  We utilize observations of 16 white dwarf stars to calculate and analyze the
oxidation states of the parent bodies accreting onto the stars. Oxygen
fugacity, a measure of overall oxidation state for rocks, is as important as
pressure and temperature in determining the structure of a planet. We find that
most of the extrasolar rocky bodies formed under oxidizing conditions, but
approximately 1/4 of the polluted white dwarfs have compositions consistent
with more reduced parent bodies. The difficulty in constraining the oxidation
states of relatively reduced bodies is discussed and a model for the
time-dependent evolution of the apparent oxygen fugacity for a hypothetical
reduced body engulfed by a WD is investigated. Differences in diffusive fluxes
of various elements through the WD envelope yield spurious inferred bulk
elemental compositions and oxidation states of the accreting parent bodies
under certain conditions. The worst case for biasing against detection of
reduced bodies occurs for high effective temperatures. For moderate and low
effective temperatures, evidence for relatively reduced parent bodies is
preserved under most circumstances for at least several characteristic
lifetimes of the debris disk.
