Mass inventory of the giant-planet formation zone in a solar nebula analogue

The initial mass distribution in the solar nebula is a critical input to planet formation models that seek to reproduce today’s Solar System1. Traditionally, constraints on the gas mass distribution are derived from observations of the dust emission from disks2,3, but this approach suffers from large uncertainties in dust opacity and gas-to-dust ratio2. On the other hand, previous observations of gas tracers only probe surface layers above the bulk mass reservoir4. Here we present the first partially spatially resolved observations of the 13C18O J = 3–2 line emission in the closest protoplanetary disk, TW Hydrae, a gas tracer that probes the bulk mass distribution. Combining it with the C18O J = 3–2 emission and the previously detected HD J = 1–0 flux, we directly constrain the mid-plane temperature and optical depths of gas and dust emission. We report a gas mass distribution with radius, R, of 13−5+8×(R/20.5au)−0.9−0.3+0.4 g cm−2 in the expected formation zone of gas and ice giants (5–21 au). We find that the mass ratio of total gas to millimetre-sized dust is 140 in this region, suggesting that at least 2.4M⊕ of dust aggregates have grown to centimetre sizes (and perhaps much larger). The radial distribution of gas mass is consistent with a self-similar viscous disk profile but much flatter than the posterior extrapolation of mass distribution in our own and extrasolar planetary systems.