The size, density, and formation of the Orcus-Vanth system in the Kuiper Belt

Abstract

The Kuiper Belt object (KBO) Orcus and its satellite Vanth form an unusual system in the Kuiper Belt. While most large KBOs have small satellites in circular orbits and smaller KBOs and their satellites tend to be much closer in size, Orcus sits in between these two regimes. Orcus is among the largest objects known in the Kuiper Belt, but the relative size of Vanth is much larger than that of the tiny satellites of the other large objects. Here, we characterize the physical and orbital characteristics of the Orcus-Vanth system in an attempt to distinguish discuss possible formation scenarios. From Hubble Space Telescope observations, we find that Orcus and Vanth have different visible colors and that Vanth does not share the water ice absorption feature seen in the infrared spectrum of Orcus. We also find that Vanth has a nearly face-on circular orbit with a period of 9.5393 ± 0.0001 days and semimajor axis of 8980 ± 20 km, implying a system mass of (6.32 ± 0.01) × 10^(20) kg or 3.8% the mass of dwarf planet Eris. From Spitzer Space Telescope observations, we find that the thermal emission is consistent with a single body with diameter 940 ± 70 km and a geometric albedo of 0.28 ± 0.04. Assuming equal densities and albedos, this measurement implies sizes of Orcus and Vanth of 900 and 280 km, respectively, and a mass ratio of 33. Assuming a factor of 2 lower albedo for the non-icy Vanth, however, implies sizes of 860 km and 380 km and a mass ratio of 12. The measured density depends on the assumed albedo ratio of the two objects but is approximately 1.5 ± 0.3 g cm^(–3), midway between typical densities measured for larger and smaller objects. The orbit and mass ratio is consistent with formation from a giant impact and subsequent outward tidal evolution, and even consistent with the system having now achieved a double synchronous state. Because of the large angle between the plane of the heliocentric orbit of Orcus and the plane of the orbit of Vanth, the system can be equally well explained, however, by initial eccentric capture, Kozai cycling to increase the eccentricity and decrease the pericenter of the orbit of Vanth, and subsequent inward tidal evolution. We discuss implications of these formation mechanisms.