2001/11/28 by David Charbonneau, Timothy M. Brown, Robert W. Noyes +2 · 18 citations
Physics and Astronomy · #Astro and Planetary Science #Astrophysics and Star Formation Studies #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/338770
published as Astrophys.J.568:377-384,2002 · 26 pages, 8 figures, accepted by ApJ 2001 November 19
arxiv created 2001/11/28 · openalex publication_date 2002/03/20 · arxiv updated 2011/07/18 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/31
We report high-precision spectrophotometric observations of four planetary transits of HD 209458, in the region of the sodium resonance doublet at 589.3 nm. We find that the photometric dimming during transit in a bandpass centered on the sodium feature is deeper by (2.32 ± 0.57) × 10 -4 relative to simultaneous observations of the transit in adjacent bands. We interpret this additional dimming as absorption from sodium in the planetary atmosphere, as recently predicted from several theoretical modeling efforts. Our model for a cloudless planetary atmosphere with a solar abundance of sodium in atomic form predicts more sodium absorption than we observe. There are several possibilities that may account for this reduced amplitude, including reaction of atomic sodium into molecular gases and/or condensates, photoionization of sodium by the stellar flux, a low primordial abundance of sodium, and the presence of clouds high in the atmosphere.