  Cold debris disks trace the limits of planet formation or migration in the
outer regions of planetary systems, and thus have the potential to answer many
of the outstanding questions in wide-orbit planet formation and evolution. We
characterized the infrared excess spectral energy distributions of 174 cold
debris disks around 546 main-sequence stars observed by both Spitzer IRS and
MIPS. We found a trend between the temperature of the inner edges of cold
debris disks and the stellar type of the stars they orbit. This argues against
the importance of strictly temperature-dependent processes (e.g. non-water ice
lines) in setting the dimensions of cold debris disks. Also, we found no
evidence that delayed stirring causes the trend. The trend may result from
outward planet migration that traces the extent of the primordial
protoplanetary disk, or it may result from planet formation that halts at an
orbital radius limited by the efficiency of core accretion.
