  Planets form in disks around young stars. Interactions with these disks cause
them to migrate and thus affect their final orbital periods. We suggest that
the connection between planets and disks may be deeper and involve a symbiotic
evolution. By contributing to the outward transport of angular momentum,
planets promote disk accretion. Here we demonstrate that planets sufficiently
massive to open gaps could be the primary agents driving disk accretion. Those
having masses below the gap opening threshold drift inward more rapidly than
the disk material and can only play a minor role in its accretion. Eccentricity
growth during gap formation may involve an even more intimate symbiosis. Given
a small initial eccentricity, just a fraction of a percent, the orbital
eccentricity of a massive planet may grow rapidly once a mass in excess of the
planet's mass has been repelled to form a gap around the planet's orbit. Then,
as the planet's radial excursions approach the gap's width, subsequent
eccentricity growth slows so that the planet's orbit continues to be confined
within the gap.
