Planetesimals are the building blocks of planets. Asteroids and comets are leftover planetesimals from the time of formation of our own solar system. The formation of km-sized or larger planetesimals remains an open problem in planet formation theories. Once objects are larger than ~10 km, gravity helps these objects grow into planets. Condensation and electrostatic surface forces can explain the growth of mm to cm-sized objects in the nebula. These “pebbles” may grow through collisions or create local gravitational instabilities to form larger planetesimals. Both gravitational instability, which forms planetesimals directly through local collapse of patches in the disk, and pairwise accretional growth of particles face difficulties producing planetesimals in the protoplanetary disk environment as it is currently understood. It is possible that some combination of these processes took place, depending on the local conditions in the protoplanetary nebula. A major source of uncertainty in the accretional growth model is the behavior of small objects and aggregates of dust colliding at the low speeds expected (~0.1 – 10 m/s). We study these collisions and the formation of planetesimals through experiments and numerical simulations.
This image of the comet 67P/Churyomov-Gerasimenko was taken by the European Space Agency’s Rosetta mission. The comet is a leftover planetesimal from the origin of the solar system. Its ice sublimates when it nears the Sun. Objects such as this are the leftover building blocks of planets. Their formation is an active area of research. Credit: European Space Agency.