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Title: Polluted white dwarfs : insights into ancient extrasolar planetary systems
Author: Harrison, John
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2020
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As the study of rocky exo-planets enters an era of characterisation, the ability to determine how similar a planet is to the Earth, and thus if it is potentially habitable, is fast approaching. An understanding of the geological processes which determine a rocky world’s bulk composition, geodynamics, and subsequent climate is of vital importance to this pursuit. In this thesis, I demonstrate how the pollution of white dwarf stars by rocky exoplanetary material offers an insight into the bulk composition of rocky exo-planets, and thus, the geological processes which occur on them. I present a model which reproduces the atmospheric metal abundances present in polluted white dwarf stars and generates constraints on the origin of the polluting material. Using this model I find that for the majority of white dwarf systems the polluting material is primitive, and thus has a composition consistent with incomplete nebula condensation from a protoplanetary disc. However, this is not the case for all systems as I also find strong evidence for the accretion of core-rich, mantle-rich, crust-rich, and crust-stripped material, supporting the idea that differentiation and collisions are common processes in exo-planetary systems, and crucially, that differentiation processes and geological processes occur in a similar fashion in exo-planetary systems as they do in the Solar System. I also present evidence that white dwarf pollutants have similar masses to large Solar System asteroids and display a diversity in their formation temperatures, ranging from water ice rich comet analogues which formed below 200K to refractory dominated bodies formed at above 1,400 K. Additionally, I show how ancient post-nebula volatilisation processes likely occurred on the pollutant of GD362, that the average accretion event lifetimes are of the order of a few million years, and that thermohaline instabilities likely do not develop in some white dwarf atmospheres. On the whole these results suggest that the rocky worlds which orbit other stars have compositions and geologies that are not dissimilar from the worlds which orbit the Sun. However, in the coming years more observations of polluted white dwarf systems will be required to further test this hypothesis.
Supervisor: Bonsor, Amy Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Exoplanets ; White Dwarf Stars ; Planetary Composition ; Planet Formation