Mutual Orbital Inclinations Between Cold Jupiters and Inner Super-Earths

Abstract

Previous analyses of Doppler and Kepler data have found that Sun-like stars hosting "cold Jupiters" (giant planets with $a\gtrsim 1\,\mathrm{AU}$) almost always host "inner super-Earths" (1-$4\,R_\oplus$, $a\lesssim1\,\mathrm{AU}$). Here, we attempt to determine the degree of alignment between the orbital planes of the cold Jupiters and the inner super-Earths. The key observational input is the fraction of Kepler stars with transiting super-Earths that also have transiting cold Jupiters. This fraction depends on both the probability for cold Jupiters to occur in such systems, and on the mutual orbital inclinations. Since the probability of occurrence has already been measured in Doppler surveys, we can use the data to constrain the dispersion of the mutual inclination distribution. We find $\sigma=11.8^{+12.7}_{-5.5}\,\mathrm{deg}$ (68% confidence) and $\sigma>3.5\,\mathrm{deg}$ (95% confidence), where $\sigma$ is the scale parameter of the Rayleigh distribution. This suggests that planetary orbits in systems with cold Jupiters tend to be coplanar - although not quite as coplanar as those in the Solar System, which have a mean inclination from the invariable plane of $1.8\,\mathrm{deg}$. We also find evidence that cold Jupiters have lower mutual inclinations relative to inner systems with higher transit multiplicity. This suggests a link between the dynamical excitation in the inner and outer systems. For example, perturbations from misaligned cold Jupiters may dynamically heat or destabilize systems of inner super-Earths.