Beyond Point Masses. V. Weywot's Non-Keplerian Orbit

TL;DR

This study analyzes the Quaoar–Weywot system with two decades of high-precision astrometry, new HST data, and occultations to reveal non-Keplerian dynamics driven by Quaoar's gravity field and surface heterogeneity. Using a Bayesian non-Keplerian orbit fitter (MultiMoon), the authors compare Keplerian models to those that include Quaoar's $J_2$ and COB-COL offsets, concluding that a constrained model (pole aligned with rings) yields $e<0.02$ and $J_2 \,\approx\,0.018^{+0.009}_{-0.008}$, consistent with a differentiated interior and a density of $\rho\approx 1751\pm13$ kg m$^{-3}$. They find robust, significant COB-COL offsets likely caused by latitudinal albedo variations and discuss their implications for orbit fitting and surface mapping, while also analyzing ring dynamics around Weywot’s mean-motion resonances and Quaoar’s spin-orbit resonances. The work places constraints on Quaoar’s interior, informs the ring confinement picture via $6:1$ MMR and $3:1$ SOR, and sets the stage for future observations (occultations, absolute astrometry, surface mapping) to refine the system’s non-Keplerian dynamics and the origins of its rings.

Abstract

We present a detailed dynamical analysis of the Quaoar-Weywot system based on nearly 20 years of high-precision astrometric data, including new HST observations and stellar occultations. Our study reveals that Weywot's orbit deviates significantly from a purely Keplerian model, requiring the inclusion of Quaoar's non-spherical gravitational field and center-of-body-center-of-light (COB-COL) offsets in our orbit models. We place a robust upper limit on Weywot's orbital eccentricity ($e<0.02$), substantially lower than previous estimates, which has important implications for the strength of mean motion resonances (MMRs) acting on Quaoar's ring system. Under the assumption that Quaoar's rings lie in its equatorial plane, we detect Quaoar's dynamical oblateness, $J_2$, at $\sim$2$σ$ confidence. The low $J_2$ value found under that assumption implies Quaoar is differentiated, with a total bulk density of $1751\pm13$ (stat.) kg m$^{-3}$. Additionally, we detect significant COB-COL offsets likely arising from latitudinal albedo variations across Quaoar's surface. These offsets are necessary to achieve a statistically robust orbit fit and highlight the importance of accounting for surface heterogeneity when modeling the orbits of dwarf planet moons. These findings improve our understanding of Quaoar's interior and surface while providing key insights into the stability and confinement mechanisms of its rings.

Figures (7)

• Figure 1: The architecture of the Quaoar system, as seen from Earth, shown to scale on 2006 Sep 21 12:00 UT. Locations of various spin-orbit and mean motion resonances are shown as colored ellipses. Quaoar pole orientation, ring width, and ring orientation are based on proudfoot2025jwst. Quaoar's shape is based on margoti2024quaoarshape, although the rotational phase is unknown, but selected to be at light curve maximum. Weywot's diameter from Fernandez-Valenzuela et al. (2025, in prep). Weywot orbit from this work.
• Figure 2: A corner plot showing the posterior distribution from our constrained orbit fit. Along the diagonal are the marginal (1D) posterior distributions for each parameter, alongside the 2D joint posterior distributions for each pair of parameters. Contours on the joint distributions show the 1$\sigma$, 2$\sigma$, and 3$\sigma$ confidence regions, and black points correspond to individual samples from the MCMC chain.
• Figure 3: Similar to Figure \ref{['fig:corner_con']}, but for the unconstrained orbit fit.
• Figure 4: Orbit fit quality ($\chi^2$ per degree of freedom) as a function of Quaoar's $J_2$ and Weywot's inclination (w.r.t. Quaoar's equatorial plane). Red dashed lines show the values expected from the undifferentiated oblate and triaxial shape models. The red cross shows the $J_2$ and inclination value measured in the constrained orbit fit. We note that the best fit quality is at $J_2 \sim 0.4$, but we limit the range of $J_2$ displayed to better show the fit quality at more realistic values of Quaoar's $J_2$. For reference, the p-value of a $\chi^2_{pdf}=1$ (1.8) is 0.46 (0.01).
• Figure 5: The COL-COB offsets implied by our constrained (left) and unconstrained (right) orbit fits. The COL is shown as a red ellipse (based on the orbit fit posterior) and the COB is marked with a black x. Quaoar is shown as a sphere with with angular diameter of 36 mas. The equator and sub-observer longitude are shown with blue lines. Over our observational baseline, a constant offset in ecliptic latitude and longitude is a good approximation of a COB-COL offset caused by longitudinal albedo features.