The Eris/Dysnomia system I: The orbit of Dysnomia

We present new results on the Eris/Dysnomia system including analysis of new images from the WFC3 instrument on the Hubble Space Telescope (HST). Seven HST orbits were awarded to program 15171 in January and February 2018, with the intervals between observations selected to sample Dysnomia over a full orbital period. Using relative astrometry of Eris and Dysnomia, we computed a best-fit Keplerian orbit for Dysnomia. Based on the Keplerian fit, we find an orbital period of 15.785899$\pm$0.000050 days, which is in good agreement with recent work. We report a non-zero eccentricity of 0.0062 at the 6.2-$\sigma$ level, despite an estimated eccentricity damping timescale of $\leq$17 Myr. Considering the volumes of both Eris and Dysnomia, the new system density was calculated to be 2.43$\pm$0.05 g cm$^{-3}$, a decrease of $\sim$4% from the previous value of 2.52$\pm$0.05 g cm$^{-3}$. The new astrometric measurements were high enough precision to break the degeneracy of the orbit pole orientation, and indicate that Dysnomia orbits in a prograde manner. The obliquity of Dysnomia's orbit pole with respect to the plane of Eris' heliocentric orbit was calculated to be 78.29$\pm$0.65$^{\circ}$ and is in agreement with previous work; the next mutual events season will occur in 2239. The Keplerian orbit fit to all the data considered in this investigation can be excluded at the 6.3-$\sigma$ level, but identifying the cause of the deviation was outside the scope of this work.