Measurements of three exo-planetesimal compositions: a planetary core, a chondritic body, and an icy Kuiper belt analogue

The study of planetesimal debris accreted by white dwarfs offers unique insights into the composition of exoplanets. Using far-ultraviolet and optical spectroscopy, we have analysed the composition of planetesimals accreted by three metal enriched H-dominated white dwarfs with effective temperatures of T_eff = 20 000 K. WD 0059+257 is accreting an object composed of 71.8 +/- 7.9 per cent Fe and Ni by mass, indicating a large core mass fraction of 69 per cent, similar to that of Mercury. We model this planetesimal as having a differentiated Earth-like composition with 65 per cent of its mantle stripped, and we find this mass loss can be caused by vaporisation of the planetesimal's mantle during post-main sequence evolution. The tentative S detection in WD 0059+257 is a possible clue to the nature of the light element in planetary cores, including that of the Earth. The volatile-rich composition of WD 1943+163 is consistent with accretion of a carbonaceous chondrite-like object, but with an extreme Si depletion. WD 1953-715 accretes a planetesimal which contains 64 +/- 21 per cent of O in the form of ices, likely H2O. This body therefore requires an initial orbit at formation beyond a radial distance of > 100 au for ice survival into the white dwarf phase. These three planetary enriched white dwarfs provide evidence of differing core fractions, volatile budgets, and initial orbital separations of the accreted planetesimals, all of which help us understand their formation and evolutionary history.