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

Abstract

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 ultrahot 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.

Evidence for Hidden Nearby Companions to Hot Jupiters

“…While the initial formation locations of hot Jupiters may be obfuscated by post-disk dynamical sculpting, compositional tracers of their birthplaces may remain undisturbed. Chemical signatures of the gas giants' initial formation locations, such as the carbon-to-oxygen ratio (C/O; Öberg et al 2011) or silicon-based tracers of refractory content (Chachan et al 2022), may remain detectable and can potentially be used to distinguish the initial locations of Jovian planet formation in the future (e.g. Line et al 2021;Reggiani et al 2022).…”