Introduction

Rather few facts can be considered as acceptable to all who are working in the field of planet and planetesimal formation. Starting there, we will explore the possible pathways as suggested by experiments. It is certainly undisputed that the regular mode of planet formation is connected to protoplanetary disks. These disks consist mostly of gas, which makes up about 99% of their mass. The remaining 1% resides in the form of dust and – depending on the temperature – in the form of ice. As terrestrial planets are mostly built from heavier elements it is natural to assume that they are somehow assembled from the dust component in the disk.

Whatever model is placed between the dust and the planets, collisions between the solid bodies are unavoidable. In fact a large part of the process of planet formation can be based on collisions which can and (at least partly) will lead to the formation of larger bodies.

In the following sections we will review experiments that have studied these collisions and eventually put these results in a rough sketch of planetesimal formation. It is sometimes argued that collisions of large bodies might be too energetic to lead to the formation of a still larger body (Youdin and Shu, 2002). As described in this chapter it is true that collisions can lead to erosion rather than growth. However, we will show that this is not necessarily so for all collisions.