The formation of planets around our own Sun and around other stars begins with dust-sized particles sticking together and gradually forming larger and larger objects. The stage of growth from pebble-sized objects to km-scale objects may follow one or more of several different routes, depending on the distance of the objects from the star and the age of the star. This project will experimentally study a broad range of collisions between pebble-sized clumps of dust, ice and rock to determine the conditions that lead to growth of larger objects. The results of these experiments will help resolve fundamental questions about a critical stage in planet formation, including the role that water ice and other biologically important molecules play in the sticking process. Many of the experiments will take place in a microgravity or free-fall environment in order to replicate the conditions in the early solar system. Video data of the collisions will be analyzed to understand the outcomes of similar collisions in the early stages of planet formation. Undergraduate and graduate students will participate in the experiments, and the video data will be shared through public websites, allowing the general public, students, and researchers to visualize this critical stage in the history of planetary systems.
The proposed activity is an experimental exploration of low-energy collisions in the protoplanetary disk to better understand the collisional evolution of pebble-sized (cm-scale) aggregates and solids, the initial building blocks of planets. The experiments will extend the current database in parameter space by including icy particulates mixed in with silicates to study the accretion efficiency in the outer solar system (beyond the frost line where water vapor condensed) and by studying collisions between large-scale aggregates at modest impact speed both with and without ice particles in the aggregates. These pebbles may grow through pairwise collisional accretion or participate in local gravitational instabilities to form larger km-scale planetesimals. 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). The proposed experiments would add much-needed data to these models to help determine the conditions under which accretional growth can produce planetesimals and the outcomes of collisions between pebbles in various nebular environments. The proposed work will involve a team of several undergraduate students and one graduate student in the design, operation, and interpretation of the experiments as well as publication of results. The experimental data consist of high speed videos that will be shared with K-12 educators and the broader research community through the public website microgravity.physics.ucf.edu.
