An absolute sodium abundance for a cloud-free ‘hot Saturn’ exoplanet
Top Cited Papers
- 7 May 2018
- journal article
- research article
- Published by Springer Science and Business Media LLC in Nature
- Vol. 557 (7706), 526-529
- https://doi.org/10.1038/s41586-018-0101-7
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 revealed only the narrow cores of these features rather than the full pressure-broadened profiles4,5,6. 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 abundances7,8,9. 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 logεNa = \({{\bf{6.9}}}_{-{\bf{0.4}}}^{+{\bf{0.6}}}\) , and use it as a proxy for the planet’s atmospheric metallicity relative to the solar value (Zp/Zʘ = \({{\bf{2.3}}}_{-{\bf{1.7}}}^{+{\bf{8.9}}}\) ). This result is consistent with the mass–metallicity trend observed for Solar System planets and exoplanets10,11,12.
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