LAUSR

In the Solar System, asteroids and Kuiper belt objects as well as comets are remnant planetesimals from the time of planet formation. Interactions between solids and gas inside a protoplanetary disc can, e.g. through the streaming instability, form gravitationally bound planetesimal-mass clouds of pebbles. Such clouds will inevitably have inelastic collisions between pebbles, lose energy and experience a runaway collapse into planetesimals. We study the collapse process with a statistical model to find the internal structure of comet-sized planetesimals. In this paper we develop a numerical model that keep track of at what depth particles inside the pebble cloud are to get the radial structure of the resulting planetesimal. We find that the interiors of a planetesimal is heavily dependent on initial pebble sizes and depth inside the planetesimal. We also look at what effect disc gas has on the collapse by adding gas drag onto particles. This both speeds up the collapse and cause lower collision speeds which results in primordial pebbles surviving the collapse. The dependence on particle sizes result in planetesimals with an interior of “onion”-like shells. Our results are in agreement with Rosetta observations of 67P/Churyumov–Gerasimenko being a porous, homogeneous pebble-pile. (Less)