  Planetary systems that orbit white dwarf stars can be studied via
spectroscopic observations of the stars themselves. Numerous white dwarfs are
seen to have accreted mostly rocky minor planets, the remnants of which are
present in the stellar photospheres. The elemental abundances in the
photospheres unveil the bulk compositions of the accreted parent bodies with a
precision far greater than can be attained with any other technique currently
available to astronomers. The most significant discovery, overall, is that
rocky extrasolar planets have bulk elemental compositions similar to those of
Earth and other rocky objects in our solar system. The white dwarf studies
reveal that many extrasolar minor planets (asteroids) are differentiated,
possessing analogs of terrestrial crust, mantle and core; this finding has
important implications for the origin of our own solar system.
