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We observed two transits of the iconic gas giant HD 209458b between 380 and780 nm, using the high-resolution ESPRESSO spectrograph. The derived planetarytransmission spectrum exhibits features at all wavelengths where the parentstar shows strong absorption lines, for example, NaI, MgI, FeI, FeII, CaI, VI,H$\alpha$, and KI. We interpreted these features as the signature of thedeformation of the stellar line profiles due to the Rossiter-McLaughlin effect,combined with the centre-to-limb effects on the stellar surface, which is inagreement with similar reports recently presented in the literature. We alsosearched for species that might be present in the planetary atmosphere but notin the stellar spectra, such as TiO and VO, and obtained a negative result.Thus, we find no evidence of any planetary absorption, including previouslyreported NaI, in the atmosphere of HD 209458b. The high signal-to-noise ratioin the transmission spectrum allows us to compare the modelled deformation ofthe stellar lines in assuming different one-dimensional stellar atmosphericmodels. We conclude that the differences among various models and observationsremain within the precision limits of the data. However, the transmission lightcurves are better explained when the centre-to-limb variation is not includedin the computation and only the Rossiter-McLaughlin deformation is considered.This demonstrates that ESPRESSO is currently the best facility for spatiallyresolving the stellar surface spectrum in the optical range using transitobservations and carrying out empirical validations of stellar models.

The atmosphere of HD 209458b seen with ESPRESSO. No detectable planetary absorptions at high resolution