Measuring Apsidal Clustering

TL;DR

This paper introduces a new method to measure apsidal clustering in the outer solar system, showing the significance has decreased with recent data, impacting the Planet X hypothesis.

Contribution

A novel conditional-likelihood method for measuring apsidal clustering that accounts for survey biases and applies to expanding TNO datasets.

Findings

Apsidal clustering significance decreased from 2.7σ to 1.9σ.

Expanded the sample of relevant TNOs from 21 to 25.

Clustering direction is now less well constrained.

Abstract

The decade-long debate over the existence of apsidal clustering in the outer solar system is poised for reignition given the plethora of distant trans-Neptunian object (TNO) discoveries expected from the forthcoming Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST). Here, we present a new conditional-likelihood method to measure apsidal clustering that is insensitive to uneven survey footprints. We calculate the long-term orbital stability of distant TNOs, which allows us to expand the known sample of relevant objects from 21 to 25. We apply our new method to this up-to-date sample, showing that the significance of the apsidal clustering in the outer solar system has fallen from $2.7\sigma$ to $1.9\sigma$, and that the direction of clustering is not well constrained. This new method is suitable for application to the growing sample of known TNOs, and the results will reveal whether the evidence for a hypothetical Planet X from apsidal clustering is real or spurious.