  Most known extrasolar planets (exoplanets) have been discovered using the
radial velocity$^{\bf 1,2}$ or transit$^{\bf 3}$ 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$^{\bf 6\rm{\bf -}\bf 9}$, 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$^{\bf 10}$. These planets are at least as numerous as the stars in
the Milky Way$^{\bf 10}$. 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 17$_{\bf -9}^{\bf +6}$%
of stars host Jupiter-mass planets (0.3--10 $\MJ$, where $\MJ {\bf = 318}$
$\Mearth$ and $\Mearth$ is Earth's mass). Cool Neptunes (10--30 $\Mearth$) and
super-Earths (5--10 $\Mearth$) are even more common: their respective
abundances per star are 52$_{\bf -29}^{\bf +22}$% and 62$_{\bf -37}^{\bf
+35}$%. We conclude that stars are orbited by planets as a rule, rather than
the exception.
