Abstract

Traditional models of planetesimal growth begin with kilometre sized bodies, which grow by pairwise collisions. The fastest growing bodies run away from the distribution, which causes larger planetesimals (100-1000 km) to form with low (less than 1 percent) efficiency. This low efficiency has been regarded as favourable, as the mass of the asteroid and Kuiper belts is similarly lower than the expectations from the minimum mass solar nebula.

However, new results suggest that the Kuiper belt region could never have had much more mass than it does today, and that extrasolar debris disks contain enough mass in larger (100~1000 km) bodies that they must have formed with high efficiency. We revisit growth starting from a kilometre, and show it always has low efficiency. We consider an alternate case, where planetesimals grow by accreting very small (cm) objects, and show that collisional cooling among the cm grains changes the character of growth, allowing it to proceed with high efficiency, satisfying the observational demands of the Kuiper belt and extrasolar debris disks.