A persistent question in exoplanet demographics is whether exoplanetary
systems form from similar compositional building blocks to our own. Polluted
white dwarf stars offer a unique way to address this question as they provide
measurements of the bulk compositions of exoplanetary material. We present a
statistical analysis of the rocks polluting oxygen-bearing white dwarfs and
compare their compositions to rocks in the Solar System. We find that the
majority of the extrasolar rocks are consistent with the composition of typical
chondrites. Measurement uncertainties prevent distinguishing between chondrites
and bulk Earth, but do permit detecting the differences between chondritic
compositions and basaltic or continental crust. We find no evidence of crust
amongst the polluted white dwarfs. We show that the chondritic nature of
extrasolar rocks is also supported by the compositions of local stars. While
galactic chemical evolution results in variations in the relative abundances of
rock-forming elements spatially and temporally on galaxy-wide scales, the
current sample of polluted white dwarfs are sufficiently young and close to
Earth that they are not affected by this process. We conclude that exotic
compositions are not required to explain the majority of observed rock types
around polluted white dwarfs, and that variations between exoplanetary
compositions in the stellar neighborhood are generally not due to significant
differences in the initial composition of protoplanetary disks. Nonetheless,
there is evidence from stellar observations that planets formed in the first
several billion years in the Galaxy have lower metal core fractions compared
with Earth on average.Comment: Accepted to PS