Astrophysique de Paris, one of the astronomers involved. “We have also found that lighter planets such as super-earths or cool Neptune are more common than heavy ones.” The exoplanets discovered so far are more likely to have a preponderance of massive giant planets, but this is due to a selection effect of the methods used, says Cassan.

Most of the exoplanets have given themselves away either because they pull their central star with their gravity and thus cause it to stagger, or because, viewed from Earth, they regularly pass in front of the star and thereby slightly weaken its brightness. Both methods naturally prefer large planets in narrow orbits and thus provide a distorted picture of the abundance of planets.

In contrast, the study by Cassan and his colleagues is based on a different method, the so-called micro-gravitational lens effect. The researchers register the deflection of light in the gravitational field of stars and planets - it causes the brightness of stars in the background of a planetary system to show characteristic fluctuations. This method allows the detection of planets over a wide range of masses and orbital diameters.

The statistical analysis of the light curves of several million stars collected in the period from 2002 to 2007 shows that 62 percent of the stars a super-earth with five to ten times the mass of the earth and 52 percent of the stars a cool Neptune-like planet with the 10- to Have 30 times the mass of the earth. Jupiter-like planets are significantly less common at 17 percent. The data suggest, the researchers say, that Earth-like planets are even more common than super-Earths. “We used to think that the earth was unique in the Milky Way,” says Daniel Kubas, another member of the team. "But there seem to be billions of planets with masses similar to Earth orbiting stars in the galaxy."