  With hundreds of exoplanets detected, it is necessary to revisit giant
planets accretion models to explain their mass distribution. In particular,
formation of sub-jovian planets remains unclear, given the short timescale for
the runaway accretion of massive atmospheres. However, gas needs to pass
through a circum-planetary disc. If the latter has a low viscosity (as expected
if planets form in "dead zones"), it might act as a bottleneck for gas
accretion. We investigate what the minimum accretion rate is for a planet under
the limit assumption that the circum-planetary disc is totally inviscid, and
the transport of angular momentum occurs solely because of the gravitational
perturbations from the star. To estimate the accretion rate, we present a
steady-state model of an inviscid circum-planetary disc, with vertical gas
inflow and external torque from the star. Hydrodynamical simulations of a
circum-planetary disc were conducted in 2D, in a planetocentric frame, with the
star as an external perturber in order to measure the torque exerted by the
star on the disc. The disc shows a two-armed spiral wave caused by stellar
tides, propagating all the way in from the outer edge of the disc towards the
planet. The stellar torque is small and corresponds to a doubling time for a
Jupiter mass planet of the order of 5 Myrs. Given the limit assumptions, this
is clearly a lower bound of the real accretion rate. This result shows that gas
accretion onto a giant planet can be regulated by circum-planetary discs. This
suggests that the diversity of masses of extra-solar planets may be the result
of different viscosities in these discs.
