Breaking Degeneracies in Formation Histories by Measuring Refractory Content in Gas Giants

Relating planet formation to atmospheric composition has been a long standing goal of the astrophysics community. So far, most modeling studies have focused on predicting the enrichment of heavy elements relative to solar and the C/O ratio in giant planet atmospheres. Although this framework provides useful constraints on the potential formation locations of gas giant exoplanets, C and O measurements alone are not enough to determine where a given gas giant planet originated. We need abundance measurements of additional elements to break these degeneracies. Here, I show that characterizing the abundances of refractory elements (e.g., silicon, iron) would be a valuable addition to our toolkit for constraining gas giant origins. Refractory elements are invariably present in the solid phase throughout the disk and their abundance in an atmosphere gives an estimate of the solid-to-gas accretion during formation. I introduce a new framework for the interpretation of atmospheric abundances of exoplanets in the form of three parameters: 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, I present a global picture of their variation with distance and time based on what we know from the solar system meteorites and updated understanding of thermal processing in protoplanetary disks. I then demonstrate that ultra hot Jupiters present an unprecedented opportunity for measuring the abundances of refractories, O and C and for probing planet formation histories. Finally, I suggest a new formation pathway for the hot Jupiters with low water abundances that emerges from this framework and does not require them to form at the CO snowline.