  The chemical composition of an exoplanet is a key ingredient in constraining
its formation history. Iron is the most abundant transition metal, but has
never been directly detected in an exoplanet due to its highly refractory
nature. KELT-9b (HD 195689b) is the archetype of the class of ultra-hot
Jupiters that straddle the transition between stars and gas-giant exoplanets
and serve as distinctive laboratories for studying atmospheric chemistry,
because of its high equilibrium temperature of 4050 +/- 180 K. These properties
imply that its atmosphere is a tightly constrained chemical system that is
expected to be nearly in chemical equilibrium and cloud-free. It was previously
predicted that the spectral lines of iron will be detectable in the visible
range of wavelengths. At these high temperatures, iron and several other
transition metals are not sequestered in molecules or cloud particles and exist
solely in their atomic forms. Here, we report the direct detection of atomic
neutral and singly-ionized iron (Fe and Fe+), and singly-ionized titanium (Ti+)
in KELT-9b via the cross-correlation technique applied to high-resolution
spectra obtained during the primary transit of the exoplanet.
