  In this letter we present the discovery of a very light planetary companion
to the star mu Ara (HD160691). The planet orbits its host once every 9.5days,
and induces a sinusoidal radial velocity signal with a semi-amplitude of 4.1
m/s, the smallest Doppler amplitude detected so far. These values imply a mass
of m2 sini = 14 earth-masses. This detection represents the discovery of a
planet with a mass slightly smaller than that of Uranus, the smallest ``ice
giant" in our Solar System. Whether this planet can be considered an ice giant
or a super-earth planet is discussed in the context of the core-accretion and
migration models.
