Collisions of Super-Earth and Sub-Neptune Planets.

Abstract

More than half of the planets so far discovered have masses between Earth and
Neptune. These ‘Super-Earth’ planets have a wide array of densities. These planets
are sufficiently massive to be able to accrete significant low density hydrogen atmospheres during formation. Post-formation they may experience density enriching erosive processes. One such erosive process is collision. After proto-planetary disc dissipation planet orbits can become unstable, leading to a period of giant impacts. These collisions preferentially eject lighter material, increasing the planet’s density.

An example of a system with planets which experienced collisional density enrichment is Kepler-107. Kepler-107c is substantially denser than its closer orbiting neighbour Kepler-107b (12.65 g cm³ as opposed to 5.3 g cm³), despite probable similar formation environments. Other erosive phenomena, e.g. photo-evaporation, are unlikely as they would affect Kepler-107b more strongly. In chapter 3 of this thesis I present simulations that show collisions can produce Kepler-107c’s enriched density.

In the rest of this thesis I examine more general simulations of Super-Earth collisions, focusing specifically on atmospheres. I show the boundary between planets merging together and bouncing off one another strongly correlates with the escape velocity from the point of closest approach. In general I find, while it requires little energy to cause some of the atmosphere to be ejected, total atmosphere ejection requires sufficient energy that the collision will also eject a significant fraction of mantle. Due to the ease of atmosphere removal, I find that all simulated collisions result in a change in both mass and composition, resulting in a corresponding increase in final planet density. These results underline the importance of giant impacts in explaining the observed Super-Earth density diversity.

Date of Award	25 Jan 2022
Original language	English
Awarding Institution	University of Bristol
Supervisor	Zoe M Leinhardt (Supervisor) & Philip Carter (Supervisor)