  Priorities in exo-planet research are rapidly moving from finding planets to
characterizing their physical properties. Of key importance is their chemical
composition, which feeds back into our understanding of planet formation. For
the foreseeable future, far-ultraviolet spectroscopy of white dwarfs accreting
planetary debris remains the only way to directly and accurately measure the
bulk abundances of exo-planetary bodies. The exploitation of this method is
limited by the sensitivity of HST, and significant progress will require a
large-aperture space telescope with a high-throughput ultraviolet spectrograph.
