  Transiting planets like HD209458b offer a unique opportunity to scrutinize
their atmospheric composition and structure. Transit spectroscopy probes the
transition region between the day and night sides, called limb. We present a
re-analysis of existing archived HST/STIS transmission spectra of HD209458b's
atmosphere. From these observations we: Identify H2 Rayleigh scattering, 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 absorptions with the presence of TiO and VO molecules in
the atmosphere of this planet.
