A Smoking Gun for Planetesimal Formation: Charge-driven Growth into a New Size Range

Collisions electrically charge grains, which promotes growth by coagulation. We present aggregation experiments with three large ensembles of basalt beads (150-180 mu m), two of which are charged, while one remains almost neutral as a control system. In microgravity experiments, free collisions within these samples are induced with moderate collision velocities (0-0.2 m s(-1)). In the control system, coagulation stops at (sub-)mm size while the charged grains continue to grow. A maximum agglomerate size of 5 cm is reached, limited only by bead depletion in the free volume. For the first time, charge-driven growth well into the centimeter range is directly proven by experiments. In protoplanetary disks, this agglomerate size is well beyond the critical size needed for hydrodynamic particle concentration as, e.g., by the streaming instabilities.

Automated summary

This experiment demonstrates that charged particles can grow through collisions and coagulation, ultimately reaching a critical size for potential hydrodynamic particle concentration. By comparing neutral and charged particles, it is shown that charged particles can continue to grow into centimeter-size agglomerates, surpassing the critical size for hydrodynamic concentration.