  The evolution of a system consisting of a protoplanetary disc with two
embedded Jupiter sized planets is studied numerically. The disc is assumed to
be flat and non-self gravitating, which is modeled by the planar
(two-dimensional) Navier-Stokes equations. The mutual gravitational interaction
of the planets and the star, and the gravitational torques of the disc acting
on the planets and the central star are included. The planets have an initial
mass of one Jupiter mass $M_{Jup}$ each and the radial distances from the star
are one and two semi-major axis of Jupiter, respectively.
  During the evolution both planets increase their mass due to accretion of gas
from the disc; after about 2500 orbital periods of the inner planet it has
reached a mass of 2.3 and the outer planet of 3.2~$M_{Jup}$. The net
gravitational torques exerted by the disc on the planets result in an inward
migration of the outer planet on time-scales comparable to the viscous
evolution time of the disc, while the semi-major axis of the inner planet
remains constant. When the distance of close approach eventually becomes
smaller than their mutual Hill radius the eccentricities increase strongly and
the system may turn unstable.
  The implications for the origin of the solar system and the extrasolar
planets are discussed.
