Do disks hold enough solids to build the observed planets?
In plain words
Weighing the dust in disks that are one to three million years old gives masses comparable to, or smaller than, the rock and metal locked in the planets found around older stars. Either planets form much earlier than assumed, the dust is being weighed wrongly, or disks are refilled from outside.
Precise statement
Millimeter continuum dust masses of Class II disks (ages $\sim 1-3\,\mathrm{Myr}$), $M_{\mathrm{dust}}\sim F_{\nu} d^2 / (\kappa_{\nu} B_{\nu}(T_{\mathrm{dust}}))$, are comparable to or lower than the heavy-element masses of exoplanet systems around older stars (Manara, Morbidelli and Guillot 2018, arXiv:1809.07374). Determine which resolves the deficit: planet cores forming before $\sim 0.1-1\,\mathrm{Myr}$ (Class 0/I stage), underestimated dust masses from optically thick emission, scattering or large grains, or replenishment of disks by late infall. The answer is a corrected disk solid-mass distribution versus age compared with the planet heavy-element distribution.
What would settle it
Multi-wavelength (millimeter to centimeter) dust-mass measurements of complete Class 0, I and II samples with radiative-transfer modeling of optical depth and scattering, set against the exoplanet heavy-element mass function.