In Cassan et al. 2012 (Nature, 481, 167), “One or more bound planets per Milky Way star from microlensing observations“) we read the following very interesting statement:
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 17 +6/-9 % of stars host Jupiter-mass planets (0.3–10 MJ , where MJ = 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 52 +22/-29 % and 62 +35/-37 %. We conclude that stars are orbited by planets as a rule, rather than the exception.