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

TL;DR

This study presents a clear optical transmission spectrum of the 'hot Saturn' WASP-96b, revealing the full pressure-broadened sodium absorption profile and enabling precise measurement of its atmospheric sodium abundance, consistent with planetary metallicity trends.

Contribution

First detection of the complete pressure-broadened sodium profile in a cloud-free exoplanet atmosphere, allowing accurate abundance and metallicity determination.

Findings

Measured sodium abundance of logε_Na=6.9+0.6-0.4

Atmospheric metallicity Z_p/Z_*=2.3+8.9/--1.7

Spectrum matches cloud-free, solar-abundance models

Abstract

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.