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

A statistical analysis of microlensing data from 2002–07 reveals that stars in the Milky Way are orbited by planets as a rule, rather than an exception. Planets common in the Milky Way Most of the extrasolar planets known so far were discovered using methods biased towards planets that are relatively close to their parent stars, and in this population about 17–30% of solar-like stars host a planet. A rather different picture emerges from an analysis of gravitational microlensing data collected between 2002 and 2007. This method probes planets that are farther away from their stars. The data reveal that it is the rule, rather than the exception, for stars in our Galaxy to host one planet or more. 'Super-Earths' are the most abundant type, being associated with around 62% of stars; 52% host cool Neptune-like planets; and 17% host 'Jupiters'. Most known extrasolar planets (exoplanets) have been discovered using the radial velocity 1 , 2 or transit 3 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 microlensing 10 . These planets are at least as numerous as the stars in the Milky Way 10 . 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.