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



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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Fig. 1: Transmission spectrum of WASP-96b compared to models.
Fig. 2: Retrieved atmospheric properties for WASP-96b.
Fig. 3: Mass–metallicity diagram for Solar System planets and exoplanets.


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This work is based on observations collected at the European Organization for Astronomical Research in the Southern Hemisphere under European Southern Observatory programme 199.C-0467(H). The research leading to these results received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement number 336792. A.J.B. is a US/UK Fulbright Scholar. J.M.G. and N.J.M. acknowledge support from a Leverhulme Trust Research Project Grant. J.K.B. is a Royal Astronomical Society Research Fellow.

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Nature thanks I. Snellen and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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N.N. led the design of the VLT FORS2 Large Programme and the scientific proposal (with contributions from N.P.G., D.K.S. and T.M.E.). N.N. led the observations, analysis, comparison with forward models and the interpretation of the result. D.K.S. led the atmospheric retrievals (with contributions from J.M.G.). J.J.F. and Z.R. provided forward atmospheric models for comparative analysis. B.S. and C.H. provided elemental abundances of the host star. N.N. wrote the manuscript (with contributions from D.K.S. and T.M.E.). J.B. and J.McC. performed independent tests on various parts of the data reduction and analysis as part of their final-year undergraduate projects under supervision from N.P.G. All authors discussed the results and commented on the draft.

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Correspondence to N. Nikolov.

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Extended data figures and tables

Extended Data Fig. 1 VLT FORS2 stellar spectra and white-light curves.

Left and right panels show the GRIS600B (blue) and GRIS600RI (red) datasets, respectively. The top row shows example stellar spectra used for relative spectrophotometric calibration. The dashed lines indicate the wavelength region used to produce the white-light curves. The second row shows normalized raw light curves for both sources. The third row shows normalized relative target-to-reference raw flux along with the marginalized Gaussian process model (A), the detrended transit light curve and model (B), and the common-mode correction (A/B). The fourth row shows the best-fit light curve residuals and 1σ error bars, obtained by subtracting the marginalized transit and systematics models from the relative target-to-reference raw flux. The two light curve residuals show dispersions of 78 and 201 parts per million, respectively.

Extended Data Fig. 2 Spectrophotometric light curves from grism 600B offset by a constant amount for clarity.

The first panel shows the raw target-to-reference flux. The second panel shows the common-mode (CM)-corrected light curves and the transit and systematics models, with the highest statistical weight. The third panel shows detrended light curves and the transit model with the highest statistical weight. The fourth panel shows residuals with 1σ error bars. The dashed lines indicate the median residual level, with dotted lines indicating the dispersion and the percentage of the theoretical photon noise limit reached (blue).

Extended Data Fig. 3

As for Extended Data Fig. 2 but for grism 600RI.

Extended Data Fig. 4 Light-curve auxiliary variables.

Shown are air mass (a, b), drifts along the cross-dispersion (c, d) and dispersion axes (e, f), FWHM (g, h) and the rate of change of the rotation angle (i, j) of the VLT FORS2 observations. Left and right columns refer to the GRIS600B and GRIS600RI observations, respectively.

Extended Data Fig. 5 Transmission spectrum of WASP-96b.

Indicated are the relative radius measurements from grism 600B (blue dots) and 600RI (red dots) along with the 1σ uncertainties, compared to the same set of models as in Fig. 1.

Extended Data Fig. 6 Na/K ratio.

Histogram of the marginalized posterior distribution of the Na to K ratio for WASP-96b. Shown are the median and 1σ levels (orange continuous and dotted lines, respectively). The solar value is indicated by the blue continuous line.

Extended Data Fig. 7 Heavy element enrichment of exoplanets relative to their stars as a function of mass.

Plotted are the Solar System planets (blue bars) and gas-giant exoplanets (grey and orange symbols). Each error bar represents the 1σ uncertainty. The blue line indicates a fit to the Solar System gas giants (pale blue symbols indicate Solar System planets).

Extended Data Fig. 8 Posterior distribution of the retrieved cloud opacity versus the sodium abundance.

VMR, vertical mixing ratio. The median and 1σ measured parameters are indicated with continuous lines and the red dot marks the intercept for clarity. The colour scale shows the normalized density of the samples in the MCMC run.

Extended Data Table 1 System parameters
Extended Data Table 2 Transmission spectrum

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Nikolov, N., Sing, D.K., Fortney, J.J. et al. An absolute sodium abundance for a cloud-free ‘hot Saturn’ exoplanet. Nature 557, 526–529 (2018).

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