Planetesimals in debris disks

Introduction Planetesimals form in gas-rich protoplanetary disks around young stars. However, protoplanetary disks fade in about 10 Myr. The planetesimals (and also many of the planets) left behind are too dim to study directly. Fortunately, collisions between planetesimals produce dusty debris disks. These debris disks trace the processes of terrestrial planet formation for 100 Myr and of exoplanetary system evolution out to 10 Gyr. This chapter begins with a summary of planetesimal formation (see Youdin, 2010; Chiang and Youdin, 2010; Johansen et al., 2015a for more detailed reviews) as a prelude to the epoch of planetesimal destruction. Our review of debris disks covers the key issues, including dust production and dynamics, needed to understand the observations. Our discussion of extrasolar debris keeps an eye on similarities to and differences from solar system dust. The Formation of Planetesimals The first step in the accretion of terrestrial planets and gas-giant cores is the growth of dust grains into planetesimals, typically defined as solids exceeding a kilometer in size. Models of planetesimal formation have three primary considerations. First, aerodynamic interactions with the gas disk guide the motions of planetesimal building blocks: dust grains, pebbles, and boulders. Second, particle sticking and related collisional evolution dominates the early stages of grain growth. Third, the final assembly of planetesimals likely involves particle concentration mechanisms, which ultimately trigger gravitational collapse. Within a protoplanetary disk, particles drift inward because they encounter a headwind of disk gas, which extracts orbital angular momentum. The headwind arises because gas gets support from radial pressure gradients, an outward acceleration on average. A meter-sized solid falls towards its star on timescales of order 100 years. Getting through the infamous “meter-sized” barrier poses a very stringent constraint on collisional growth (Adachi et al., 1976). Radial drift is fastest for solids that are optimally coupled to the gas, meaning the dimensionless drag constant τ^s = Ω^stop is unity. The stopping time, t^stop, is the e-folding timescale for gas drag to damp particle motion, and Ω is the orbital frequency. Larger particles have larger τ^s, but the radial location and disk model also matter.