Observational trends in planet formation

Most of the fine structure seen in young circumstellar disks, or protoplanetary disks, is often attibuted to gravitational interactions with compact bodies, some as small superearths. Hydrodynamical models satisfactorily account for the observations, thus yielding constraints on the embedded planetary systems and their environmental conditions. However, the same observations raise questions on the actual process leading to planet formation and on the scarcity of the putative protoplanets. Although only a handful of directly imaged candidate protoplanets exist, most are Halpha sources, indicative of accretion. Among these, PDS70c stands out as the only example known to have a radio counterpart. In contrast to optical measurements, which are affected by extinction, the radio signal directly probes the protoplanetary environment, provided the emission mechanisms are known. The multi-frequency and variability data available in PDS70c are particularly informative. These data rule out thermal emission from a dusty environment, and instead link the radio signal from PDS70c to a standing shock at the surface of a circumplanetary disk.