Sodium Absorption from the Exoplanetary Atmosphere of HD 189733b Detected in the Optical Transmission Spectrum

We present the first ground-based detection of sodium absorption in the transmission spectrum of an extrasolar planet. Absorption due to the atmosphere of the extrasolar planet HD 189733b is detected in both lines of the Na I doublet. High spectral resolution observations were taken of 11 transits with the High Resolution Spectrograph ( HRS) on the 9.2 m Hobby-Eberly Telescope ( HET). The Na I absorption in the transmission spectrum due to HD 189733b is (-67.2 +/- 20.7) x 10(-5) deeper in the "narrow" spectral band that encompasses both lines relative to adjacent bands. The 1 sigma error includes both random and systematic errors, and the detection is > 3 sigma. This amount of relative absorption in Na I for HD 189733b is similar to 3 times larger than that detected for HD 209458b by Charbonneau et al. ( 2002) and indicates that these two hot Jupiters may have significantly different atmospheric properties.