Using White dwarfs to dissect planetary material and reveal core formation

Polluted white dwarfs that have accreted planetary material provide a unique opportunity to probe the interiors of exoplanetary bodies. The compositions of these bodies reveal key geological processes that operate in exoplanetary systems. Core-mantle differentiation, the process by which iron cores form, is of particular interest due to its wide ranging implications for interior structure and habitability. Bayesian modelling of a sample of white dwarfs shows that core-mantle differentiation is invoked for a significant proportion of pollutants, implying that it is a common process in exoplanetary systems. We can also use Bayesian modelling to infer the conditions under which differentiation takes place, including the mass of the differentiated body. Bodies with larger masses have higher internal pressures. This affects the distribution of Ni, Cr and Si between core and mantle, and hence the composition of any fragments derived from these bodies. By modelling core formation, we can therefore use detections of Ni, Cr and Si in white dwarfs to infer mass. We use this novel method to find 3 systems whose abundances are best explained by the accretion of fragments of small parent bodies, and 2 systems which imply accretion of fragments of Earth-sized bodies. This provides evidence that core-mantle differentiated bodies in exoplanetary systems span a range of masses.