Moutou et al. (2001) looked for absorption by species such as Na,H,He,CH+,CO+,N2+, and H2O+ in the upper atmosphere of HD209458b. These observations were followed by Moutou et al. (2003) who attempted to measure the transit depth in the He 1083 nm line that was predicted to be significant by Seager and Sasselov (2000). The most recent searches were reported by Winn et al. (2004) and Narita et al. (2005) who looked for transits in the Na D, Li, Hα, Hβ, Hγ, Fe, and Ca absorption lines. So far none of the ground-based searches have led to a detection of the upper atmosphere. However, the non-detection is based on only a few observations that have proven difficult to analyze, and the search should continue.
He
line
1083 nm
nm