Causal evidence for the primordiality of colours in trans-Neptunian objects

TL;DR

This study uses a data-driven causal analysis to demonstrate that the colours of Trans-Neptunian Objects are primarily primordial, reflecting their formation locations rather than evolutionary processes.

Contribution

It introduces a model-agnostic causal inference approach that reveals the primordial nature of TNO colours without relying on orbital or physical modeling.

Findings

TNO colours are causally linked to their inclination distribution.

The model predicts an unknown confounder consistent with Neptune's influence.

TNO colours are mainly primordial, not altered by subsequent evolution.

Abstract

The origins of the colours of Trans-Neptunian Objects (TNOs) represent a crucial unresolved question, central to understanding the history of our Solar System. Recent observational surveys revealed correlations between the eccentricity and inclination of TNOs, and their colours. This rekindled the long-standing debate on whether these colours reflect the conditions of TNO formation or their subsequent evolution. We address this question using a model-agnostic, data-driven approach that unanimously converges to a common causal graph from the analysis of two different datasets, each from two different conditional independence test methods. For evaluation, we demonstrate how our model is consistent with the currently-accepted paradigms of TNOs' dynamical histories, without involving any orbital modelling or physics-based assumptions. Our causal model (with no knowledge of the existence of Neptune) predicts the need for an unknown confounding variable, consistent with Neptune's effects. The model predicts that the colour of TNOs is the root cause of their inclination distribution, rather than the other way around. This strongly suggests that the colours of TNOs reflect an underlying dynamical property, most likely their formation location. Our model excludes formation scenarios that invoke substantial colour modification by subsequent evolution. We conclude that the colours of TNOs are predominantly primordial.