The satellite of ( 225088 ) 2007 OR _ { 10 } was discovered on archival Hubble Space Telescope images and along with new observations with the WFC3 camera in late 2017 we have been able to determine the orbit .
The orbit ’ s notable eccentricity , e \approx 0.3 , 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 , \mathrm { D _ { eff } } < 100 km .
Based on the newly determined system mass of 1.75 \cdot 10 ^ { 21 } kg , 2007 OR _ { 10 } is the fifth most massive dwarf planet after Eris , Pluto , Haumea and Makemake .
The newly determined orbit has also been considered as an additional option in our radiometric analysis , provided that the moon orbits in the equatorial plane of the primary .
Assuming a spherical shape for the primary this approach provides a size of 1230 \pm 50 km , with a slight dependence on the satellite orbit orientation and primary rotation rate chosen , and a bulk density of 1.75 \pm 0.07 g cm ^ { -3 } for the primary .
A previous size estimate that assumed an equator-on configuration ( 1535 ^ { +75 } _ { -225 } km ) would provide a density of 0.92 _ { -0.14 } ^ { +0.46 } g cm ^ { -3 } , unexpectedly low for a 1000 km-sized dwarf planet .