The Intrinsic Neptune Trojan Orbit Distribution: Implications for the Primordial Disk and Planet Migration

TL;DR

This study uses a Bayesian approach to debias the orbit distribution of Neptune Trojans, revealing they are dynamically excited and similar to Jupiter Trojans, with implications for the primordial disk and Neptune's migration history.

Contribution

It provides the first survey-agnostic debiased orbit distribution of Neptune Trojans and compares them to Jupiter Trojans, informing models of Neptune's migration and the primordial disk.

Findings

Neptune Trojans have an inclination width >11° at 95% confidence.

Debiased distributions of Neptune and Jupiter Trojans are very similar.

Migration models suggest a pre-heated disk is needed for observed Trojan inclinations.

Abstract

The present-day orbit distribution of the Neptune Trojans is a powerful probe of the dynamical environment of the outer solar system during the late stages of planet migration. In this work, I conservatively debias the inclination, eccentricity, and libration amplitude distributions of the Neptune Trojans by reducing a priori unknown discovery and follow-up survey properties to nuisance parameters and using a likelihood-free Bayesian rejection sampler for parameter estimation. Using this survey-agnostic approach, I confirm that the Neptune Trojans are a dynamically excited population: at $>$95% confidence, the Neptune Trojans' inclination width must be $\sigma_i > 11^\circ$. For comparison and motivation purposes, I also model the Jupiter Trojan orbit distributions in the same basis and produce new estimates of their parameters (Jupiter Trojan $\sigma_i=14.4^\circ \pm 0.5^\circ$, $\sigma_{L11} = 11.8^\circ \pm 0.5^\circ$, and $\sigma_e = 0.061\pm 0.002$). The debiased inclination, libration amplitude, and eccentricity distributions of the Neptune Trojans are nominally very similar to those of the Jupiter Trojans. I use these new constraints to inform a suite of simulations of Neptune Trojan capture by an eccentric, rapidly-migrating Neptune from an initially dynamically-hot disk. These simulations demonstrate that if migration and eccentricity-damping timescales were short ($\tau_a \lesssim 10$ Myr, $\tau_e \lesssim 1$ Myr), the disk that Neptune migrated into must have been pre-heated (prior to Neptune's appearance) to a width comparable to the Neptune Trojans' extant width to produce a captured population with an inclination distribution width consistent with that of the observed population.