  We carry out three dimensional smoothed particle hydrodynamics simulations to
study the impact of planet-disc interactions on a gravitationally unstable
protoplanetary disc. We find that the impact of a planet on the disc's
evolution can be described by three scenarios. If the planet is sufficiently
massive, the spiral wakes generated by the planet dominate the evolution of the
disc and gravitational instabilities are completely suppressed. If the planet's
mass is too small, then gravitational instabilities are unaffected. If the
planet's mass lies between these extremes, gravitational instabilities are
weakened. We present mock Atacama Large Millimeter/submillimeter Array (ALMA)
continuum observations showing that the observability of large-scale spiral
structures is diminished or completely suppressed when the planet is massive
enough to influence the disc's evolution. Our results show that massive discs
that would be expected to be gravitationally unstable can appear axisymmetric
in the presence of a planet. Thus, the absence of observed large-scale spiral
structures alone is not enough to place upper limits on the disc's mass, which
could have implications on observations of young Class I discs with rings &
gaps.
