Analogs of the early solar system.

Within the last few decades, the existence of protoplanetary disks has been inferred on the basis of emission from T Tauri stars that does not arise from a stellar photosphere. More recently, high-resolution interferometric techniques have resolved the dust continuum emission, and millimeter arrays have imaged circumstellar molecular gas. These measurements corroborate the disk interpretation; many T Tauri stars are surrounded by centrifugally supported circumstellar disks with radial sizes of order 100 AU. Further proof issues from Hubble Space Telescope images of disks that are illuminated externally. The morphology of circumstellar dust is revealed in striking detail and affirms the prevalence and dimensions of disks imaged at longer wavelengths. The fate of circumstellar material around young stars must be understood in order to discern the degree to which these disks are proto-planetary. Observational studies of circumstellar disks which are in the beginning of a dispersal phase are challenging and place great demands on astronomical techniques. Nevertheless, the connection between disks and the formation of extra-solar planets is supported by increasing circumstantial evidence. Optically thin dust continuum emission persists in T Tauri stars and is detected around some young main sequence stars. Since the dust is subject to rapid dispersal by radiation pressure and Poynting-Robertson drag, some mechanism of replenishment is required. Disks around nearby young main sequence stars show evidence for inner voids and disk asymmetries that should also disappear on short timescales. The presence of large orbiting bodies which collide and interact with the resulting debris can explain both the persistence of optically thin dust and the maintenance of otherwise-ephemeral dynamical features. Together with recent detections of extra-solar planets, these observations lend some support to the hypothesis that circumstellar disks commonly give birth to planetary systems.