Where are the Extrasolar Mercuries?

TL;DR

This study analyzes the oxidation states of rocky bodies accreting onto white dwarfs, revealing most formed under oxidizing conditions, with some evidence of reduced bodies, and discusses the challenges in accurately determining their compositions.

Contribution

It introduces a method to assess oxidation states of extrasolar rocky bodies using white dwarf observations and models their evolution, highlighting the prevalence of oxidizing conditions and detection biases.

Findings

Most extrasolar rocky bodies formed under oxidizing conditions.

Approximately 25% of polluted white dwarfs have reduced parent bodies.

Detection biases favor oxidized bodies at high temperatures.

Abstract

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.