Planet Formation

Terrestrial planets like Earth, Mars or Venus start out as dust particles in protoplanetary disks, flat objects of gas and dust around young forming starts (s. image to the right, HL Tau, ALMA Partnership et al. 2015, ApJ, 808, 10). These dust particles collide with each other, initially stick together and grow to larger aggregates. However, there are several barriers on the way to large bodies. These barriers and processes to jump over them are part of our experimental research (s. image below, Wurm and Teiser 2021, Nature Reviews Physics, 3, 405).

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Limits of simple hit-and-stick growth

The regular van der Waals forces between dust grains are relatively small. At some point, they are no longer sufficient to hold growing aggregates together after a collision. This limit is at about 1 millimeter. It is possible to go beyond without collisions by concentrating aggregates to the limit of gravitational collapse into planetesimals. However, this usually requires particles larger than the 1 millimeter limit. As the conditions in disks vary quite significantly in different locations, one might ask which influence high and low temperatures or electric and magnetic fields might have on the limiting particle size. The animation to the right from an experimental study by Kruss and Wurm (2018, ApJ, 869, 45) e.g. shows an example where iron rich dust aggregates form larger clusters as a magnetic field is turned on. This might e.g. set the bias to form iron rich planets like Mercury, especially at the Curie-line (Bogdan et al. 2023, A&A, 670, A6). Also, particles at this closer distance to a star loose all water from their surface, which makes them much stickier. Further out, ices dominate.

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A boost in growth by tribocharging

Outstanding process with the potential to shift the limits of growth strongly is tribocharging. As two particles collide, they regularly exchange charges. This way an ensemble of neutral particles can evolve into an ensemble of charged particles with a complex charge pattern on their surfaces. Like in a salt crystal, the attractive parts of the Coulomb forces can bind particles in aggregates much stronger now. This way, stable clusters of particles form. The image to the right shows an example of particles that were tribocharged and then grew into a single large cluster (Teiser et al. 2021, ApJL, 908, L22).

These studies regularly rely on microgravity (e.g. in the drop tower in Bremen) and are currently funded by the DLR space administration with funds provided by the Federal Ministry of Economic Affairs and Climate Action. In this context we also carried out an experiment on a suborbital flight with longer duration of microgravity, supported by ESA. The electrostatic growth phase can shift the Limits into the centimeter or decimeter range of cluster sizes.

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Ionisation of protoplanetary disks

Quite a number of processes in protoplanetary disks – from chemistry to the motion of molecules interacting with the disk‘s magnetic fields depend on the degree of ionization of the disk. This depends on the temperature and high energy radiation. However, and this is a rather new development, it also depends on the collisions of solid particles. As outlined above, grains exchange charge in collisions but this charge transfer can include the surrounding gas phase and charge the gas. We currently study these processes in laboratory experiments.

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Erosion of small bodies in protoplanetary disks

A planetesimal of km-size, formed by the sand sized grains mentioned above, are not safe yet. They are still very fragile as self-gravity is still weak on their surface. All processes that can remove particles can lead to their destruction. An all time classics is wind, which also acts in protoplanetary disks. It is more extreme though at low ambient pressure with high flow velocities. We study such erosion in microgravity experiments funded by the DLR space administration with funds provided by the Federal Ministry for Economic Affairs and Climate Action and in dedicated low pressure wind tunnels. Also electrostatic fields, occuring naturally in the setting given above, can disassemble a body (Onyeagusi et al. 2024).

Lifting grains on Mars

Erosion processes are important for planetary surfaces. However, the conditions for erosion mechanisms are not the same on all planets. Mars, e.g., has only a thin atmosphere and the dynamical pressure of wind is relatively low. Still, the planet can enshroud itself in a global dust cloud. In this context, we study how „thermal creep“ an effect occuring within the pores of the Martian soil, kan lead to the lifting of particles. Again, we carry out ground based laboratory experiments and expeirments under microgravity or rather Martian gravity funded by the DLR space administration with funds provided by the Federal Ministry for Economic Affairs and Climate Action.

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Particle Levitation

We discovered that temperature gradients over dust particles may force therm to levitate at low pressures. Thermal creep forces (Knudsen compressor) lead to an overpressure below the particle. In 1909 Knudsen already found that temperature gradients may lead to creeping of gas along non uniformly heated tubes. The pores within a dust particle aggregate can be interpreted as a collection of micro-tubes. A gas soakage through the particle lets the particle hover. We perform leviation experiments at high (800 K) but also at low (70 K) temperatures to study e.g. collisions or photophoresis.

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