We describe calculations of the evolution of an ensemble of small planetesi
mals in the outer solar system. In a solar nebula with a mass of several ti
mes the minimum mass solar nebula, objects with radii of 100-1000 km can fo
rm on timescales of 10-100 Myr. Model luminosity functions derived from the
se calculations agree with current observations of bodies beyond the orbit
of Neptune (Kuiper Belt objects). New surveys with current and planned inst
ruments can place better constraints on the mass and dynamics of the solar
nebula by measuring the luminosity function at red magnitudes, m(R) greater
than or equal to 28.