  AU Mic~b is a Neptune size planet on a 8.47-day orbit around the nearest
pre-main sequence ($\sim$20 Myr) star to the Sun, the bright (V=8.81) M dwarf
AU Mic. The planet was preliminary detected in Doppler radial velocity time
series and recently confirmed to be transiting with data from the TESS mission.
AU Mic~b is likely to be cooling and contracting and might be accompanied by a
second, more massive planet, in an outer orbit. Here, we present the
observations of the transit of AU Mic~b using ESPRESSO on the VLT. We obtained
a high-resolution time series of spectra to measure the Rossiter-McLaughlin
effect and constrain the spin-orbit alignment of the star and planet, and
simultaneously attempt to retrieve the planet's atmospheric transmission
spectrum. These observations allow us to study for the first time the early
phases of the dynamical evolution of young systems. We apply different
methodologies to derive the spin-orbit angle of AU Mic~b, and all of them
retrieve values consistent with the planet being aligned with the rotation
plane of the star. We determine a conservative spin-orbit angle $\lambda$ value
of $-2.96^{+10.44}_{-10.30}$, indicative that the formation and migration of
the planets of the AU Mic system occurred within the disk. Unfortunately, and
despite the large SNR of our measurements, the degree of stellar activity
prevented us from detecting any features from the planetary atmosphere. In
fact, our results suggest that transmission spectroscopy for recently formed
planets around active young stars is going to remain very challenging, if at
all possible, for the near future.
