The majority of pre-Main-Sequence stars of low mass (M < 3 Msun) are thought to have circumstellar disks of gas and dust. These are the precursors of planetary systems. Planets form in these disks, and the process is tightly related to the formation and evolution of dust. Circumstellar disks probably only last a few million years before dissipation. An understanding of the structure and evolution of disks is necessary to explain their ubiquity, as well as the processes and timescales associated with the formation of planets. Masses of circumstellar disks are uncertain — they are thought to be on the order of a few percent of the stellar mass, but the dispersion in that value is likely to be very large. With a similarly large dispersion, their sizes are on the order of a few hundred AU. Their temperatures are consistent with their being heated by the central star, most of their material glowing at temperatures of ~100 K or below. Thus, their dust emission falls mostly in the mid and FIR. Disks are frequently seen via the scattered starlight, as in the famous case of β Pic, and their gas content can be mapped with mm array telescopes. However, a key observational component for the study of the physics of these structures is the measurement of the dust mass, which needs to be done by detecting their thermal emission, best observed in the submm regime where it is thought to be optically thin. SIRTF and SOFIA will allow much progress to be made in that area. However, even at 40 µm those telescopes will be severely resolution-limited. High resolution is not only important in detecting thermal gradients in disks and therefore allowing a more accurate estimate of the dust mass, but also in detecting the tidal distortions in the disk structure that may be produced by the assembly of planets.
The combination of angular resolution, operation at multiple submm bands from 200 to 850 µm (thus allowing characterization of the temperature variations in the disk), and the large FOV of the AT is the key to its extraordinary potential for work in this field. The recent detection of circumstellar disks at 850 µm in two nearby brown dwarfs by Klein et al. (2003), which also illustrates the likelihood of planet detection in those systems, shows further promise in this rapidly expanding field of research.