The outer Solar System is a fossil record of our earliest dynamical history. Wide-field surveys have revealed a remarkable inventory of extreme trans-Neptunian objects (TNOs), including Sednoids (Sedna-like objects), that conventional formation models struggle to explain. In this talk, I will present a dynamical pathway for the origin of the Sednoid population involving a temporary super-Earth-mass rogue planet in the early Solar System. Numerical simulations show that such a planet can efficiently populate Sednoid-like orbits during its residence over a few hundred million years before eventual ejection. Backward integrations of the known Sednoids further hint at a primordial apsidal alignment approximately 4.2 Gyr ago, potentially preserving a dynamical footprint of this lost planet. I will discuss the implications of this scenario for the early instability and architecture of the Solar System, as well as its observationally testable predictions. Upcoming surveys and facilities, including FOSSIL-SSR (PI: me), Rubin/LSST, and JWST, are expected to greatly expand the known distant TNO population over the coming decade, providing the first real opportunity to distinguish between competing models for the origin of the Solar System’s most extreme small bodies.
