Hyperbolic Orbits in the Solar System: Interstellar Origin or Perturbed Oort Cloud Comets?

TL;DR

This paper analyzes the origins of hyperbolic objects in the Solar System, deriving probability distributions to distinguish between interstellar and Oort cloud sources, and applies this to known objects like 'Oumuamua and Borisov.

Contribution

It provides analytical models to estimate the likelihood of hyperbolic objects originating from interstellar space or the Oort cloud, based on their orbital parameters.

Findings

'Oumuamua and Borisov are most likely of interstellar origin.

Oort cloud objects can be perturbed into hyperbolic orbits by low-mass objects.

Likelihood of interstellar origin increases with decreasing eccentricity and perihelion.

Abstract

We study the dynamical properties of objects in hyperbolic orbits passing through the inner Solar system in the context of two different potential sources: interstellar space and the Oort cloud. We analytically derive the probability distributions of eccentricity, $e$, and perihelion distance, $q$, for each source and estimate the numbers of objects produced per unit of time as a function of these quantities. By comparing the numbers from the two sources, we assess which origin is more likely for a hyperbolic object having a given eccentricity and perihelion distance. We find that the likelihood that a given hyperbolic object is of interstellar origin increases with decreasing eccentricity and perihelion. Conversely, the likelihood that a hyperbolic object has been scattered from the Oort cloud by a passing star increases with decreasing eccentricity and increasing perihelion. By carefully considering their orbital elements, we conclude that both 1I/2017 U1 'Oumuamua ($e\simeq$ 1.2 and $q\simeq$ 0.26 au) and 2I/2019 Q4 Borisov ($e\simeq$ 3.3 and $q\simeq$ 2 au) are most likely of interstellar origin, not scattered from the Oort cloud. However, we also find that Oort cloud objects can be scattered into hyperbolic orbits like those of the two known examples, by sub-stellar and even sub-Jovian mass perturbers. This highlights the need for better characterization of the low mass end of the free-floating brown dwarf and planet population.