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Observations of stellar surfaces - except for the Sun - are hampered by theirtiny angular extent, while observed spectral lines are smeared by averagingover the stellar surface, and by stellar rotation. Exoplanet transits can beused to analyze stellar atmospheric structure, yielding high-resolution spectraacross spatially highly resolved stellar surfaces, free from effects of spatialsmearing and the rotational wavelength broadening present in full-disk spectra.During a transit, stellar surface portions successively become hidden, anddifferential spectroscopy between various transit phases provides spectra ofthose surface segments then hidden behind the planet. The small area subtendedby even a large planet (about 1% of a main-sequence star) offers high spatialresolution but demands very precise observations. We demonstrate thereconstruction of photospheric FeI line profiles at a spectral resolutionR=80,000 across the surface of the solar-type star HD209458. Any detailedunderstanding of stellar atmospheres requires modeling with 3-dimensionalhydrodynamics. The properties predicted by such models are mapped onto theprecise spectral-line shapes, asymmetries and wavelength shifts, and theirvariation from the center to the limb across any stellar disk. This methodprovides a tool for testing and verifying such models. The method will soonbecome applicable to more diverse types of stars, thanks to new spectrometerson very large telescopes, and since ongoing photometric searches are expectedto discover additional bright host stars of transiting exoplanets.

Exoplanet transits enable high-resolution spectroscopy across spatially resolved stellar surfaces