  One of the striking discoveries of protoplanetary disc research in recent
years are the spiral arms seen in several transitional discs in polarised
scattered light. An interesting interpretation of the observed spiral features
is that they are density waves launched by one or more embedded (proto-)planets
in the disc. In this paper we investigate whether planets can be held
responsible for the excitation mechanism of the observed spirals. We use
locally isothermal hydrodynamic simulations as well as analytic formulae to
model the spiral waves launched by planets. Then H-band scattered light images
are calculated using a 3D continuum radiative transfer code to study the effect
of surface density and pressure scale height perturbation on the detectability
of the spirals. We find that a relative change of about 3.5 in the surface
density is required for the spirals to be detected with current telescopes in
the near-infrared for sources at the distance of typical star-forming regions
(140pc). This value is a factor of eight higher than what is seen in
hydrodynamic simulations. We also find that a relative change of only 0.2 in
pressure scale height is sufficient to create detectable signatures under the
same conditions. Therefore, we suggest that the spiral arms observed to date in
protoplanetary discs are the results of changes in the vertical structure of
the disc (e.g. pressure scale height perturbation) instead of surface density
perturbations.
