The Spitzer Space Telescope, with its sensitive mid-infrared (5 – 35 micron) detectors, allowed to characterize the inner few Astronomical Units (AU) in an unprecedented sample of disks in the closest star-forming regions around the Sun. We have literally gone from samples of a few hundred objects to samples of a few thousand. Not only that, Spitzer has discovered a wild variety of inner disk architectures in all those young stars in nearby star-forming regions. In particular, it has confirmed the belief from previous observations that the age of the central star is not defining necessarily the evolutionary status of the disk, but that there are other physical variables involved in the process in a way that we still do not understand.
The work we started as part of the “Cores 2 Disks” (c2d) Spitzer Legacy Program is a systematic characterization of all the disks around the 5 large, well-known nearby star-forming regions that this program observed, to characterize a realistic range of initial conditions of the protoplanetary disks before the planet-forming mechanism(s) start to operate. This sample contains over 1000 young stellar objects (YSOs) with a very rich distribution of cluster and dispersed population.