[P-PS03] Small Solar System Bodies: New perspectives on the origin and evolution of the Solar System

[PPS03-06] A Rogue Planet Hypothesis for the Formation of the Outer Solar System

*Yukun Huang¹ (1.National Observatory of Japan)

Keywords:Kuiper Belt, Trans-Neptunian Objects, Formation of the Solar System, Planetary Dynamics

Over the past two decades, outer Solar System surveys have greatly expanded our inventory of Trans-Neptunian Objects (TNOs), revealing complex structures in the Kuiper Belt that challenge our understanding of the early Solar System. Notably, there is a population of objects in distant mean-motion resonances with Neptune, a substantial detached population not dynamically coupled with Neptune, and the existence of sednoids—extremely distant TNOs with high perihelia, including Sedna, 2012 VP113, and Leleākūhonua (2015 TG387). We propose that a super-Earth-mass planet, temporarily present in the early Solar System—dubbed a “rogue planet”—can account for these observed features. Our simulations demonstrate that such a rogue planet can sufficiently populate the distant Kuiper Belt while preserving the low-inclination cold classical belt, consistent with recent observation surveys. One observable constraint to test the rogue planet hypothesis is the primordial orbital alignment of sednoids. By integrating the orbits of the three sednoids backward over the age of the Solar System, we find that their apsidal lines tightly clustered only once, around 4.5 Gyr ago, at a perihelion longitude of 200°, indicating a primordial alignment imprinted during the planet formation epoch. These findings constrain the mass and lifetime of the rogue planet and suggest that alternative models—such as a still-present Planet Nine or early stellar encounters—may not adequately explain the observed features. If future sednoid discoveries confirm this primordial alignment, it would strongly support the rogue planet hypothesis as a coherent explanation for the formation and evolution of the outer Solar System.