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 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; J J Spake

doi:10.1038/s41586-018-0101-7

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

Nature. 2018 May;557(7706):526-529. doi: 10.1038/s41586-018-0101-7. Epub 2018 May 7.

Authors

N Nikolov¹, D K Sing^{2

3}, J J Fortney⁴, J M Goyal², B Drummond², T M Evans², N P Gibson⁵, E J W De Mooij^{6

7}, Z Rustamkulov⁴, H R Wakeford⁸, B Smalley⁹, A J Burgasser¹⁰, C Hellier⁹, Ch Helling^{11

12}, N J Mayne², N Madhusudhan¹³, T Kataria¹⁴, J Baines⁵, A L Carter², G E Ballester¹⁵, J K Barstow¹⁶, J McCleery⁵, J J Spake²

Affiliations

¹ Physics and Astronomy, University of Exeter, Exeter, UK. nikolov.nkn@gmail.com.
² Physics and Astronomy, University of Exeter, Exeter, UK.
³ Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA.
⁴ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA, USA.
⁵ Astrophysics Research Centre, School of Mathematics and Physics, Queens University Belfast, Belfast, UK.
⁶ School of Physical Sciences, Dublin City University, Glasnevin, Ireland.
⁷ Centre for Astrophysics & Relativity, Dublin City University, Glasnevin, Ireland.
⁸ Space Telescope Science Institute, Baltimore, MA, USA.
⁹ Astrophysics Group, Keele University, Keele, UK.
¹⁰ Department of Physics, University of California, San Diego, CA, USA.
¹¹ Centre for Exoplanet Science, SUPA, School of Physics and Astronomy, University of St Andrews, St Andrews, UK.
¹² Anton Pannekoek Institute for Astronomy, University of Amsterdam, Amsterdam, The Netherlands.
¹³ Institute of Astronomy, University of Cambridge, Cambridge, UK.
¹⁴ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA.
¹⁵ Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA.
¹⁶ Physics and Astronomy, University College London, London, UK.

PMID: 29736017
DOI: 10.1038/s41586-018-0101-7

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^1-3. However, observations have revealed only the narrow cores of these features rather than the full pressure-broadened profiles^4-6. 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^7-9. 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 = [Formula: see text], and use it as a proxy for the planet's atmospheric metallicity relative to the solar value (Z_p/Z_ʘ = [Formula: see text]). This result is consistent with the mass-metallicity trend observed for Solar System planets and exoplanets^10-12.

Publication types

Research Support, U.S. Gov't, Non-P.H.S.
Research Support, Non-U.S. Gov't