One or more bound planets per Milky Way star from micolensing observations

Paris, January 11, 2012. In the Milky Way, at least one planet orbits around every star. This estimate of the abundance of planets outside the System Solaire was obtained thanks to the statistical study conducted by Arnaud Cassan, researcher at the Institut d’Astrophysique de Paris (UPMC/CNRS), as part of an international collaboration. It highlights a high proportion of planets cousins to the Earth by their mass. Published on January 12 in the journal Nature, these results are the fruit of six years of research. observations of millions of stars using a high-performance method based on the gravitational microlensing effect.

“This artists’s cartoon view gives an impression of how common planets are around the stars in the Milky Way. The planets, their orbits and their host stars are all vastly magnified compared to their real separations. A six-year search that surveyed millions of stars using the microlensing technique concluded that planets around stars are the rule rather than the exception. The average number of planets per star is greater than one.” Credit: ESO/M. Kornmesser

Apart from the eight planets of the Solar System, those scattered in the Milky Way (exoplanets) are they abundant or rare? To answer this fundamental question in astronomy, the authors have relied on observations made from 2002 to 2007 by a global network of telescopes in the framework of OGLE(1) and PLANET(2) collaborations. Two French laboratories are involved in the published study, the Institut d’Astrophysique de Paris (UPMC/CNRS) and the Institute for Research in Astrophysics and Planetology (CNRS/University Toulouse III Paul Sabatier).

The observations were based on the gravitational microlens technique, which relies on the apparent amplification of the brightness of a background star when a closer body crosses its line of sight. Avoiding the bias of other existing methods (radial velocity and transit techniques), it allowed the observation of planets even at great distances from their star, not limited to massive planets or planets close to their star.

Astronomers have combined the information obtained from the tracking of millions of of stars to existing results. Their statistical study has thus made it possible to link the number of planets per star, their masses and the distance between a star and a planet. Their work has established that the number of exoplanets is at least equal to the number of number of stars in the Milky Way. They also showed that about one sixth of the stars have a giant planet like Jupiter as a companion, about half of them are accompanied by a planet similar to Neptune, and that about two-thirds of the stars are associated with super-Earths… (cousins of the Earth but 5 to 10 times more massive). This study suggests that many Earth-like planets could exist and be discovered… in the years to come.


(1) OGLE : Optical Gravitational Lensing
(2) PLANET : Probing Lensing Anomalies NETwork Experiment

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