There are theoretical arguments that the disk should be truncated:
A passing star tidally strips the Kuiper belt
(Melita el al. 2001; Kobayashi & Ida 2001; Levison, Morbidelli, & Dones 2004).
An edge formed prior to planetesimal formation due to aerodynamic drag.
(Youdin & Shu 2002)
An edge formed during planet accretion due to size-dependent radial gas drag.
(Weidenschilling 2003)
Nearby early-type stars photo-evaporated the outer regions of the solar nebula.
(Hollenbach & Adams 2003; Matsuyama et al. 2003)
Magneto-hydrodynamic instabilities in the outer regions of the
disk prevented the formation of planetesimals in these regions
(Gammie 1996; Stone et al. 1998). (Note: many of these do not truncate nebula, but only the region where planetesimals formed.)
Also we see truncated disks
Example: AU Mic
M star: 50% mass of the Sun, 10-15Myr old.
Break in surface density at 35AU. (Augereau & Beust 2006).
Beyond break, distribution is well modeled as dust being blown out.
