  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.
