The Spitzer Space Telescope has had a profound impact in the study of protoplanetary disks by providing the first real evidence that many disks have gaps and holes in them due to planet formation, even at very early ages. (see, e.g., Calvet et al. 2002). An example of this is shown left, where UX Tau has both a near-infrared excess, indicative of hot dust interior to 1 AU, while showing a gap in the dust emission implying that the outer dusty disk is truncated interior to about 50 AU. A likely possibility is that giant planets have opened up this gap in the disk, as relatively massive objects such as low-mass stellar companions would probably shut off mass accretion from the outer disk, while UX Tau and other “transitional disks” are still accreting (see below). Identifying these systems from spectral energy distributions is the first step towards subsequent imaging with ALMA and the EVLA to study disk properties in the earliest epochs of planet formation. Work done by former Michigan graduate student Catherine Espaillat, working with Nuria Calvet; Espaillat et al. 2007.
The combination of large gaps in disks while accretion onto the central star still occurs is a challenge to understand theoretically. Zhu+11 we looked at the behavior of multiple planet systems in viscous disks. An individual giant planet clears only a narrow gap. More planets make larger gaps, but to explain the large gaps implied by the Spitzer observations seemingly would require an implausible number of giant planets. A solution which would require fewer planets, of lower mass, can be found if the disks have very low viscosities, as discussed in the section on signatures of planet formation.
One mechanism which helps explain gaps in dust emission relative to gas flows is the interaction of dust with gas. The disk gas tends to rotate slightly below Keplerian due to an outward gas pressure gradient, while the dust, if sufficiently large, will try to orbit at the Keplerian rate (since it isn’t supported by gas pressure). The resulting transfer of angular momentum results in dust of a particular size range, which can be mm-cm sized, to drift toward pressure maxima. A planet carving out a gap produces a gas pressure maximum outside its orbit which can trap dust and form a ring, as shown above in the figure from Zhu+12.