  Relating planet formation to atmospheric composition has been a long-standing
goal of the planetary science community. So far, most modeling studies have
focused on predicting the enrichment of heavy elements and the C/O ratio in
giant planet atmospheres. Although this framework provides useful constraints
on the potential formation locations of gas giant exoplanets, carbon and oxygen
measurements alone are not enough to determine where a given gas giant planet
originated. Here, we show that characterizing the abundances of refractory
elements (e.g., silicon, iron) can break these degeneracies. Refractory
elements are present in the solid phase throughout most of the disk and their
atmospheric abundances therefore reflect the solid-to-gas accretion ratio
during formation. We introduce a new framework that parameterizes the
atmospheric abundances of gas giant exoplanets in the form of three ratios:
Si/H, O/Si, and C/Si. Si/H traces the solid-to-gas accretion ratio of a planet
and is loosely equivalent to earlier notions of 'metallicity'. For O/Si and
C/Si, we present a global picture of their variation with distance and time
based on what we know from the solar system meteorites and an updated
understanding of the variations of thermal processing within protoplanetary
disks. We show that ultra-hot Jupiters are ideal targets for atmospheric
characterization studies using this framework, as we can measure the abundances
of refractories, oxygen and carbon in the gas phase. Finally, we propose that
hot Jupiters with silicate clouds and low water abundances might have accreted
their envelopes between the soot line and the water snowline.
