One formula to describe the struggle between planetesimals and gas drag: Insights from microgravity experiments

Wind erosion is a serious threat to the evolution of planetesimals in the early stages of planet formation. Since they move around the star at a different orbital velocity than the surrounding gas, they feel a permanent headwind of at least 50 m/s 1. In the case of eccentric orbits, this difference in velocity can increase significantly. Planetesimals are not solid rocks, but rather a collection of loosely bound dust aggregates 2 that are held together by the low self-gravity. Although the pressure in protoplanetary disks is very low, wind erosion can dismantle planetesimals. As a result, their existence is ruled out in some areas close to the star 3,4.In a series of wind tunnel experiments in the laboratory and on microgravity platforms such as the drop tower Bremen and parabolic flights, we have recreated the conditions on planetesimals more and more realistically. Under pressures down to 1 Pa and gravity down to 10-5g, we observed and analysed wind-induced erosion of granular particles. Starting with simple glass spheres, we finally recreated the surface of a pebble pile planetesimal with millimetre sized SiO2 dust aggregates. These aggregates were produced by the collision of micrometre sized SiO2 particles in analogy to the dust growth up to the bouncing barrier in protoplanetary disks.As a result of our experiments, we found a single formula that predicts the erosion threshold of granular particles on the surface of a planetesimal. Its overall size, the volume and density of the pebbles forming it and its position in the protoplanetary disk can be set as parameters. Together with a suitable disk model, this allows us to define stable and unstable orbits for the evolution of planetesimals into full grown planets.References1 Weidenschilling S. J., 1977, MNRAS, 180, 572 Wahlberg Jansson K., Johansen A., Bukhari Syed M., Blum J., 2017, ApJ, 835, 1093 Cedenblad L., Schaffer N., Johansen A., Mehlig B., Mitra D., 2021, ApJ, 921, 1234 Schnau, L., Teiser, J., Demirci, T., Joeris, K., Onyeagusi, F.C., Fritscher, M., Wurm, G., 2023, AA, 672 A169