OSSOS IX: two objects in Neptune's 9:1 resonance -- implications for resonance sticking in the scattering population

TL;DR

This paper reports the discovery of two objects in Neptune's 9:1 resonance, estimates the resonance population beyond 100 au, and discusses implications for resonance sticking and the Kuiper belt's history.

Contribution

First robust estimate of Neptune's 9:1 resonance population beyond 100 au based on OSSOS detections, supporting resonance sticking as a key process.

Findings

Two objects securely in Neptune's 9:1 resonance at ~130 au.

Estimated resonance population of ~11,000 objects with D>100 km.

Resonance occupation timescales of ~1 Gyr consistent with transient sticking.

Abstract

We discuss the detection in the Outer Solar System Origins Survey (OSSOS) of two objects in Neptune's distant 9:1 mean motion resonance at semimajor axis $a\approx~130$~au. Both objects are securely resonant on 10~Myr timescales, with one securely in the 9:1 resonance's leading asymmetric libration island and the other in either the symmetric or trailing asymmetric island. These objects are the largest semimajor axis objects with secure resonant classifications, and their detection in a carefully characterized survey allows for the first robust resonance population estimate beyond 100~au. The detection of these objects implies a 9:1 resonance population of $1.1\times10^4$ objects with $H_r<8.66$ ($D~\gtrsim~100$~km) on similar orbits (95\% confidence range of $\sim0.4-3\times10^4$). Integrations over 4~Gyr of an ensemble of clones spanning these objects' orbit fit uncertainties reveal that they both have median resonance occupation timescales of $\sim1$~Gyr. These timescales are consistent with the hypothesis that these objects originate in the scattering population but became transiently stuck to Neptune's 9:1 resonance within the last $\sim1$~Gyr of solar system evolution. Based on simulations of a model of the current scattering population, we estimate the expected resonance sticking population in the 9:1 resonance to be 1000-4500 objects with $H_r<8.66$; this is marginally consistent with the OSSOS 9:1 population estimate. We conclude that resonance sticking is a plausible explanation for the observed 9:1 population, but we also discuss the possibility of a primordial 9:1 population, which would have interesting implications for the Kuiper belt's dynamical history.