A New High Perihelion Inner Oort Cloud Object: 2015 TG387

Abstract

Inner Oort Cloud objects (IOCs) are Trans-Plutonian for their entire orbits. They are beyond the strong gravitational influences of the known planets yet close enough to the Sun that outside forces are minimal. Here we report the discovery of the third known IOC after Sedna and 2012 VP113, called 2015 TG387. 2015 TG387 has a perihelion of $65 \pm 1$ au and semi-major axis of $1170 \pm 70$ au. The longitude of perihelion angle, $\bar{\omega}$, for 2015 TG387 is between that of Sedna and 2012 VP113, and thus similar to the main group of clustered extreme trans-Neptunian objects (ETNOs), which may be shepherded into similar orbital angles by an unknown massive distant planet, called Planet X or Planet Nine. 2015 TG387's orbit is stable over the age of the solar system from the known planets and Galactic tide. When including outside stellar encounters over 4 Gyrs, 2015 TG387's orbit is usually stable, but its dynamical evolution depends on the stellar encounter scenarios used. Surprisingly, when including a massive Planet X beyond a few hundred au on an eccentric orbit that is anti-aligned in longitude of perihelion with most of the known ETNOs, we find 2015 TG387 is typically stable for Planet X orbits that render the other ETNOs stable as well. Notably, 2015 TG387's argument of perihelion is constrained and its longitude of perihelion librates about 180 degs from Planet X's longitude of perihelion, keeping 2015 TG387 anti-aligned with Planet X over the age of the solar system. We find a power law slope near 3 for the semi-major axis distribution of IOCs, meaning there are many more high than low semi-major axis IOCs. There are about 2 million IOCs larger than 40 km, giving a mass of $10^{22}$ kg. The IOCs inclination distribution is similar to the scattered disk, with an average inclination of 19 degs.