  We present a detailed study of the metal-polluted DB white dwarf SDSS
J0845+2257 (Ton 345). Using high-resolution HST/COS and VLT spectroscopy, we
have detected hydrogen and eleven metals in the atmosphere of the white dwarf.
The origin of these metals is almost certainly the circumstellar disc of dusty
and gaseous debris from a tidally-disrupted planetesimal, accreting at a rate
of 1.6E10 gs^-1. Studying the chemical abundances of the accreted material
demonstrates that the planetesimal had a composition similar to the Earth,
dominated by rocky silicates and metallic iron, with a low water content. The
mass of metals within the convection zone of the white dwarf corresponds to an
asteroid of at least ~130-170 km in diameter, although the presence of ongoing
accretion from the debris disc implies that the planetesimal was probably
larger than this. While a previous abundance study of the accreted material has
shown an anomalously high mass fraction of carbon (15 percent) compared to the
bulk Earth, our independent analysis results in a carbon abundance of just 2.5
percent. Enhanced abundances of core material (Fe, Ni) suggest that the
accreted object may have lost a portion of its mantle, possibly due to stellar
wind stripping in the asymptotic giant branch. Time-series spectroscopy reveals
variable emission from the orbiting gaseous disc, demonstrating that the
evolved planetary system at SDSS J0845+2257 is dynamically active.
