  We investigate the origin of the measured over-abundance of alkali metals in
the atmospheres of hot gas giants, relative to both their host stars and their
atmospheric water abundances. We show that formation exterior to the water snow
line followed by inward disc-driven migration results in excess accretion of
oxygen-poor, refractory-rich material from within the snow-line. This naturally
leads to enrichment of alkali metals in the planetary atmosphere relative to
the bulk composition of its host star but relative abundances of water that are
similar to the stellar host. These relative abundances cannot be explained by
in situ formation which places the refractory elements in the planetary deep
interior rather than the atmosphere. We therefore suggest that the measured
compositions of the atmospheres of hot Jupiters are consistent with significant
migration for at least a subset of hot gas giants. Our model makes robust
predictions about atmospheric composition that can be confirmed with future
data from JWST and Ariel.
