Carbon to oxygen ratios in extrasolar planetesimals

TL;DR

This study measures carbon and oxygen ratios in debris from extrasolar planetesimals around white dwarfs, finding no evidence for carbon-rich compositions and suggesting similarities to Solar System materials.

Contribution

First comprehensive analysis of C/O ratios in extrasolar planetesimal debris, revealing a bimodal distribution and constraining the occurrence of carbon-rich systems.

Findings

No carbon-rich planetesimals detected (C/O<0.8).

C/O ratios similar to Solar System objects, bimodal distribution.

Less than 17% of systems are carbon-rich with 2 sigma confidence.

Abstract

Observations of small extrasolar planets with a wide range of densities imply a variety of planetary compositions and structures. Currently, the only technique to measure the bulk composition of extrasolar planetary systems is the analysis of planetary debris accreting onto white dwarfs, analogous to abundance studies of meteorites. We present measurements of the carbon and oxygen abundances in the debris of planetesimals at ten white dwarfs observed with the Hubble Space Telescope, along with C/O ratios of debris in six systems with previously reported abundances. We find no evidence for carbon-rich planetesimals, with C/O<0.8 by number in all 16 systems. Our results place an upper limit on the occurrence of carbon-rich systems at <17 percent with a 2 sigma confidence level. The range of C/O of the planetesimals is consistent with that found in the Solar System, and appears to follow a bimodal distribution: a group similar to the CI chondrites, with log(<C/O>)=-0.92, and oxygen-rich objects with C/O less than or equal to that of the bulk Earth. The latter group may have a higher mass fraction of water than the Earth, increasing their relative oxygen abundance.