How many exoplanets in our Galaxy?

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.

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