Formation of Pebble-Pile Planetesimals and the Interior Structure of Comets

Abstract: Planets form in protoplanetary discs of gas, dust and ice around newborn stars. In the Solar System, not only planets are found as a result from the time of planet formation, but also remnant planetesimals in the form of asteroids, Kuiper belt objects and comets. Recent observations, e.g. by the space mission Rosetta, have found that comets are porous objects barely able to hold themselves together by gravity.Gravitationally bound clouds of mm- to dm-sized pebbles can form in the protoplanetary disc by interactions between solids and the gas, e.g. through the streaming instability. Such clouds will have collisions between pebbles resulting in energy dissipation and inevitably a collapse into a solid planetesimal.In paper I we develop a statistical model to investigate, with numerical simulations, the collapse of pebble clouds into planetesimals. We find that low-mass planetesimals, e.g. comets, are porous pebble-piles with this formation mechanism. Paper II and III investigate the role of fragmenting collisions in planetesimal formation, both with laboratory experiments and numerical simulations.The above model assumes an isolated, homogeneous planetesimal. In paper IV we find that the interior structure of comets can vary with depth ('onion'-like shells) and that gas in the protoplanetary disc can affect the collapse significantly.