An absolute sodium abundance for a cloud-free 'hot Saturn' exoplanet

Broad absorption signatures from alkali metals, such as the sodium (Na I) and potassium (K I) resonance doublets, have long been predicted in the optical atmospheric spectra of cloud-free irradiated gas giant exoplanets(1-3). However, observations have revealed only the narrow cores of these features rather than the full pressure-broadened profiles(4-6). Cloud and haze opacity at the day-night planetary terminator are considered to be responsible for obscuring the absorption-line wings, which hinders constraints on absolute atmospheric abundances(7-9). Here we report an optical transmission spectrum for the 'hot Saturn' exoplanet WASP-96b obtained with the Very Large Telescope, which exhibits the complete pressure-broadened profile of the sodium absorption feature. The spectrum is in excellent agreement with cloud-free, solar-abundance models assuming chemical equilibrium. We are able to measure a precise, absolute sodium abundance of log epsilon(Na) = 6.9(-0.4)(+0.6), and use it as a proxy for the planet's atmospheric metallicity relative to the solar value (Z(p)/Z(circle dot) = 2.3(-1.7)(+8.9)). This result is consistent with the mass-metallicity trend observed for Solar System planets and exoplanets(10-12).