Causal Evidence for the Primordiality of Colors in Trans-Neptunian Objects

TL;DR

This study uses a data-driven causal modeling approach to demonstrate that the colors of Trans-Neptunian Objects are primarily primordial, reflecting their formation conditions rather than collisional evolution or irradiation effects.

Contribution

It introduces a model-agnostic causal graph approach to determine the origin of TNO colors, providing new evidence for their primordial nature and predicting an unknown perturbing body.

Findings

TNO colors are the root cause of their inclination distribution.

The causal model predicts Neptune as an unknown perturbing body.

Colors of TNOs are predominantly primordial, not altered by irradiation.

Abstract

The origins of the colors of Trans-Neptunian Objects (TNOs) represent a crucial unresolved question, central to understanding the history of our Solar System. Recent observational surveys have revealed correlations between the eccentricity and inclination of TNOs and their colors. This has rekindled the long-standing debate on whether these colors reflect the conditions of TNO formation or their subsequent collisional evolution. In this study, we address this question with 98.7% certainty, using a model-agnostic, data-driven approach based on causal graphs. First, as a sanity check, we demonstrate how our model can replicate the currently accepted paradigms of TNOs' dynamical history, blindly and without any orbital modeling or physics-based assumptions. In fact, our causal model (with no knowledge of the existence of Neptune) predicts the existence of an unknown perturbing body, i.e., Neptune. We then show how this model predicts, with high certainty, that the color of TNOs is the root cause of their inclination distribution, rather than the other way around. This strongly suggests that the colors of TNOs reflect an underlying dynamical property, most likely their formation location. Moreover, our causal model excludes formation scenarios that invoke substantial color modification by subsequent irradiation. We therefore conclude that the colors of TNOs are predominantly primordial.