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An absolute sodium abundance for a cloud-free ‘hot Saturn’ exoplanet
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An absolute sodium abundance for a cloud-free ‘hot Saturn’ exoplanet
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Journal Article
An absolute sodium abundance for a cloud-free ‘hot Saturn’ exoplanet
2018
Overview
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 log
ε
Na
=
6.9
-
0.4
+
0.6
, and use it as a proxy for the planet’s atmospheric metallicity relative to the solar value (
Z
p
/
Z
ʘ
=
2.3
-
1.7
+
8.9
). This result is consistent with the mass–metallicity trend observed for Solar System planets and exoplanets
10
–
12
.
The optical transmission spectrum for the ‘hot Saturn’ exoplanet WASP-96b reveals a clear atmosphere, an atmospheric sodium abundance and hence its metallicity, which is consistent with the metallicity trend observed in Solar System planets and exoplanets.
