ALMA surveys have suggested that the dust in Class II disks may not be enough to explain the averaged solid mass in exoplanets , under the assumption that the mm disk continuum emission is optically thin .
This optically thin assumption seems to be supported by recent DSHARP observations where the measured optical depths of spatially resolved disks are mostly less than one .
However , we point out that dust scattering can considerably reduce the emission from an optically thick region .
If that scattering is ignored , the optical depth will be considerably underestimated .
An optically thick disk with scattering can be misidentified as an optically thin disk .
Dust scattering in more inclined disks can reduce the intensity even further , making the disk look even fainter .
The measured optical depth of \sim 0.6 in several DSHARP disks can be naturally explained by optically thick dust with an albedo of \sim 0.9 at 1.25 mm .
Using the DSHARP opacity , this albedo corresponds to a dust population with the maximum grain size ( s _ { max } ) of 0.1-1 mm .
For optically thick scattering disks , the measured spectral index \alpha can be either larger or smaller than 2 depending on if the dust albedo increases or decreases with wavelength .
Using the DSHARP opacity , \alpha < 2 corresponds to s _ { max } of 0.03-0.3 mm .
We describe how this optically thick scattering scenario could explain the observed scaling between submm continuum sizes and luminosities , and might help ease the tension between the dust size constraints from polarization and dust continuum measurements .
We suggest that a significant amount of disk mass can be hidden from ALMA observations at short millimeter wavelengths .
For compact disks smaller than 30 au , we can easily underestimate the dust mass by more than a factor of 10 .
Longer wavelength observations ( e.g .
VLA or SKA ) are desired to probe the dust mass in disks .