  In the past few years, the study of exoplanets has evolved from being pure
discovery, then being more exploratory in nature and finally becoming very
quantitative. In particular, transmission spectroscopy now allows the study of
exoplanetary atmospheres. Such studies rely heavily on space-based or large
ground-based facilities, because one needs to perform time-resolved, high
signal-to-noise spectroscopy. The very recent exchange of the prisms of the
FORS2 atmospheric diffraction corrector on ESO's Very Large Telescope should
allow us to reach higher data quality than was ever possible before. With
FORS2, we have obtained the first optical ground-based transmission spectrum of
WASP-19b, with 20 nm resolution in the 550--830 nm range. For this planet, the
data set represents the highest resolution transmission spectrum obtained to
date. We detect large deviations from planetary atmospheric models in the
transmission spectrum redwards of 790 nm, indicating either additional sources
of opacity not included in the current atmospheric models for WASP-19b or
additional, unexplored sources of systematics. Nonetheless, this work shows the
new potential of FORS2 for studying the atmospheres of exoplanets in greater
detail than has been possible so far.
