  We present the first detection of atomic emission lines from the atmosphere
of an exoplanet. We detect neutral iron lines from the day-side of KELT-9b (Teq
$\sim$ 4, 000 K). We combined thousands of spectrally resolved lines observed
during one night with the HARPS-N spectrograph (R $\sim$ 115, 000), mounted at
the Telescopio Nazionale Galileo. We introduce a novel statistical approach to
extract the planetary parameters from the binary mask cross-correlation
analysis. We also adapt the concept of contribution function to the context of
high spectral resolution observations, to identify the location in the
planetary atmosphere where the detected emission originates. The average
planetary line profile intersected by a stellar G2 binary mask was found in
emission with a contrast of 84 $\pm$ 14 ppm relative to the planetary plus
stellar continuum (40 $\pm$ 5$\%$ relative to the planetary continuum only).
This result unambiguously indicates the presence of an atmospheric thermal
inversion. Finally, assuming a modelled temperature profile previously
published (Lothringer et al. 2018), we show that an iron abundance consistent
with a few times the stellar value explains the data well. In this scenario,
the iron emission originates at the $10^{-3}$-$10^{-5}$ bar level.
