  We present the first detection of the 13C17O J=3-2 transition toward the HL
Tau protoplanetary disc. We find significantly more gas mass (at least a factor
of ten higher) than has been previously reported using C18O emission. This
brings the observed total disc mass to 0.2 M, which we consider to be a
conservative lower limit. Our analysis of the Toomre Q profile suggests that
this brings the disc into the regime of gravitational instability. The radial
region of instability (50-110 au) coincides with the location of a proposed
planet-carved gap in the dust disc and a spiral in the gas. We, therefore,
propose that if the origin of the gap is confirmed to be due to a forming giant
planet, then it is likely to have formed via the gravitational fragmentation of
the protoplanetary disc.
