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

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N. Nikolov, D. K. Sing, J. J. Fortney, J. M. Goyal, B. Drummond, T. M. Evans, N. P. Gibson, E. J. W. De Mooj, Z. Rustamkulov, H. R. Wakeford, B. Smalley, A. J. Burgasser, C. Hellier, Christiane Helling, N. J. Mayne, N. Madhusudhan, T. Kataria, J. Baines, A. L. Carter, G. E. Ballester & 3 others J. K. Barstow, J. McCleery, J. J. Spake

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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 = 6.9^+0.6_−0.4, and use it as a proxy for the planet’s atmospheric metallicity relative to the solar value (Zp/Zʘ = 2.3^+8.9_−1.7). This result is consistent with the mass–metallicity trend observed for Solar System planets and exoplanets10,11,12.


Original languageEnglish
Pages (from-to)526-529
Publication statusPublished - 7 May 2018

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