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

N. Nikolov (), D. K. Sing, J. J. Fortney, J. M. Goyal, B. Drummond, T. M. Evans, N. P. Gibson, E. J. W. Mooij, Z. Rustamkulov, H. R. Wakeford, B. Smalley, A. J. Burgasser, C. Hellier, Ch. Helling, N. J. Mayne, N. Madhusudhan, T. Kataria, J. Baines, A. L. Carter, G. E. Ballester, J. K. Barstow, J. McCleery and J. J. Spake
Additional contact information
N. Nikolov: University of Exeter
D. K. Sing: University of Exeter
J. J. Fortney: University of California
J. M. Goyal: University of Exeter
B. Drummond: University of Exeter
T. M. Evans: University of Exeter
N. P. Gibson: Queens University Belfast
E. J. W. Mooij: Dublin City University
Z. Rustamkulov: University of California
H. R. Wakeford: Space Telescope Science Institute
B. Smalley: Keele University
A. J. Burgasser: University of California
C. Hellier: Keele University
Ch. Helling: University of St Andrews
N. J. Mayne: University of Exeter
N. Madhusudhan: Institute of Astronomy, University of Cambridge
T. Kataria: Jet Propulsion Laboratory, California Institute of Technology
J. Baines: Queens University Belfast
A. L. Carter: University of Exeter
G. E. Ballester: Lunar and Planetary Laboratory, University of Arizona
J. K. Barstow: University College London
J. McCleery: Queens University Belfast
J. J. Spake: University of Exeter

Nature, 2018, vol. 557, issue 7706, 526-529

Abstract: 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 exoplanets1–3. However, observations have revealed only 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 to be responsible for obscuring the absorption-line wings, which hinders constraints on absolute atmospheric abundances7–9. 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 = $${{\bf{6.9}}}_{-{\bf{0.4}}}^{+{\bf{0.6}}}$$ 6.9 - 0.4 + 0.6 , and use it as a proxy for the planet’s atmospheric metallicity relative to the solar value (Zp/Zʘ = $${{\bf{2.3}}}_{-{\bf{1.7}}}^{+{\bf{8.9}}}$$ 2.3 - 1.7 + 8.9 ). This result is consistent with the mass–metallicity trend observed for Solar System planets and exoplanets10–12.

Date: 2018
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DOI: 10.1038/s41586-018-0101-7

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