Such was the knowledge astronomers had of the formation of planetary systems when high angular resolution instruments first made their appearance. These tools opened the door to the study of protoplanetary disks (or circumstellar disks), but also to the different stages of planetary formation or the architecture of a planetary system. Two main research fields took than shape: determining the overall architecture of protoplanetary disks and gaining an understanding of the evolution of the dust grains within them, in other words the large scale structures and the small scale interactions.

From the physics of protoplanetary disks...


To take the large scale structure first of all, few things were known about protoplanetary disks before the advent of high angular resolution techniques: lines of silicate in stellar spectra already betrayed their presence, but they did not allow the degeneracy which contaminated the theoretical models to be lifted. It was in particular impossible to simultaneously know the distance to the star and the temperature of these silicate grains: grains of different types can have the same temperature without necessarily being the same distance from the star. Without these distances it was impossible to gain a representation of the disk.

High angular resolution imaging has lifted this degeneracy to the point of enabling the characteristics of the grains to be determined, such as their colour, size and composition. It has also revealed the architecture of these disks, which are today classified into two large categories: disks which are opened out or flared and relatively flat disks. Several more exotic details have also brightened up the observational data: certain disks have spinal arms, holes or gaps which could indicate the presence of objects which are being formed.