  We investigate the response of an accretion disk to the presence of a
perturbing protoplanet embedded in the disk through time dependent
hydrodynamical simulations. The disk is treated as a two-dimensional viscous
fluid and the planet is kept on a fixed orbit. We run a set of simulations
varying the planet mass, and the viscosity and temperature of the disk. All
runs are followed until they reach a quasi-equilibrium state.
  We find that for planetary masses above a certain minimum mass, already 3
M_Jup for a viscosity of nu = 10^{-5}, the disk makes a transition from a
nearly circular state into an eccentric state. Increasing the planetary mass
leads to a saturation of disk eccentricity with a maximum value of around 0.25.
The transition to the eccentric state is driven by the excitation of an m=2
spiral wave at the outer 1:3 Lindblad resonance. The effect occurs only if the
planetary masses are large enough to clear a sufficiently wide and deep gap to
reduce the damping effect of the outer 1:2 Lindblad resonance. An increase in
viscosity and temperature in the disk, which both tend to close the gap, have
an adverse influence on the disk eccentricity.
  In the eccentric state the mass accretion rate onto the planet is greatly
enhanced, an effect that may ease the formation of massive planets beyond about
5 M_Jup that are otherwise difficult to reach.
