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ALMA surveys have suggested that the dust in Class II disks may not be enoughto explain the averaged solid mass in exoplanets, under the assumption that themm disk continuum emission is optically thin. This optically thin assumptionseems to be supported by recent DSHARP observations where the measured opticaldepths of spatially resolved disks are mostly less than one. However, we pointout that dust scattering can considerably reduce the emission from an opticallythick region. If that scattering is ignored, the optical depth will beconsiderably underestimated. An optically thick disk with scattering can bemisidentified as an optically thin disk. Dust scattering in more inclined diskscan reduce the intensity even further, making the disk look even fainter. Themeasured optical depth of $\sim$0.6 in several DSHARP disks can be naturallyexplained by optically thick dust with an albedo of $\sim$0.9 at 1.25 mm. Usingthe DSHARP opacity, this albedo corresponds to a dust population with themaximum grain size ($s_{max}$) of 0.1-1 mm. For optically thick scatteringdisks, the measured spectral index $\alpha$ can be either larger or smallerthan 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 theobserved scaling between submm continuum sizes and luminosities, and might helpease the tension between the dust size constraints from polarization and dustcontinuum measurements. We suggest that a significant amount of disk mass canbe hidden from ALMA observations at short millimeter wavelengths. For compactdisks smaller than 30 au, we can easily underestimate the dust mass by morethan a factor of 10. Longer wavelength observations (e.g. VLA or SKA) aredesired to probe the dust mass in disks.

One Solution to the Mass Budget Problem for Planet Formation: Optically Thick Disks with Dust Scattering