  Phases of gravitational instability are expected in the early phases of disk
evolution, when the disk mass is still a substantial fraction of the mass of
the star. Disk fragmentation into sub-stellar objects could occur in the cold
exterior part of the disk. Direct detection of massive gaseous clumps on their
way to collapse into gas giant planets would offer an unprecedented test of the
disk instability model. Here we use state-of-the-art 3D radiation-hydro
simulations of disks undergoing fragmentation into massive gas giants,
post-processed with the RADMC-3D ray-tracing code to produce dust continuum
emission maps. These are then fed into the Common Astronomy Software
Applications (CASA) ALMA simulator. The synthetic maps show that both overdense
spiral arms and actual clumps at different stages of collapse can be detected
with the Atacama Large Millimetre/sub-millimetre Array (ALMA) in the full
configuration at the distance of the Ophiuchus star forming region (125 pc).
The detection of clumps is particularly effective at shorter wavelengths (690
GHz) combining two resolutions with multi-scale clean. Furthermore, we show
that a flux-based estimate of the mass of a protoplanetary clump can be from
comparable to a factor of 3 higher than the gravitationally bound clump mass.
The estimated mass depends on the assumed opacity, and on the gas temperature,
which should be set using the input of radiation-hydro simulations. We conclude
that ALMA has the capability to detect "smoking gun" systems that are a
signpost of the disk instability model for gas giant planet formation.
