Orbital clustering of distant Kuiper Belt Objects by hypothetical Planet 9. Secular or resonant ?

TL;DR

This paper reevaluates the orbital clustering of distant Kuiper Belt Objects attributed to a hypothetical Planet 9, demonstrating that secular dynamics alone can explain observed features without exclusively relying on mean-motion resonances.

Contribution

It introduces a numerical approach to the secular Hamiltonian analysis, revealing new libration islands and clarifying the roles of secular and resonant mechanisms in KBO clustering.

Findings

Secular dynamics can produce anti-aligned, high-eccentricity libration islands matching observations.

Series expansion of the secular Hamiltonian is not convergent, requiring numerical computation.

Resonant motion can also generate similar libration islands, influencing the interpretation of KBO clustering mechanisms.

Abstract

Statistical analysis of the orbits of distant Kuiper Belt Objects (KBOs) have led to suggest that an additional planet should reside in the Solar System. According to recent models, the secular action of this body should cause orbital alignment of the KBOs. It was recently claimed that the KBOs concerned by this dynamics are presumably trapped in mean motion resonances with the suspected planet. I reinvestigate here the secular model underlying this idea. The original analysis was done expanding and truncating the secular Hamiltonian. I show that this is inappropriate here, as the series expansion is not convergent. I present a study based on numerical computation of the Hamiltonian with no expansion. I show in phase-space diagrams the existence of apsidally anti-aligned, high eccentricity libration islands that were not present in the original modelling, but that match numerical simulations. These island were claimed to correspond to bodies trapped in mean-motion resonances with the hypothetical planet, and match the characteristics of the distant KBOs observed. My main result is that regular secular dynamics can account for the anti-aligned particles itself as well as mean-motion resonances. I also perform a semi-analytical study of resonant motion and show that some resonance are actually capable of producing the same libration islands. I discuss then the relative importance of both mechanisms.