Reassessing the origin and evolution of Ecliptic Comets in the Planet-9 Scenario

Abstract

A group of newly observed extreme trans-Neptunian objects show an unexpected level of orbital confinement, characterized by an approximate alignment of the orbital angular momentum vectors and apsidal lines. It has been proposed that a yet undiscovered giant planet (named Planet-9,) exists in the outer parts of the solar system and is causing this clustering. Initial studies suggested that Planet-9 could be as massive as 15$M_{\oplus }$. In this mass range, however, this planet tends to strongly interact with scattered disk objects (SDOS; $50<a<1000$ au) and influence the dynamics and the orbits of a population of short period comets known as ecliptic comets. The outcome of this interaction is a population of ecliptic comets with orbital inclinations broadly inconsistent with observations. In this work, we model the formation and long-term dynamical evolution of trans-Neptunian object populations and Oort cloud during the solar system dynamical instability phase considering a revised set of mass and orbital parameters for Planet-9. In our simulation, Planet-9, is assumed to have a mass of $m_9\sim 7.5M_{\oplus }$, a moderately inclined orbit with $i_9\sim 20$ deg, semi-major axis $a_9\sim 600$ au, and orbital eccentricity of $e_9\sim 0.3$. Our results show that a relatively less massive Planet-9 is broadly consistent with the inclination distribution of trans-Neptunian objects and the observed number of ecliptic comets ($D>10$ km) in the solar system. Furthermore, our results indicate that under the influence of Planet-9, distant Kuiper belt objects with $40<q<100\mathrm{au}$ and $200<a<500\mathrm{au}$ that are significantly inclined, are more likely to be apsidally aligned with the planet rather than anti-aligned, with an anti-aligned-to-aligned population ratio of approximately 0.5-0.7. Objects within this semi-major axis and perihelion range and with orbital inclinations lower than $\lesssim$ 20 deg (comparable to that assumed for Planet-9), however, exhibit significant apsidal anti-alignment. Within this low-inclination subset, the ratio of anti-aligned to aligned populations is approximately 2-4. These findings provide a novel observational direction that could help refine the search for this putative planet.