The mass and density of the dwarf planet 2007 OR10

The satellite of 2007 OR10 was discovered on archival Hubble Space Telescope images in 2017. With new observations taken with the WFC3 camera of the HST in late 2017 we were able to confirm the presence of the satellite and determine the orbit. The orbit's notable eccentricity, e=0.31, may be a consequence of an intrinsically eccentric orbit and slow tidal evolution, but may also be caused by the Kozai mechanism. Dynamical considerations also suggest that the moon is small, D < 100 km. Based on the newly determined system mass of 1.75×10²¹kg, 2007 O10 is the fifth most massive dwarf planet after Eris, Pluto, Haumea and Makemake. We also revisited the radiometric size estimate of the primary using the assumption that the moon orbits in the equatorial plane of the primary. This approach provides a size range of 1210 km < D < 1295 km, and a bulk density of 1.72±0.16 g cm^-3 for the primary. A previous size estimate that had assumed an equator-on configuration (D = 1535⁺⁷⁵_-225 km) provides a density of 0.92_-0.14^+0.46 g cm^-3, unexpectedly low for a 1000 km-sized dwarf planet. 2007 OR10 and the satellite have the larest color difference, Δ(V-R)=0.43±0.17 among binary transneptunian objects.