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

TL;DR

This study demonstrates experimentally that electrostatic charging significantly enhances grain coagulation, enabling growth into centimeter sizes, which is crucial for planetesimal formation in protoplanetary disks.

Contribution

The paper provides the first direct experimental evidence that charge-driven coagulation can produce centimeter-sized aggregates, surpassing previous growth limitations.

Findings

Charged grains grow up to 5 cm in microgravity experiments.

Neutral grains stop coagulating at sub-millimeter sizes.

Charge-driven growth exceeds critical sizes for planetesimal formation.

Abstract

Collisions electrically charge grains which promotes growth by coagulation. We present aggregation experiments with three large ensembles of basalt beads ($150\,\mu\mathrm{m} - 180\,\mu\mathrm{m})$, two of which are charged, while one remains almost neutral as control system. In microgravity experiments, free collisions within these samples are induced with moderate collision velocities ($0 - 0.2 \,\mathrm{m\,s}^{-1}$). 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.