Dynamics and accretion of planetesimals

In the standard scenario of planet formation, planets are formed from a protoplanetary disk that consists of gas and dust. The building blocks of solid planets are called planetesimals; they are formed by coagulation of dust. We review the basic dynamics and accretion of planetesimals by showing N-body simulations. The orbits of planetesimals evolve through two-body gravitational relaxation: viscous stirring increases the random velocity and dynamical friction realizes the equiparation of the random energy. In the early stage of planetesimal accretion, the growth mode of planetesimals is runaway growth, where larger planetesimals grow faster than smaller ones. When a protoplanet (a runaway-growing planetesimal) exceeds a critical mass, the growth mode shifts to oligarchic growth, where similar-sized protoplanets grow while keeping a certain orbital separation. The final stage of terrestrial planet formation is collision among protoplanets, known as giant impacts. We also summarize the dynamical effects of disk gas on planets and the core accretion model for the formation of gas giants, and discuss the diversity of planetary systems.