Details

Autor(en) / Beteiligte

• Nikolov, N.
• Sing, D. K.
• Fortney, J. J.
• Goyal, J. M.
• Drummond, B.
• Evans, T. M.
• Gibson, N. P.
• De Mooij, E. J. W.
• Rustamkulov, Z.
• Wakeford, H. R.
• Smalley, B.
• Burgasser, A. J.
• Hellier, C.
• Helling, Ch
• Mayne, N. J.
• Madhusudhan, N.
• Kataria, T.
• Baines, J.
• Carter, A. L.
• Ballester, G. E.
• Barstow, J. K.
• McCleery, J.
• Spake, J. J.

Titel

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

Ist Teil von

• Nature (London), 2018-05, Vol.557 (7706), p.526-529

Ort / Verlag

London: Nature Publishing Group UK

Erscheinungsjahr

2018

Beschreibungen/Notizen

• 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.