Growing up is hard to do, especially for
baby planets. Now, scientists may have uncovered the solution to one puzzle
about protoplanetary growing pains.

An obstacle to planetary formation,
known as the bouncing barrier, hinders the clumping of dust particles that
eventually form planets. But electric charge can provide extra stickiness that those
cosmic motes need
for clumps to keep
growing, scientists report December 9 in Nature
Physics
. Testing that explanation required vigorously shaking thousands of
small glass beads and catapulting them more than 100 meters skyward in an
attempt to mimic planets’ birthplaces, protoplanetary disks.

In the pancakes of dust and gas known as
protoplanetary disks, the seeds of planets collide and stick, forming larger and
larger clumps. But, according to experiments and simulations, once particles are
a millimeter or so in size, their growth stalls as they bounce off one another,
rather than sticking. It’s a quandary that has stymied attempts to simulate how
planets form.

Somehow, the dust particles overcome the
bouncing barrier, resulting in a cosmos peppered with a wide variety of worlds (SN: 1/8/19).
“We see exoplanets, so there must be a way to get bigger particles,” says
experimental astrophysicist Tobias Steinpilz of the University of
Duisburg-Essen in Germany.

So Steinpilz and colleagues set out to
form analogs of planetary seeds. Instead of protoplanetary dust grains, the
researchers used glass beads, each a bit less than half a millimeter in
diameter. Collisions between those beads would mimic colliding dust particles
in the protoplanetary disk. But there was one catch: Earth’s gravity. “That
overpowers everything we want to see,” Steinpilz says.

glass beads
Identical glass beads clump together due to their electric charges, as shown in a simulation (top) and an experiment (bottom).T. Steinpilz et al/Nature Physics 2019

So the researchers launched their experiment
with a catapult inside the 120-meter-tall Bremen Drop Tower in Germany, letting the apparatus containing the beads, a camera and
other measurement equipment, fly upward and back down. During its approximately
nine-second flight, the device was effectively weightless.

Prior to the launch, the researchers
shook the beads, mimicking the collisions that particles in a protoplanetary
disk would experience over time. That movement caused the beads to build up electric
charges, some negative and some positive. When the beads went weightless, they formed
clumps — some consisting of over a thousand beads — thanks to electric forces
between the charged beads, the researchers determined.

The results “clearly show that electrostatic
forces help grow beyond the bouncing barrier in lab conditions,” says
astronomer Richard Booth of the University of Cambridge. But, he notes, “there
is a question of trying to extrapolate these lab conditions to what we see in
protoplanetary disks.” In particular, protoplanetary disks consist of dust
grains made of natural materials rather than glass.

Steinpilz’s team also performed shaking
experiments with basalt spheres, which are more similar to the particles in a
real protoplanetary disk. Basalt particles charged up even more than the glass
beads, the team found, suggesting that the effect might be even stronger in
protoplanetary disks.

Other barriers remain for developing planets,
though. High-speed particles, for example, can collide and break larger clumps apart,
so growing up still takes grit.