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 exoplanets1,2,3. However, observations have only revealed the narrow cores of these features rather than the full pressure-broadened profiles4-6. Cloud and haze opacity at the day-night planetary terminator are considered responsible for obscuring the absorption-line wings, which hinders constraints on absolute atmospheric abundances7-9. Here we present an optical transmission spectrum for the 'hot-Saturn' 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+0.6-0.4, and use it as a proxy to the planet's atmospheric metallicity relative to the solar value (Z_p/Z_\star=2.3+8.9/--1.7). This result is consistent with the mass-metallicity trend observed for solar-system planets and exoplanets10-12.
@article{arxiv.1806.06089,
title = {An absolute sodium abundance for a cloud-free 'hot Saturn' exoplanet},
author = {Nikolay Nikolov and David K. Sing and Jonathan J. Fortney and Jayesh M. Goyal and Benjamin Drummond and Tom M. Evans and Neale P. Gibson and Ernst J. W. De Mooij and Zafar Rustamkulov and Hannah R. Wakeford and Barry Smalley and Adam J. Burgasser and Coel Hellier and Christiane Helling and Nathan J. Mayne and Nikku Madhusudhan and Tiffany Kataria and Josef Baines and Aarynn L. Carter and Gilda E. Ballester and Joanna K. Barstow and Jack McCleery and Jessica J. Spake},
journal= {arXiv preprint arXiv:1806.06089},
year = {2018}
}