SIMBAD references

We present the first detection of the ^{13}C{17}O J = 3-2 transition toward the HL Tau protoplanetary disc. We find significantly more gas mass (at least a factor of 10 higher) than has been previously reported using C^{18}O 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.