Atmospheric composition and structure of HD209458b

Transiting planets like HD209458b offer a unique opportunity to scrutinize their at- mospheric composition and structure. Transit spectroscopy probes the transition region between the day and night sides, called limb. We present a re-analysis of existing HST/STIS transmission spectra of HD209458bs atmosphere. From these observations we: Identify H2 Rayleigh scatter- ing, derive the absolute Sodium abundance and quantify its depletion in the upper atmosphere, extract a stratospheric T-P profile with a temperature inversion and explain broad band ab- sorptions with the presence of TiO and VO molecules in the atmosphere of this planet.