The first macroscopic bodies in protoplanetary disks are dust aggregates .
We report on a number of experimental studies with dust aggregates formed from micron-size quartz grains .
We confirm in laboratory collision experiments an earlier finding that producing macroscopic bodies by the random impact of sub-mm aggregates results in a well-defined upper-filling factor of 0.31 \pm 0.01 .
Compared to earlier experiments , we increase the projectile mass by about a factor of 100 .
The collision experiments also show that a highly porous dust-aggregate can retain its highly porous core if collisions get more energetic and a denser shell forms on top of the porous core .
We measure the mechanical properties of cm-sized dust samples of different filling factors between 0.34 and 0.50 .
The tensile strength measured by a Brazilian test , varies between 1 kPa and 6 kPa .
The sound speed is determined by a runtime measurement to range between 80 m/s and 140 m/s while Young ’ s modulus is derived from the sound speed and varies between 7 MPa and 25 MPa .
The samples were also subjected to quasi-static omni- and uni-directional compression todetermine their compression strengths and flow functions .
Applied to planet formation , our experiments provide basic data for future simulations , explain the specific collisional outcomes observed in earlier experiments , and in general support a scenario where collisional growth of planetesimals is possible .