An absolute sodium abundance for a cloud-free 'hot Saturn' exoplanet.
Nature 2018;
557:526-529. [PMID:
29736017 DOI:
10.1038/s41586-018-0101-7]
[Citation(s) in RCA: 88] [Impact Index Per Article: 14.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/27/2017] [Accepted: 01/30/2018] [Indexed: 11/09/2022]
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 exoplanets1-3. However, observations have revealed only the narrow cores of these features rather than the full pressure-broadened profiles4-6. 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 abundances7-9. 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 = [Formula: see text], and use it as a proxy for the planet's atmospheric metallicity relative to the solar value (Zp/Zʘ = [Formula: see text]). This result is consistent with the mass-metallicity trend observed for Solar System planets and exoplanets10-12.
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