  Large Kuiper Belt Objects are conventionally thought to have formed out of a
massive planetesimal belt that is a few thousand times its current mass. Such a
picture, however, is incompatible with multiple lines of evidence. Here, we
present a new model for the conglomeration of Cold Classical Kuiper belt
objects, out of a solid belt only a few times its current mass, or a few
percent of the solid density in a Minimum Mass Solar Nebula. This is made
possible by depositing most of the primordial mass in grains of size centimetre
or smaller. These grains collide frequently and maintain a dynamically cold
belt out of which large bodies grow efficiently: an order-unity fraction of the
solid mass can be converted into large bodies, in contrast to the ~0.1%
efficiency in conventional models. Such a light belt may represent the true
outer edge of the Solar system, and it may have effectively halted the outward
migration of Neptune. In addition to the high efficiency, our model can also
produce a mass spectrum that peaks at an intermediate size, similar to the
observed Cold Classicals, if one includes the effect of cratering collisions.
In particular, the observed power-law break observed at ~30 km for Cold
Classicals, one that has been interpreted as a result of collisional erosion,
may be primordial in origin.
