Outside of Neptune's orbit, near Pluto, lies the dwarf planet Haumea, one of the most interesting bodies ever discovered. Its triaxial ellipsoid shape implies a very fast rotation, and it has two large moons, named Hi'iaka and Namaka, which have similarities to the Earth-Moon system. There are many indications of a collisional formation, but the formation of this system is still not fully understood. For example, it is not known why the moons are so far from Haumea or why they are in non-circular orbits. Cuk et. al. 2014 clearly demonstrated that to better explain the evolution of this system, we must first improve our estimates of the orbits and masses of Haumea and its two moons.
MacDonald, Mariah, "The Masses and Orbits of the Moons of Dwarf Planet Haumea" (2015). Aerospace, Physics, and Space Science Student Publications. 20.