Rocky Extrasolar Planetary Compositions Derived from Externally-Polluted White Dwarfs

TL;DR

This study analyzes the compositions of exoplanetary material accreted onto white dwarfs, revealing similarities and differences with Earth's bulk composition, and supports the idea that some rocky bodies formed inside snow-lines with little ice.

Contribution

First detailed abundance analysis of polluted white dwarfs showing their accreted material's similarity to terrestrial planets, aiding understanding of exoplanet compositions.

Findings

Accreted mass comparable to large asteroids (~10^22 g).

Some white dwarf pollutants have compositions similar to Earth's bulk material.

Evidence suggests some parent bodies formed inside snow-lines with minimal ice.

Abstract

We report Keck High Resolution Echelle Spectrometer data and model atmosphere analysis of two helium-dominated white dwarfs, PG1225-079 and HS2253+8023, whose heavy pollutions most likely derive from the accretion of terrestrial-type planet(esimal)s. For each system, the minimum accreted mass is ~10^22 g, that of a large asteroid. In PG1225-079, Mg, Cr, Mn, Fe and Ni have abundance ratios similar to bulk Earth values, while we measure four refractory elements, Ca, Sc, Ti and V, all at a factor of ~2-3 higher abundance than in the bulk Earth. For HS2253+8023 the swallowed material was compositionally similar to bulk Earth in being more than 85% by mass in the major element species, O, Mg, Si, and Fe, and with abundances in the distinctive proportions of mineral oxides - compelling evidence for an origin in a rocky parent body. Including previous studies we now know of four heavily polluted white dwarfs where the measured oxygen and hydrogen are consistent with the view that the parents bodies formed with little ice, interior to any snow-line in their nebular environments. The growing handful of polluted white dwarf systems with comprehensive abundance measurements form a baseline for characterizing rocky exoplanet compositions that can be compared with bulk Earth.