Hot Jupiters Have Giant Companions: Evidence for Coplanar High-Eccentricity Migration

This study considers the characteristics of planetary systems with giant planets based on a population-level analysis of the California Legacy Survey planet catalog. We identified three characteristics common to hot Jupiters. First, while not all hot Jupiters have a detected outer giant planet companion ($M \sin i$ = 0.3--30 $M_{\textrm{Jup}}$), such companions are ubiquitous when survey completeness corrections are applied for orbital periods out to 40,000 days. Giant harboring systems without a hot Jupiter also host at least one outer giant planet companion per system. Second, the mass distributions of hot Jupiters and other giant planets are indistinguishable. However, within a planetary system that includes a hot Jupiter, the outer giant planet companions are at least $3\times$ more massive than the inner hot Jupiters. Third, the eccentricity distribution of the outer companions in hot Jupiter systems (with an average model eccentricity of $\langle e\rangle=0.34\pm0.05$) is different from the corresponding outer planets in planetary systems without hot Jupiters ($\langle e\rangle=0.19\pm0.02$). We conclude that the existence of two gas giants, where the outermost planet has an eccentricity $\ge0.2$ and is $3\times$ more massive, are key factors in the production of a hot Jupiter. Our simple model based on these factors predicts that $\sim$10\% of warm and cold Jupiter systems will by chance meet these assembly criteria, which is consistent with our measurement of $16\pm6\%$ relative occurrence of hot Jupiter systems to all giant-harboring systems. We find that these three features favor coplanar high-eccentricity migration as the dominant mechanism for hot Jupiter formation.