The bulk composition of exo-planets

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. Mass and radius measurements of transiting planets yield bulk densities, from which interior structures and compositions can be deduced (Valencia et al. 2010). However, those results are model-dependent and subject to degeneracies (Rogers & Seager 2010; Dorn et al. 2015). Transmission spectroscopy can provide insight into the atmospheric compositions (Sing et al. 2013; Deming et al. 2013), though cloud decks detected in a number of super earths systematically limit the use of this method (Kreidberg et al. 2014). 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.