  We investigate the planetary migration of low-mass planets
($M_p\in[1,15]M_\oplus$, here $M_\oplus$ is the Earth mass) in a gaseous disc
containing a previously formed gap. We perform high-resolution 3D simulations
with the FARGO3D code. To create the gap in the surface density of the disc, we
use a radial viscosity profile with a bump, which is maintained during the
entire simulation time. We find that when the gap is sufficiently deep, the
spiral waves excited by the planet trigger the Rossby wave instability, forming
cyclonic (underdense) vortices at the edges of the gap. When the planet
approaches the gap, it interacts with the vortices, which produce a complex
flow structure around the planet. Remarkably, we find a widening of the
horseshoe region of the planet produced by the vortex at the outer edge of the
gap, which depending on the mass of the planet differs by at least a factor of
two with respect to the standard horseshoe width. This inevitably leads to an
increase in the corotation torque on the planet and produces an efficient trap
to halt its inward migration. In some cases, the planet becomes locked in
corotation with the outer vortex. Under this scenario, our results could
explain why low-mass planets do not fall towards the central star within the
lifetime of the protoplanetary disc. Lastly, the development of these vortices
produces an asymmetric temporal evolution of the gap, which could explain the
structures observed in some protoplanetary discs.
