No evidence for a metallicity-dependent enhancement of distant giant companions to close-in small planets in the California Legacy Survey

Abstract

Understanding the relationship between close-in small planets (CS) and distant giants (DG) is central to understanding the formation of planetary systems like our own. Most studies of this connection have found evidence for a positive correlation, though significant statistical and systematic uncertainties remain due to differences in sample size, target selection bias, and even the definitions of `close-in small' and `distant giant' planets. Recently, Bryan & Lee (2024) conducted a study of 184 stars hosting super-Earths ($M \sin i = 1-20 \, M_{E}$ or $R = 1-4 \, R_{E}$) to determine the effect of stellar metallicity on the prevalence of distant giant companions ($M \sin i = 0.5-20 \, M_{Jup}$, $a = 1-10$ AU). They found that such giants are twice as common in the presence of inner planets, but \textit{only} in metal-rich systems: P(DG|CS, [Fe/H]>0) = $28^{+4.9}_{-4.6}\%$ vs. P(DG|[Fe/H]>0) = $14.3 ^{+2.0}_{-1.8}\%$. Further, they found that this correlation disappears for metal-poor stars: P(DG|CS, [Fe/H]$\leq$0) = $4.5^{+2.6}_{-1.9}\%$ vs. P(DG|[Fe/H]$\leq$0) = $5.0^{+1.6}_{-1.3}\%$. We conducted an analogous study to test whether this effect was present in the California Legacy Survey sample. Using the same planet definitions, we did not find evidence for an enhancement in metal-rich systems: P(DG|CS, [Fe/H]>0) = $14^{+12}_{-8}\%$ vs. P(DG|[Fe/H]>0) = $16.4^{+2.5}_{-2.4}\%$. We also found a $2\sigma$ tension between our conditional rate and a 2x enhancement. The discrepancy between our findings and theirs may arise from a variety of sources, underscoring the need for large exoplanet surveys to overcome both small number statistics and sample inhomogeneities.