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

Abstract

Most known extrasolar planets (exoplanets) have been discovered using the radial velocity^1,2 or transit³ methods. Both are biased towards planets that are relatively close to their parent stars, and studies find that around 17–30% (refs 4, 5) of solar-like stars host a planet. Gravitational microlensing^6,7,8,9, on the other hand, probes planets that are further away from their stars. Recently, a population of planets that are unbound or very far from their stars was discovered by microlensing¹⁰. These planets are at least as numerous as the stars in the Milky Way¹⁰. Here we report a statistical analysis of microlensing data (gathered in 2002–07) that reveals the fraction of bound planets 0.5–10 au (Sun–Earth distance) from their stars. We find that of stars host Jupiter-mass planets (0.3–10 M_J, where M_J = 318 M_⊕ and M_⊕ is Earth’s mass). Cool Neptunes (10–30 M_⊕) and super-Earths (5–10 M_⊕) are even more common: their respective abundances per star are and . We conclude that stars are orbited by planets as a rule, rather than the exception.