In protoplanetary disks, solid objects (so-called planetesimals) are formed from dust. Micrometre-sized dust grains grow into millimetre-sized aggregates. Once those aggregates have diameters exceeding a few centimetres, they become subject to concentration mechanisms such as the streaming instability, permitting the formation of self-gravitating clusters, which might eventually collapse into kilometre-sized planetesimals. However, for the streaming instability to set in, clumps spanning sizes from centimetres to decimetres are required in the centre of a protoplanetary disk. In the size range between millimetres and centimetres, aggregates bounce off each other rather than sticking together, and growth is stalled. Here we show in microgravity experiments that collisions between millimetre-sized grains lead to sufficient electrical charging for aggregation to bridge this gap between the bouncing barrier and the onset of the streaming instability. We computationally simulate aggregation and find that models agree with the experimental data only if electrical charging is present. We therefore propose that collisional charging may promote early growth in the size gap that current models of planetesimal formation cannot account for.

在原行星盘中，固体物体（所谓的小行星）是由灰尘形成的。微米级的尘粒会长成毫米级的聚集体。一旦这些聚集体的直径超过几厘米，它们就会受到诸如流不稳定之类的集中机制的影响，从而允许形成自重星团，最终可能塌陷成几千米大小的小行星。但是，要引起流不稳定，在原行星盘的中心需要大小从厘米到分米的团块。在毫米到厘米之间的大小范围内，聚集体彼此反弹而不是粘在一起，并且生长停滞。在这里，我们在微重力实验中表明，毫米大小的晶粒之间的碰撞会导致足够的电荷聚集，从而弥合反弹势垒与流动不稳定性开始之间的间隙。我们通过计算模拟聚集，发现只有在存在电荷的情况下，模型才与实验数据相符。因此，我们提出碰撞充电可能会促进尺寸差距的早期增长，而当前的小行星形成模型无法解释这种差距。