  Protoplanetary discs are now routinely observed and exoplanets, after the
numerous indirect discoveries, are starting to be directly imaged. To better
understand the planet formation process, the next step is the detection of
forming planets or of signposts of young planets still in their disc, such as
gaps. A spectacular example is the ALMA science verification image of HL Tau
showing numerous gaps and rings in its disc. To study the observability of
planet gaps, we ran 3D hydrodynamical simulations of a gas and dust disc
containing a 5 M J gap-opening planet and characterised the spatial
distribution of migrating, growing and fragmenting dust grains. We then
computed the corresponding synthetic images for ALMA. For a value of the dust
fragmentation threshold of 15 m s --1 for the collisional velocity, we identify
for the first time a self-induced dust pile up in simulations taking
fragmentation into account. This feature, in addition to the easily detected
planet gap, causes a second apparent gap that could be mistaken for the
signature of a second planet. It is therefore essential to be cautious in the
interpretation of gap detections.
