It takes millions of years for a micrometer-sized dust grain to become a planet with a diameter of 10,000 kilometers. It all begins in a disc-shaped cloud of gas (99%) and dust (1%), the protoplanetary disc: here, the dust particles collide and form agglomerates. Clouds of these agglomerates eventually collapse into larger bodies, called planetesimals, which can already have a diameter of one to one hundred kilometers. Through gravity, the planetesimals attract further matter, grow into protoplanets and later into full-fledged planets.

During the processes in the disk, the particles override a collision barrier. “Actually, dust grains larger than about one millimeter cannot grow at all because they either bounce off each other or break apart,” explain astrophysicists Prof. Dr. Gerhard Wurm and PD Dr. Jens Teiser. ”But because they keep colliding, they become charged differently and then attract each other.”

The team had already observed adhesion due to electrostatic charge in previous drop tower experiments. Because only about nine seconds of measurement time in microgravity were possible there, they were unable to examine the final size and stability of the growing bodies. The experiments in the current study were conducted quite differently: they took place during the suborbital flight of a sounding rocket of the European Space Agency (ESA). “While the rocket climbed to and returned from an altitude of 270 kilometers, it offered us six minutes of microgravity to control and monitor our experiments from the ground,” says Teiser.

The UDE team was thus able to directly observe the growth of compact agglomerates of about three centimeters in size and to measure exactly the maximum speed at which individual particles may collide without destroying anything. “The agglomerates are so stable that they can withstand the bombardment of individual particles at up to 0.5 meters per second. Anything faster erodes them,” emphasizes astrophysicist Wurm. ”In addition, we have carried out numerical simulations that show that the collisions do indeed result in a strong electrostatic charge and attraction.”

“We were surprised to find such specific speeds for erosion,” adds Teiser. ‘Especially since they are close to the values used in previous simulations for fragmentation, i.e. the breaking up of particles or objects.” This means that the physical conditions are similar to those under which material in the disc-shaped cloud around a young star is eroded or broken up.

The results of the UDE team are incorporated into physical models of protoplanetary disks and particle growth and thus help to understand the details of planet formation.

The research was funded by the German Aerospace Center (DLR) and the Federal Ministry for Economic Affairs and Climate Action.