Planet Formation in the Outer Solar System

This paper reviews coagulation models for planet formation in the Kuiper belt, emphasizing links to recent observations of our and other solar systems. At heliocentric distances of 35–50 AU, single-annulus and multiannulus planetesimal accretion calculations produce several 1000 km or larger planets and many 50–500 km objects on timescales of 10–30 Myr in a minimum-mass solar nebula. Planets form more rapidly in more massive nebulae. All models yield two power-law cumulative size distributions, with for radii q N ∝ r q p 3.0–3.5 C km and for radii km. These size distributions are consistent with observations of Kuiper 2.5 r 10 N ∝ r r 1 C belt objects acquired during the past decade. Once large objects form at 35–50 AU, gravitational stirring leads to a collisional cascade where 0.1–10 km objects are ground to dust. The collisional cascade removes 80%–90% of the initial mass in the nebula in ∼1 Gyr. This dust production rate is comparable to rates inferred for a Lyr, b Pic, and other extrasolar debris disk systems.