The role of giant impacts in the formation of the outer solar system and exosystems

Giant impacts (collisions between planet-sized bodies) are the most violent events in planet formation. For a few hours, collisions that form greater than Earth-mass bodies release more energy than the Sun. Large fractions of the icy and/or rocky mantles of the colliding bodies are melted and vaporised, and the huge torques exerted can leave the post-impact body rapidly rotating.

The mass of the largest body may either increase or decrease, depending on the amount of material ejected from the system. Impacts can fundamentally alter the trajectory of a planet's evolution, as the ratios of atmosphere, crust, mantle and core all change and systems of moons can be formed. For example, giant impacts are thought to be responsible for the high obliquity of Uranus, the high density of Mercury, and the formation of Earth's Moon.

This project will explore how collisions shape the planets in our solar system and in exosystems. Using state-of-the-art giant impact simulations, the Lucy will investigate the range of dynamic and thermodynamic outcomes of collisions between water-rich bodies, and develop 'scaling laws' that relate the outcomes to the impact parameters. Although generally a common occurrence, the rates and energies of impacts vary between different planet formation models.

By analysing the frequency and parameters of impacts in each scenario, Lucy will ascertain how giant impacts would sculpt planetary systems in different cases. Furthermore, by determining the observational signatures of post-impact bodies and synestias, the project will aim to provide the tools necessary to identify synestias, and other post-impact bodies, in other solar systems.