  Protoplanets may be born into dust-rich environments if planetesimals formed
through streaming or gravitational instabilities, or if the protoplanetary disc
is undergoing mass loss due to disc winds or photoevaporation. Motivated by
this possibility, we explore the interaction between low mass planets and dusty
protoplanetary discs with focus on disc-planet torques. We implement Lin &
Youdin's newly developed, purely hydrodynamic model of dusty gas into the PLUTO
code to simulate dusty protoplanetary discs with an embedded planet. We find
that for imperfectly coupled dust and high metallicity, e.g. Stokes number
$10^{-3}$ and dust-to-gas ratio $\Sigma_\mathrm{d}/\Sigma_\mathrm{g}=0.5$ , a
`bubble' develops inside the planet's co-orbital region, which introduces
unsteadiness in the flow. The resulting disc-planet torques sustain large
amplitude oscillations that persists well beyond that in simulations with
perfectly coupled dust or low dust-loading, where co-rotation torques are
always damped. We show that the desaturation of the co-rotation torques by
finite-sized particles is related to potential vorticity generation from the
misalignment of dust and gas densities. We briefly discuss possible
implications for the orbital evolution of protoplanets in dust-rich discs. We
also demonstrate Lin & Youdin's dust-free framework reproduces previous results
pertaining to dusty protoplanetary discs, including dust-trapping by pressure
bumps, dust settling, and the streaming instability.
