A New Dance Partner for Uranus: Discovering a Minor Planet in Outer Resonance

Daniel Bamberger and colleagues present the discovery and analysis of a minor planet, known as 2015 OU194, that appears to be locked in a rare orbital pattern -- called a mean motion resonance -- with Uranus. This work uncovers the first known example of such an “outer” resonance involving Uranus and explores two other possible candidates in similar orbits.

Introduction: The Chaotic Lives of Centaurs

The authors begin by discussing the peculiar region of the Solar System that lies between Saturn and Neptune, where many small icy objects called centaurs orbit. Centaurs are known for having chaotic and unstable paths through space, which makes them fascinating and challenging to study. One way that some of these objects can become temporarily stable is through a mean motion resonance (MMR) -- a gravitational relationship where the minor planet completes a certain number of orbits in the same time it takes a planet to complete a different number of orbits. For example, if the small body completes 3 orbits while Uranus completes 4, it would be in what is called a 3:4 resonance, or “U3/4.” Until now, all known Uranus resonances involved objects orbiting inside its path; this paper examines the first known case of an object in an outer resonance with Uranus, beyond its orbit.

Observations: Unearthing Archival Data

2015 OU194 was initially discovered on 12 July 2015 by the Subaru Telescope in Hawaii and observed again the following year. However, Bamberger and colleagues dug deeper, uncovering additional images of the object taken in 2017 and 2018 that had not been used before. These extra observations extended the “data-arc” -- the span of time over which we have tracked the object -- from just 1 year to 3.5 years, allowing for a much more accurate calculation of its orbit. Using 34 observations and orbital modeling software, they determined that 2015 OU194 has a nearly circular orbit with a semi-major axis of about 23.33 astronomical units (AU), and takes roughly 113 years to orbit the Sun. The object’s colors, measured in several filters, are typical for inactive centaurs or trans-Neptunian objects, suggesting no unusual surface activity.

Simulations: Testing the Stability of the Resonance

With the extended observations in hand, the team ran computer simulations to test whether 2015 OU194 is truly in a stable U3/4 resonance. They created two “clones” of the object’s orbit (slightly varied versions within observational error) and tracked all three over a simulated period stretching a million years into the past and half a million years into the future. The results showed that all three versions remain trapped in the U3/4 resonance over these timescales, with a resonant angle that oscillates (or librates) around 180°, rarely swinging too far from this value. This small amplitude of libration suggests the resonance is relatively stable -- a surprising finding given how chaotic centaur orbits usually are.

Searching for Other Resonant Objects

Curious whether 2015 OU194 is alone in this rare configuration, the authors looked at two other known minor planets with similar orbits: 2013 RG98 and 2014 NX65. Their simulations showed that 2013 RG98 also enters the U3/4 resonance for hundreds of thousands of years, though its resonance is less stable, with the resonant angle varying more wildly. The third object, 2014 NX65, does not stay in the U3/4 resonance; instead, it bounces between interacting with Uranus and Neptune in various ways, demonstrating more chaotic behavior. Thus, 2015 OU194 stands out for its particularly calm and enduring resonance.

Conclusions: A Unique Finding

Bamberger and colleagues conclude that 2015 OU194 is unique among the minor planets currently known in the outer Solar System: it occupies an outer resonance with Uranus, has a nearly circular orbit, and displays a stable and low-amplitude resonance over long timescales. This discovery opens the door to studying how such resonances can shape the populations of centaurs and hints that other, similar objects may yet be found.

Source: Bamberger