Outer Solar System Perihelion Gap Formation through Interactions with a Hypothetical Distant Giant Planet

Abstract

Abstract Among the outer solar system minor planet orbits there is an observed gap in perihelion between roughly 50 and 65 au at eccentricities e ≳ 0.65. Through a suite of observational simulations, we show that the gap arises from two separate populations, the Extreme Trans-Neptunian Objects (ETNOs; perihelia q ≳ 40 au and semimajor axes a ≳ 150 au) and the Inner Oort Cloud objects (IOCs; q ≳ 65 au and a ≳ 250 au), and is very unlikely to result from a realistic single, continuous distribution of objects. We also explore the connection between the perihelion gap and a hypothetical distant giant planet, often referred to as Planet 9 or Planet X, using dynamical simulations. Some simulations containing Planet X produce the ETNOs, the IOCs, and the perihelion gap from a simple Kuiper-Belt-like initial particle distribution over the age of the solar system. The gap forms as particles scattered to high eccentricity by Neptune are captured into secular resonances with Planet X where they cross the gap and oscillate in perihelion and eccentricity over hundreds of kiloyears. Many of these objects reach a minimum perihelia in their oscillation cycle within the IOC region increasing the mean residence time of the IOC region by a factor of approximately five over the gap region. Our findings imply that, in the presence of a massive external perturber, objects within the perihelion gap will be discovered, but that they will be only ∼20% as numerous as the nearby IOC population (65 au ≲ q ≲ 100 au).