Rings in the ALMA continuum image of protoplanetary disks are the locations where pebbles accumulate, which is beneficial for planetesimal formation and subsequent planet assembly. We investigate the viability of planet formation inside ALMA rings in which pebbles are trapped by either a Gaussian-shape pressure bump or by the strong dust backreaction. Planetesimals form at the midplane of the ring via streaming instability. By conducting N-body simulations, we study the growth of these planetesimals by collisional mergers and pebble accretion. Thanks to the high concentration of pebbles in the ring, the growth of planetesimals by pebble accretion becomes efficient as soon as they are born. We find that type-I migration plays a decisive role in the evolution of rings and planets. For disks where planets can migrate inward from the ring, a steady state is reached where the ring spawns ~20 earth-mass planetary cores as long as rings are fed with materials from the outer disk. This "planet factory" ring can potentially explain the observed "fine-tuned" optical depths in the DSHARP large program. In contrast, in the absence of a planet removal mechanism (migration), a single massive planet will form and destroy the ring. A wide and massive planetesimals belt will be left at the location of the planet-forming ring.
Our work raises the hypothesis that the pebble/debris-disk rings are potentially related to the planet system interior to the ring location. As the planets spawned by the distant ALMA rings in our study are massive. We compare our simulation outputs with several observed distant exoplanet systems.