The Elemental Compositions of Extrasolar Planetesimals

TL;DR

Most polluted white dwarfs accrete asteroid-like bodies with compositions similar to Earth's crust, rich in oxygen, magnesium, silicon, and iron, but generally deficient in carbon and water, indicating rocky, terrestrial-like planetesimals.

Contribution

This study provides the first compositional analysis of extrasolar planetesimals, revealing their similarity to Earth's bulk composition and their typical deficiency in carbon and water.

Findings

Extrasolar planetesimals are at least 85% composed of O, Mg, Si, and Fe.

Carbon is usually less than 1% of the mass, often deficient compared to the Sun.

Less than 1% water by mass is found in the accreted extrasolar asteroids.

Abstract

Evidence is now compelling that most externally-polluted white dwarfs derive their heavy atoms by accretion from asteroids - the building blocks of rocky planets. Optical and ultraviolet spectroscopy of a small sample of suitable white dwarf stars shows that to zeroth order, the accreted extrasolar parent bodies compositionally resemble bulk Earth. (1) Extrasolar planetesimals are at least 85% by mass composed of O, Mg, Si and Fe. (2) Compared to the Sun, C is often deficient, usually by at least a factor of 10 and therefore comprises less than 1% of an extrasolar planetesimal's mass. At least to-date, C has never been found to be enhanced as would be expected if carbon-rich planetesimals have formed. (3) While there may be individual exceptions, considered as a whole, the population of extrasolar asteroids accreted onto a well-defined sample of local white dwarf stars is less than 1% water by mass.