Recent high-resolution interferometric observations of protoplanetary disks at (sub-)millimeter wavelengths reveal omnipresent substructures, such as rings, spirals, and asymmetries. A detailed investigation of eight rings detected in five disks by the DSHARP survey came to the conclusion that all rings are just marginally optically thick with optical depths between 0.2 and 0.5 at a wavelength of 1.25 mm. This surprising result could either be coincidental or indicate that the optical depth in all of the rings is regulated by the same process. We investigated if ongoing planetesimal formation could explain the fine-tuned optical depths in the DSHARP rings by removing dust and transforming it into invisible planetesimals. We performed a one-dimensional simulation of dust evolution in the second dust ring of the protoplanetary disk around HD 163296, including radial transport of gas and dust, dust growth and fragmentation, and planetesimal formation via gravitational collapse of sufficiently dense pebble concentrations. We show that planetesimal formation can naturally explain the observed optical depths if streaming instability regulates the midplane dust-to-gas ratio to unity. Furthermore, our simple monodisperse analytical model supports the hypothesis that planetesimal formation in dust rings should universally limit their optical depth to the observed range.
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