  We investigate the minimum planet mass that produces observable signatures in
infrared scattered light and submm continuum images and demonstrate how these
images can be used to measure planet masses to within a factor of about two. To
this end we perform multi-fluid gas and dust simulations of discs containing
low mass planets, generating simulated observations at $1.65 \mu$m, $10 \mu$m
and $850 \mu$m. We show that the minimum planet mass that produces a detectable
signature is $\sim 15 M_\oplus$: this value is strongly dependent on disc
temperature and changes slightly with wavelength (favouring the submm). We also
confirm previous results that there is a minimum planet mass of $\sim 20
M_\oplus$ that produces a pressure maximum in the disc: only planets above this
threshold mass generate a dust trap that can eventually create a hole in the
submm dust. Below this mass, planets produce annular enhancements in dust
outward of the planet and a reduction in the vicinity of the planet. These
features are in steady state and can be understood in terms of variations in
the dust radial velocity, imposed by the perturbed gas pressure radial profile,
analogous to a traffic jam. We also show how planet masses can be derived from
structure in scattered light and sub-mm images. We emphasise that simulations
with dust need to be run over thousands of planetary orbits so as to allow the
gas profile to achieve a steady state and caution against the estimation of
planet masses using gas only simulations.
