  In the innermost regions of protoplanerary discs, the solid-to-gas ratio can
be increased considerably by a number of processes, including photoevaporative
and particle drift. MHD disc models also suggest the existence of a dead-zone
at $R\lesssim 10$ AU, where the regions close to the midplane remain laminar.
In this context, we use two-fluid hydrodynamical simulations to study the
interaction between a low-mass planet ($\sim 1.7 \;{\rm M_\oplus}$) on a fixed
orbit and an inviscid pebble-rich disc with solid-to-gas ratio $\epsilon\ge
0.5$. For pebbles with Stokes numbers St=0.1, 0.5, multiple dusty vortices are
formed through the Rossby Wave Instability at the planet separatrix. Effects
due to gas drag then lead to a strong enhancement in the solid-to-gas ratio,
which can increase by a factor of $\sim 10^3$ for marginally coupled particles
with St=0.5. As in streaming instabilities, pebble clumps reorganize into
filaments that may plausibly collapse to form planetesimals. When the planet is
allowed to migrate in a MMSN disc, the vortex instability is delayed due to
migration but sets in once inward migration stops due a strong positive pebble
torque. Again, particle filaments evolving in a gap are formed in the disc
while the planet undergoes an episode of outward migration. Our results suggest
that vortex instabilities triggered by low-mass planets could play an important
role in forming planetesimals in pebble-rich, inviscid discs, and may
significantly modify the migration of low-mass planets. They also imply that
planetary dust gaps may not necessarily contain planets if these migrated away.
