Signatures of an Earth-class Planet in the Distant Solar System

Abstract

Edgeworth-Kuiper belt formation models fail to explain the architecture of the Solar system beyond 50 au, leaving the origins of detached objects (perihelion q > 40 au), high-inclination populations (inclination i > 45°), and extreme trans-Neptunian objects a persistent mystery. To resolve these anomalies, we performed comprehensive 4.5 Gyr N-body simulations incorporating grainy Neptune migration alongside two scenarios: primordial scattering planets (Mars- to Earth-mass embryos) and a distant, stable Earth-class planet with 1–2 M⊕ (hereafter ‘Planet X2’). The primordial scattering planet scenario failed to reproduce the observed distant populations in the modern Solar system because the embryos were typically ejected within a few to tens of megayears, yielding results similar to our control models without extra planets. In contrast, a 2-M⊕ Planet X2 orbiting at a semimajor axis a = 350–500 au (q = 200 au, i = 1°–15°) generates a substantial detached population—including a crucial low-inclination component beyond ∼150–200 au, creates a robust high-inclination reservoir, and replicates the orbital niches of all known extreme TNOs, such as Sedna and 2012 VP113. The model’s distant populations match all intrinsic population ratios and bulk orbital properties. Additionally, the model successfully accounts for inner TNO populations within ∼100 au. An evaluation using the Dark Energy Survey Simulator demonstrates that our preferred models satisfy observational distributions for detected inclination and perihelion populations, although a discrepancy in the semimajor axis distribution highlights an area for future refinement. Overall, these findings provide evidence for an undiscovered planet and offer testable predictions to guide future surveys, such as the Vera C. Rubin Observatory’s LSST.