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.
@article{arxiv.1301.5562,
title = {The Elemental Compositions of Extrasolar Planetesimals},
author = {M. Jura},
journal= {arXiv preprint arXiv:1301.5562},
year = {2015}
}
Comments
10 pages, 3 figures, review presented at IAU Symposium No. 293, Beijing, August 2012, Formation, Detection and Characterization of Extrasolar Habitable Planets