A New Window into Planet Formation and Migration: Refractory-to-Volatile Elemental Ratios in Ultra-hot Jupiters

A primary goal of exoplanet characterization is to use a planet's current composition understand how that planet formed. For example, the C/O ratio has long been recognized as carrying important information on chemistry volatile species. Refractory elements, like Fe, Mg, and Si, are usually not considered in this conversation because they condense into solids Fe(s) or MgSiO$_3$ would be removed from observable, gaseous atmosphere exoplanets cooler than about 2000~K. However, planets hotter 2000~K, called ultra-hot Jupiters (UHJs), warm enough largely avoid condensation refractory In paper, we explore insight measurement abundances can provide origins. Through refractory-to-volatile elemental abundance ratios, estimate atmospheric rock-to-ice fraction constrain formation migration scenarios. We first relate present-day its using various compositional models for rocky icy components protoplanetary disk. discuss potential confounding factors sequestration heavy metals core condensation. then show such retrievals low-resolution UV-IR transmission spectrum WASP-121b with PETRA, which 5.0$^{+6.0}_{-2.7}\times$ solar greater 2/3. This result consistent significant enrichment by planetismals. Lastly, rich future measuring ratios arrival JWST combining observations at low- high-resolution.