  One class of protoplanetary disc models, the X-wind model, predicts strongly
subkeplerian orbital gas velocities, a configuration that can be sustained by
magnetic tension. We investigate disc-planet interactions in these subkeplerian
discs, focusing on orbital migration for low-mass planets and gap formation for
high-mass planets. We use linear calculations and nonlinear hydrodynamical
simulations to measure the torque and look at gap formation. In both cases, the
subkeplerian nature of the disc is treated as a fixed external constraint. We
show that, depending on the degree to which the disc is subkeplerian, the
torque on low-mass planets varies between the usual Type I torque and the
one-sided outer Lindblad torque, which is also negative but an order of
magnitude stronger. In strongly subkeplerian discs, corotation effects can be
ignored, making migration fast and inward. Gap formation near the planet's
orbit is more difficult in such discs, since there are no resonances close to
the planet accommodating angular momentum transport. In stead, the location of
the gap is shifted inwards with respect to the planet, leaving the planet on
the outside of a surface density depression. Depending on the degree to which a
protoplanetary disc is subkeplerian, disc-planet interactions can be very
different from the usual Keplerian picture, making these discs in general more
hazardous for young planets.
