Orchestra: Coagulation

Growth by collision and merger

  • Statistical approach to inelastic collisions
  • Fragmentation algorithm for collision outcomes
  • Fokker-Planck approach to gravitational encounters and elastic collisions


  • Essential for 1015 or more planetesimals
  • Calculate evolution of small planets accurately
  • Calculate collisional cascade well


  • Fail when most of the mass in a few large objects
  • Cannot predict final mass of planet
  • Cannot model formation of gas giant planets


  • Divide disk into concentric annuli
  • Within each annulus, mass bins from 10-15 kg to 1027 kg
  • Particles in all annuli interact
  • Collision rates: particle-in-a-box with gravitational focusing
  • Dynamics: dynamical fraction, gas drag, P-R drag, viscous stirring

Code is extensively tested

Photo credit: Scott Kenyon & Ben Bromley