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Searching for signs of geological and biological evolution in exoplanetary systems using white dwarfs
Lead Supervisor: Amy Bonsor, Institute of Astronomy
Co-supervisors: Craig Walton, Department of Earth Sciences/ETH Zurich; Laura Rogers, Institute of Astronomy

Brief summary
Probing exogeology and signatures of biology in planetary material accreted by white dwarfs. Bulk abundances of planetary material seen in the atmospheres of some white dwarfs indicate geological process, including notably core-mantle-crustal differentiation. Key elemental species such as Fe, Ca, Si, Mg indicate the nature of the bodies. Trace species such as Li, Ni, Cr, Mn, P will be used to probe in further detail the evolutionary state of the material. In particular, the project will assess the ability of white dwarfs to probe the P content of planetary crust, seen to increase during Earth’s history due to the presence of complex life.

Importance of the area of research concerned
Although complex life clearly exists on Earth, the exact pathway to its existence is yet to be fully understood. Exoplanets provide the perfect opportunity to study what happened in our history that provided a safe haven for life. 

Although we now detect many rocky exoplanets, planetary material in the atmospheres of white dwarfs provides a unique means to probe the geological evolution of such planets. Spectroscopy reveals the bulk elemental composition of exoplanets, including Ca, Mg, Fe, P, C, S, Ni, Li etc. White dwarfs provide clear evidence of iron-core and crustal formation. 

This project focuses on the crustal reservoir as a unique probe of the geological conditions required for life. The project will investigate what can be uncovered by white dwarf observations, including potential signatures of the presence of biology. For example, the continental crust’s bulk Phosphorus (P) underwent a 3-fold enrichment following the evolution of animal life on Earth. 

What will the student do?
The project will be split into two parts: firstly analysing the range of elemental abundances available from white dwarfs presented in the literature, including objects such as NLTT 43806, which show evidence for the accretion of crustal material. Secondly making predictions for future observations with the capacity to detect particular geochemical signatures, including those related to the presence of biology. 

The student will create forward models predicting the composition of crustal material, based on various initial conditions and geochemical scenarios. These will be incorporated into existing models that aim to find the most likely explanation for the elemental abundances seen in white dwarf atmospheres, based on Bayesian analysis. The models will be used to determine an observational strategy best suited to exploring the evolution of crustal material. 

Strong numerical and computational skills, most likely from a physics, maths or geosciences background would be an advantage.

Walton C.~R., Hao J., Huang F., Jenner F.~E., Williams H., Zerkle A.~L., Lipp A., et al., 2023, Evolution of the crustal phosphorus reservoir, SciA, 9, eade6923. doi:10.1126/sciadv.ade6923
Buchan A.~M., Bonsor A., Shorttle O., Wade J., Harrison J., Noack L., Koester D., 2022, Planets or asteroids? A geochemical method to constrain the masses of White Dwarf pollutants, MNRAS, 510, 3512. doi:10.1093/mnras/stab3624
Jura M., Klein B., Xu S., Young E.~D., 2014, A Pilot Search for Evidence of Extrasolar Earth-analog Plate Tectonics, ApJL, 791, L29. doi:10.1088/2041-8205/791/2/L29

Requirements as to the educational background of candidates that would be suitable for the project
Strong numerical and computational skills, most likely from a physics, maths or geosciences background.

You can find out about applying for this project on the Leverhulme Centre for Life in the Universe widening participation PhD Studentships page.