Abstract: In the past decade, ALMA observations have revealed that a large fraction of protoplanetary disks contains rings in the dust continuum and gas molecular line emission. Dust trapping induced by gap-opening planets is a popular explanation for these rings. However, it is unclear whether pressure bumps can survive for evolutionary time scales. In addition, the formation of planets is inefficient at large distances. Recently, the Molecules with ALMA at Planet-forming Scales (MAPS) ALMA Large Program reported a high number of line emission substructures coincident with dust rings and gaps. To test the robustness of the claimed correlation, we compare the observed spatial overlap fraction in substructures with those from the null hypothesis, in which the overlap is assumed to arise from the random placement of line-emission substructures. Our results reveal that there is no statistically significant evidence for a universal correlation between line-emission and continuum substructures, further questioning the frequently made link between continuum rings and pressure bumps.
We investigate an alternative scenario, which involves only dust-gas interactions in a smooth gas disc. We postulate that dust rings are a manifestation of dense, clumpy mid-plane pebbles that are actively forming planetesimals. The clumpy medium itself hardly experiences radial drift, but clumps lose mass by planetesimal formation, and may therefore be the places where planets form. The clumpy ring picture may reconcile the chickens and eggs problem of rings and planets, and can naturally explain the “fine-tuned” optical depths problem of the observed dust rings.