  ALMA has spatially resolved over 200 annular structures in protoplanetary
discs, many of which are suggestive of the presence of planets. Constraining
the mass of these putative planets is quite degenerate for it depends on the
disc physical properties, and for simplicity a steady-state is often assumed
whereby the planet position is kept fixed and there is a constant source of
dust at the outer edge of the disc. Here we argue against this approach by
demonstrating how the planet and dust dynamics can lift degeneracies of such
steady-state models. We take main disc parameters from the well-known
protoplanetary disc HD 163296 with a suspected planet at $R\approx 86$~au as an
example. By running gas and dust hydrodynamical simulations post-processed with
dust radiative transfer calculations, we first find steady-state disc and
planet parameters that reproduce ALMA continuum observations fairly well. For
the same disc mass, but now allowing the planet to migrate in the simulation,
we find that the planet undergoes runaway migration and reaches the inner disc
in $\sim 0.2$ Myr. Further, decreasing the disc mass slows down planet
migration, but it then also increases the dust's radial drift, thereby
depleting the disc dust faster. We find that the opposing constraints of planet
migration and dust drift require the disc mass to be at most $0.025~M_{\odot}$,
must less massive than previously estimated, and for the dust to be porous
rather than compact. We propose that similar analysis should be extended to
other sources with suspected planetary companions.
