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Colloquium
ALMA rings as planet factories

Abstract

Almost 10 years ago, ALMA first discovered substructure in the HL tau system, in the forms of bright rings and dark lanes. This and other substructure is now ubiquitously seen in continuum and line emission. The standard (but not exclusive) explanation for substructure is sculpting by planets. Massive planets will open gaps in the disk, creating pressure maxima that collects solids, and changes the local gas rotation speeds, which can be traced by molecular line observations. Nevertheless, some of these indicators for planets are contradictory and direct evidence for planets in the form of H-alpha detection are rare. Here, in a turnaround of the classical chicken-egg dilemma, we instead propose that these dense ALMA rings are the ideal sites to form planets. We show that, inside the ring, the threshold of the pebble accretion process is met as soon as planetesimals form and that planets grow rapidly. After reaching ~10 Earth mass, planets migrate away from the ring, which continues to produce planets as long as the pebble supply lasts. Next, I will highlight the role of pebble accretion in changing the chemical composition of the gas through ice sublimation. I will conclude by applying these ideas to the MWC-480 system.