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

## 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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## References

1. 1.

Seager, S. & Sasselov, D. Theoretical transmission spectra during extrasolar giant planet transits. Astrophys. J. 537, 916–921 (2000).

2. 2.

Sudarsky, D. et al. Albedo and reflection spectra of extrasolar giant planets. Astrophys. J. 538, 885–903 (2000).

3. 3.

Burrows, A. et al. The near-infrared and optical spectra of methane dwarfs and brown dwarfs. Astrophys. J. 531, 438–446 (2000).

4. 4.

Charbonneau, D. et al. Detection of an extrasolar planet atmosphere. Astrophys. J. 568, 377–384 (2002).

5. 5.

Sing, D. K. et al. A continuum from clear to cloudy hot-Jupiter exoplanets without primordial water depletion. Nature 529, 59–62 (2016).

6. 6.

Wyttenbach, A. et al. Hot exoplanet atmospheres resolved with transit spectroscopy (HEARTS). I. Detection of hot neutral sodium at high altitudes on WASP-49b. Astron. Astrophys. 602, A36 (2017).

7. 7.

Fortney, J. J. et al. On the indirect detection of sodium in the atmosphere of the planetary companion to HD 209458. Astrophys. J. 589, 615–622 (2003).

8. 8.

Line, M. R. & Parmentier, V. The influence of nonuniform cloud cover on transit transmission spectra. Astrophys. J. 820, 78 (2016).

9. 9.

Benneke, B. & Seager, S. Atmospheric retrieval for super-Earths: uniquely constraining the atmospheric composition with transmission spectroscopy. Astrophys. J. 753, 100 (2012).

10. 10.

Kreidberg, L. et al. A precise water abundance measurement for the hot Jupiter WASP-43b. Astrophys. J. 793, 27 (2014).

11. 11.

Line, M. R. et al. No thermal inversion and a solar water abundance for the hot Jupiter HD 209458b from HST/WFC3 spectroscopy. Astrophys. J. 152, 203 (2016).

12. 12.

Wakeford, H. et al. HAT-P-26b: a Neptune-mass exoplanet with a well-constrained heavy element abundance. Science 356, 628–631 (2017).

13. 13.

Hellier, C. et al. Transiting hot Jupiters from WASP-South, Euler and TRAPPIST: WASP-95b to WASP-101b. Mon. Not. R. Astron. Soc. 440, 1982–1992 (2014).

14. 14.

Fortney, J. J., Lodders, K., Marley, M. S. & Freedman, R. S. A unified theory for the atmospheres of the hot and very hot Jupiters: two classes of irradiated atmospheres. Astrophys. J. 678, 1419–1435 (2008).

15. 15.

Collins, G. W. The Fundamentals of Stellar Astrophysics (W. H. Freeman and Co., New York, 1989).

16. 16.

Allard, N. F. et al. A new model for brown dwarf spectra including accurate unified line shape theory for the Na I and K I resonance line profiles. Astron. Astrophys. 411, 473–476 (2003).

17. 17.

Burrows, A. & Volobuyev, M. Calculations of the far-wing line profiles of sodium and potassium in the atmospheres of substellar-mass objects. Astrophys. J. 583, 985–995 (2003).

18. 18.

Johnas, C. M. S. The effects of new Na I D line profiles in cool atmospheres. Astron. Astrophys. 466, 323–325 (2007).

19. 19.

Burgasser, A. J. The spectra of T dwarfs. II. Red optical data. Astrophys. J. 594, 510–524 (2003).

20. 20.

Nikolov, N. et al. VLT FORS2 comparative transmission spectroscopy: detection of Na in the atmosphere of WASP-39b from the ground. Astrophys. J. 832, 191 (2016).

21. 21.

Helling, Ch. et al. The mineral clouds on HD 209458b and HD 189733b. Mon. Not. R. Astron. Soc. 460, 855–883 (2016).

22. 22.

Tremblin, P. Fingering convection and cloudless models for cool brown dwarf atmospheres. Astrophys. J. 804, 17 (2015).

23. 23.

Asplund, M. et al. The chemical composition of the Sun. Annu. Rev. Astron. Astrophys. 47, 481–522 (2009).

24. 24.

Fortney, J. J. et al. A framework for characterizing the atmospheres of low-mass low-density transiting planets. Astrophys. J. 775, 80 (2013).

25. 25.

Pollack, J. B. et al. Formation of the giant planets by concurrent accretion of solids and gas. Icarus 124, 62–85 (1996).

26. 26.

Mordasini, C. et al. Extrasolar planet population synthesis. IV. Correlations with disk metallicity, mass, and lifetime. Astron. Astrophys. 541, A97 (2012).

27. 27.

Wong, M. H. et al. Updated Galileo probe mass spectrometer measurements of carbon, oxygen, nitrogen, and sulfur on Jupiter. Icarus 171, 153–170 (2004).

28. 28.

Fletcher, L. N. et al. Methane and its isotopologues on Saturn from Cassini/CIRS observations. Icarus 199, 351–367 (2009).

29. 29.

Karkoschka, E. & Tomasko, M. G. The haze and methane distributions on Neptune from HST-STIS spectroscopy. Icarus 211, 780–797 (2011).

30. 30.

Sromovsky, L. A. et al. Methane on Uranus: the case for a compact CH4 cloud layer at low latitudes and a severe CH4 depletion at high-latitudes based on re-analysis of Voyager occultation measurements and STIS spectroscopy. Icarus 215, 292–312 (2011).

31. 31.

Appenzeller, I. et al. Successful commissioning of FORS1—the first optical instrument on the VLT. The Messenger 94, 1–6 (1998).

32. 32.

Gibson, N. P. et al. VLT/FORS2 comparative transmission spectroscopy II: confirmation of a cloud deck and Rayleigh scattering in WASP-31b, but no potassium? Mon. Not. R. Astron. Soc. 467, 4591–4605 (2017).

33. 33.

Roeser, S. et al. The PPMXL catalogue of positions and proper motions on the ICRS. Combining USNO-B1.0 and the two micron all sky survey (2MASS). Astron. J. 139, 2440–2447 (2010).

34. 34.

Nikolov, N. et al. Hubble Space Telescope hot Jupiter transmission spectral survey: a detection of Na and strong optical absorption in HAT-P-1b. Mon. Not. R. Astron. Soc. 437, 46–66 (2014).

35. 35.

Gibson, N. P. et al. A Gaussian process framework for modelling instrumental systematics: application to transmission spectroscopy. Mon. Not. R. Astron. Soc. 419, 2683–2694 (2012).

36. 36.

Evans, T. M. et al. An ultrahot gas-giant exoplanet with a stratosphere. Nature 548, 58–61 (2017).

37. 37.

Nikolov, N. et al. Hubble PanCET: an isothermal day-side atmosphere for the bloated gas-giant HAT-P-32Ab. Mon. Not. R. Astron. Soc. 474, 1705–1717 (2018).

38. 38.

Ambikasaran, S. et al. Fast direct methods for Gaussian processes. IT Process Automation Manager (ITPAM) 38, 252–265 (2015).

39. 39.

Foreman-Mackey, D. George: Gaussian process regression. Astrophys. Source Code Library ascl.soft11015 (2015).

40. 40.

Foreman-Mackey, D. et al. emcee: the MCMC hammer. Proc. Astron. Soc. Pacif. 125, 306–312 (2013).

41. 41.

Foreman-Mackey, D. corner.py: scatterplot matrices in Python. J. Open Source Softw. 1, 24 (2016).

42. 42.

Mandel, K. & Agol, E. Analytic lightcurves for planetary transit searches. Astrophys. J. 580, L171–L175 (2002).

43. 43.

Magic, Z. et al. The Stagger-grid: a grid of 3D stellar atmosphere models. IV. Limb darkening coefficients. Astron. Astrophys. 573, A90 (2015).

44. 44.

Claret, A. A new non-linear limb-darkening law for LTE stellar atmosphere models II. Geneva and Walraven systems: calculations for −5.0 ≤ log[M/H] ≤ +1, 2000 K ≤T eff ≤ 50000 K at several surface gravities. Astron. Astrophys. 401, 657–660 (2003).

45. 45.

Espinoza, N. & Jordan, A. Limb darkening and exoplanets: testing stellar model atmospheres and identifying biases in transit parameters. Mon. Not. R. Astron. Soc. 450, 1879–1899 (2015).

46. 46.

Espinoza, N. & Jordan, A. Limb darkening and exoplanets—II. Choosing the best law for optimal retrieval of transit parameters. Mon. Not. R. Astron. Soc. 457, 3573–3581 (2016).

47. 47.

Markwardt, C. B. Non-linear least-squares fitting in IDL with MPFIT. Astron. Soc. Pacif. Conf. Ser. 411, 251–254 (2009).

48. 48.

Sedaghati, E. et al. Detection of titanium oxide in the atmosphere of a hot Jupiter. Nature 549, 238–241 (2017).

49. 49.

Sedaghati, E. et al. Potassium detection in the clear atmosphere of a hot-Jupiter. FORS2 transmission spectroscopy of WASP-17b. Astron. Astrophys. 596, A47 (2016).

50. 50.

Lendl, M. FORS2 observes a multi-epoch transmission spectrum of the hot Saturn-mass exoplanet WASP-49b. Astron. Astrophys. 587, A67 (2016).

51. 51.

Southworth, J. Homogeneous studies of transiting extrasolar planets—I. Light-curve analyses. Mon. Not. R. Astron. Soc. 386, 1644–1666 (2008).

52. 52.

Mallonn, M. et al. Transmission spectroscopy of the inflated exo-Saturn HAT-P-19b. Astron. Astrophys. 580, A60 (2015).

53. 53.

Stevenson, K. B. A search for water in the atmosphere of HAT-P-26b using LDSS-3C. Astrophys. J. 817, 141 (2016).

54. 54.

Nikolov, N. et al. HST hot-Jupiter transmission spectral survey: haze in the atmosphere of WASP-6b. Mon. Not. R. Astron. Soc. 447, 463–478 (2015).

55. 55.

Rackham, B. et al. ACCESS I: an optical transmission spectrum of GJ 1214b reveals a heterogeneous stellar photosphere. Astrophys. J. 834, 151 (2017).

56. 56.

Huitson, C. M. Gemini/GMOS transmission spectral survey: complete optical transmission spectrum of the hot Jupiter WASP-4b. Astron. J. 154, 95–113 (2017).

57. 57.

Akaike, H. A new look at the statistical model identification. IEEE Trans. Automatic Control 19, 716–723 (1974).

58. 58.

Gibson, N. P. Reliable inference of exoplanet light-curve parameters using deterministic and stochastic systematics models. Mon. Not. R. Astron. Soc. 445, 3401–3414 (2014).

59. 59.

Schwarz, G. Estimating the dimension of a model. Ann. Stat. 6, 461–464 (1978).

60. 60.

Pont, F. et al. The effect of red noise on planetary transit detection. Mon. Not. R. Astron. Soc. 373, 231–242 (2006).

61. 61.

Lodders, K. Alkali element chemistry in cool dwarf atmospheres. Astrophys. J. 519, 793–801 (1999).

62. 62.

Lodders, K. & Fegley, B. Atmospheric chemistry in giant planets, brown dwarfs, and low-mass dwarf stars. I. Carbon, nitrogen, and oxygen. Icarus 155, 393–424 (2002).

63. 63.

Freedman, R. S. et al. Line and mean opacities for ultracool dwarfs and extrasolar planets. Astrophys. J. Suppl. Ser. 174, 504–513 (2008).

64. 64.

Madhusudhan, N. & Seager, S. A temperature and abundance retrieval method for exoplanet atmospheres. Astrophys. J. 707, 24–39 (2009).

65. 65.

Amundsen, D. S. et al. Accuracy tests of radiation schemes used in hot Jupiter global circulation models. Astron. Astrophys. 564, A59 (2014).

66. 66.

Amundsen, D. S. et al. Treatment of overlapping gaseous absorption with the correlated-k method in hot Jupiter and brown dwarf atmosphere models. Astron. Astrophys. 598, A97 (2017).

67. 67.

Tremblin, P. et al. Advection of potential temperature in the atmosphere of irradiated exoplanets: a robust mechanism to explain radius inflation. Astrophys. J. 841, 30 (2017).

68. 68.

Drummond, B. The effects of consistent chemical kinetics calculations on the pressure-temperature profiles and emission spectra of hot Jupiters. Astron. Astrophys. 594, A69 (2016).

69. 69.

Goyal, J. M. A library of ATMO forward model transmission spectra for hot Jupiter exoplanets. Mon. Not. R. Astron. Soc. 474, 5158–5185 (2017).

70. 70.

Lecavelier des Etangs, A. et al. Rayleigh scattering in the transit spectrum of HD 189733b. Astron. Astrophys. 481, L83–L86 (2008).

71. 71.

Eastman, J., Gaudi, B. S. & Agol, E. EXOFAST: a fast-exoplanetary fitting suite in IDL. Publ. Astron. Soc. Pacif. 125, 83–112 (2013).

72. 72.

Drummond, B. et al. Observable signatures of wind-driven chemistry with a fully consistent three-dimensional radiative hydrodynamics model of HD 209458b. Astrophys. J. 855, 31 (2018).

73. 73.

Wakeford, H. et al. The complete transmission spectrum of WASP-39b with a precise water constraint. Astron. J. 155, 29–43 (2018).

74. 74.

Thorngren, D. P. et al. The mass-metallicity relation for giant planets. Astrophys. J. 831, 64 (2016).

## Acknowledgements

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.

### Reviewer information

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

## Author information

### Affiliations

1. #### Physics and Astronomy, University of Exeter, Exeter, UK

• N. Nikolov
• , D. K. Sing
• , J. M. Goyal
• , B. Drummond
• , T. M. Evans
• , N. J. Mayne
• , A. L. Carter
•  & J. J. Spake

• D. K. Sing
3. #### Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA, USA

• J. J. Fortney
•  & Z. Rustamkulov
4. #### Astrophysics Research Centre, School of Mathematics and Physics, Queens University Belfast, Belfast, UK

• N. P. Gibson
• , J. Baines
•  & J. McCleery
5. #### School of Physical Sciences, Dublin City University, Glasnevin, Ireland

• E. J. W. De Mooij
6. #### Centre for Astrophysics & Relativity, Dublin City University, Glasnevin, Ireland

• E. J. W. De Mooij
7. #### Space Telescope Science Institute, Baltimore, MA, USA

• H. R. Wakeford
8. #### Astrophysics Group, Keele University, Keele, UK

• B. Smalley
•  & C. Hellier
9. #### Department of Physics, University of California, San Diego, CA, USA

• A. J. Burgasser
10. #### Centre for Exoplanet Science, SUPA, School of Physics and Astronomy, University of St Andrews, St Andrews, UK

• Ch. Helling
11. #### Anton Pannekoek Institute for Astronomy, University of Amsterdam, Amsterdam, The Netherlands

• Ch. Helling

• T. Kataria
14. #### Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA

• G. E. Ballester
15. #### Physics and Astronomy, University College London, London, UK

• J. K. Barstow

### Contributions

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.

### Competing interests

The authors declare no competing interests.

### Corresponding author

Correspondence to N. Nikolov.

## Extended data figures and tables

1. ### 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.

2. ### 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).

3. ### Extended Data Fig. 3

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

4. ### 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.

5. ### 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.

6. ### 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.

7. ### 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).

8. ### 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.