Comet Formation in the Framework of Streaming Instability

The bodies of the solar system formed 4.6 Gyr ago in the protoplanetary disk around the young protosun. Starting with submicrometre-sized dust and ice grains, collisions and coalescence led to the formation of aggregates, planetesimals, and eventually planets. Comets are believed to be the kilometre-sized icy planetesimals that were not accreted into larger bodies and have survived until today. It is poorly understood how bodies manage to grow to sizes larger than about millimetre to decimetre because bouncing and fragmentation terminate growth well below kilometres. Streaming instability arising from the coupling between the gas and millimetre- to decimetre-sized dust aggregates via drag is a promising mechanism to bridge this gap. The instability produces locally high dust densities which collapse due to self-gravity and form planetesimals in the size range kilometres to a few hundred kilometres. Planetesimal formation through streaming instability predicts objects with characteristic properties of comets. This thesis investigates the formation of comets in the framework of the streaming instability. Aggregate growth in the solar nebula is modelled to find the specific properties of the aggregates that eventually trigger the streaming instability. Simulations of the gravitational collapse of a cloud of porous aggregates (pebble cloud) are conducted to address the questions how aggregate properties change during the collapse and whether or not the resulting planetesimal has the properties of a comet.