On the dynamical origins of retrograde Jupiter Trojans and their connection to high-inclination TNOs

Abstract

Over the course of the last decade, observations of highly inclined (orbital inclination i > 60$^\circ$) trans-Neptunian objects (TNOs) have posed an important challenge to current models of solar system formation (Levison et al. in Icarus 196(1):258–273, 2008; Nesvorný in Astron J 150:73, 2015). These remarkable minor planets necessitate the presence of a distant reservoir of strongly out-of-plane TNOs, which itself requires some dynamical production mechanism (Gladman et al. in Astron J Lett 697:L91–L94, 2009; Gomes et al. in Icarus 258:37–49, 2015; Batygin and Brown in Astrophys J 833(1):L3, 2016). A notable recent addition to the census of high-i minor bodies in the solar system is the retrograde asteroid 514107 Ka’epaoka’awela, which currently occupies a 1:-1 mean motion resonance with Jupiter at i = 163$^\circ$ (Wiegert et al. in Nature 543:687–689, 2017). In this work, we delineate a direct connection between retrograde Jupiter Trojans and high-i Centaurs. First, we backpropagate a large sample of clones of Ka’epaoka’awela for 100 Ma numerically and demonstrate that long-term stable clones tend to decrease their inclination steadily until it concentrates between 90$^\circ$ and 135$^\circ$, while their eccentricity and semi-major axis increase, placing many of them firmly into the trans-Neptunian domain. Importantly, the clones show significant overlap with the synthetic high-i Centaurs generated in Planet 9 studies (Batygin et al. in Phys Rep 805:1–53, 2019), and hint at the existence of a relatively prominent, steady-state population of minor bodies occupying polar trans-Saturnian orbits. Second, through direct numerical forward modeling, we delineate the dynamical pathway through which conventional members of the Kuiper Belt’s scattered disk population can become retrograde Jovian Trojan resonators in the presence of Planet 9.