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. 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; J. K. Barstow; J. McCleery; J. J. Spake

doi:10.1038/s41586-018-0101-7

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. 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. BallesterJ. K. Barstow, J. McCleery, J. J. Spake

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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 exoplanets^1,2,3. However, observations have revealed only the narrow cores of these features rather than the full pressure-broadened profiles^4,5,6. 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 abundances^7,8,9. 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 logε_Na= 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 exoplanets^10,11,12.

Original language	English
Pages (from-to)	526-529
Number of pages	18
Journal	Nature
Volume	557
Issue number	7706
Early online date	7 May 2018
DOIs	https://doi.org/10.1038/s41586-018-0101-7
Publication status	Published - 24 May 2018

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© 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. This work has been made available online in accordance with the publisher’s policies. This is the author created accepted version manuscript following peer review and as such may differ slightly from the final published version. The final published version of this work is available at https://doi.org/10.1038/s41586-018-0101-7
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Nikolov, N., Sing, D. K., Fortney, J. J., Goyal, J. M., Drummond, B., Evans, T. M., Gibson, N. P., De Mooj, E. J. W., Rustamkulov, Z., Wakeford, H. R., Smalley, B., Burgasser, A. J., Hellier, C., Helling, C., Mayne, N. J., Madhusudhan, N., Kataria, T., Baines, J., Carter, A. L., ... Spake, J. J. (2018). An absolute sodium abundance for a cloud-free 'hot Saturn' exoplanet. Nature, 557(7706), 526-529. https://doi.org/10.1038/s41586-018-0101-7

@article{f84187cdc4ed4d17a719a9bf197ac890,

title = "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 {\textquoteleft}hot Saturn{\textquoteright} 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{\textquoteright}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.",

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

note = "The research leading to these results received funding from the European Research Council under the European Union{\textquoteright}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.",

year = "2018",

month = may,

day = "24",

doi = "10.1038/s41586-018-0101-7",

language = "English",

volume = "557",

pages = "526--529",

journal = "Nature",

issn = "0028-0836",

publisher = "Nature publishing group",

number = "7706",

}

Nikolov, N, Sing, DK, Fortney, JJ, Goyal, JM, Drummond, B, Evans, TM, Gibson, NP, De Mooj, EJW, Rustamkulov, Z, Wakeford, HR, Smalley, B, Burgasser, AJ, Hellier, C, Helling, C, Mayne, NJ, Madhusudhan, N, Kataria, T, Baines, J, Carter, AL, Ballester, GE, Barstow, JK, McCleery, J & Spake, JJ 2018, 'An absolute sodium abundance for a cloud-free 'hot Saturn' exoplanet', Nature, vol. 557, no. 7706, pp. 526-529. https://doi.org/10.1038/s41586-018-0101-7

TY - JOUR

T1 - An absolute sodium abundance for a cloud-free 'hot Saturn' exoplanet

AU - Nikolov, N.

AU - Sing, D. K.

AU - Fortney, J. J.

AU - Goyal, J. M.

AU - Drummond, B.

AU - Evans, T. M.

AU - Gibson, N. P.

AU - De Mooj, E. J. W.

AU - Rustamkulov, Z.

AU - Wakeford, H. R.

AU - Smalley, B.

AU - Burgasser, A. J.

AU - Hellier, C.

AU - Helling, Christiane

AU - Mayne, N. J.

AU - Madhusudhan, N.

AU - Kataria, T.

AU - Baines, J.

AU - Carter, A. L.

AU - Ballester, G. E.

AU - Barstow, J. K.

AU - McCleery, J.

AU - Spake, J. J.

N1 - 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.

PY - 2018/5/24

Y1 - 2018/5/24

N2 - 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.

AB - 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.

U2 - 10.1038/s41586-018-0101-7

DO - 10.1038/s41586-018-0101-7

M3 - Article

SN - 0028-0836

VL - 557

SP - 526

EP - 529

JO - Nature

JF - Nature

IS - 7706

ER -

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

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