Water Fractions in Extrasolar Planetesimals

TL;DR

This study uses polluted white dwarfs to estimate the water content of extrasolar planetesimals, finding most are dry and supporting the significance of snow lines in planet formation.

Contribution

It provides the first comprehensive analysis of water fractions in extrasolar planetesimals using a large sample of polluted white dwarfs.

Findings

Most extrasolar planetesimals are dry, with little water content.

The population of polluted white dwarfs supports the importance of snow lines in planet formation.

Uncertainties in accretion history can complicate interpretations of water content.

Abstract

With the goal of using externally-polluted white dwarfs to investigate the water fractions of extrasolar planetesimals, we assemble from the literature a sample that we estimate to be more than 60% complete of DB white dwarfs warmer than 13,000 K, more luminous than 3 ${\times}$ 10$^{-3}$ L$_{\odot}$ and within 80 pc of the Sun. When considering all the stars together, we find the summed mass accretion rate of heavy atoms exceeds that of hydrogen by over a factor of 1000. If so, this sub-population of extrasolar asteroids treated as an ensemble has little water and is at least a factor of 20 drier than CI chondrites, the most primitive meteorites. In contrast, while an apparent "excess" of oxygen in a single DB can be interpreted as evidence that the accreted material originated in a water-rich parent body, we show that at least in some cases, there can be sufficient uncertainties in the time history of the accretion rate that such an argument may be ambiguous. Regardless of the difficulty associated with interpreting the results from an individual object, our analysis of the population of polluted DBs provides indirect observational support for the theoretical view that a snow line is important in disks where rocky planetesimals form.