Detection of an Extrasolar Planet Atmosphere

TLDR

Possible explanations for the weaker sodium absorption include chemical conversion, photoionization, low sodium abundance, or high‑altitude clouds. The study used high‑precision spectrophotometry of four HD 209458 transits centered on the sodium doublet at 589.3 nm. The transit light curve shows a 2.32 × 10⁻⁴ deeper dip in the sodium band, indicating sodium absorption, but the observed strength is weaker than predicted by a cloudless solar‑abundance model.

Abstract

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.