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 n ot arise from a stellar photosphere. More recently, high-resolution in terferometric techniques have resolved the dust continuum emission, an d millimeter arrays have imaged circumstellar molecular gas. These mea surements 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 Telescop e images of disks that are illuminated externally. The morphology of c ircumstellar dust is revealed in striking detail and affirms the preva lence and dimensions of disks imaged at longer wavelengths. The fate o f circumstellar material around young stars must be understood in orde r to discern the degree to which these disks are proto-planetary. Obse rvational studies of circumstellar disks which are in the beginning of a dispersal phase are challenging and place great demands on astronom ical techniques. Nevertheless, the connection between disks and the fo rmation of extra-solar planets is supported by increasing circumstanti al evidence. Optically thin dust continuum emission persists in T Taur i stars and is detected around some young main sequence stars. Since t he dust is subject to rapid dispersal by radiation pressure and Poynti ng-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. T he presence of large orbiting bodies which collide and interact with t he resulting debris can explain both the persistence of optically thin dust and the maintenance of otherwise-ephemeral dynamical features. T ogether with recent detections of extra-solar planets, these observati ons lend some support to the hypothesis that circumstellar disks commo nly give birth to planetary systems.