  We investigate the interaction of gas and dust in a protoplanetary disk in
the presence of a massive planet using a new two-fluid hydrodynamics code. In
view of future observations of planet-forming disks we focus on the condition
for gap formation in the dust fluid. While only planets more massive than 1
Jupiter mass (MJ) open up a gap in the gas disk, we find that a planet of 0.1
MJ already creates a gap in the dust disk. This makes it easier to find
lower-mass planets orbiting in their protoplanetary disk if there is a
significant population of mm-sized particles.
