# The mass and density of the dwarf planet (225088) 2007 OR10

## TL;DR

This study determines the orbit, size, and density of dwarf planet (225088) 2007 OR10, revealing it as the fifth most massive dwarf planet with a dense, spherical primary, based on satellite observations and radiometric analysis.

## Contribution

The paper provides the first detailed orbit and size measurements of 2007 OR10 using Hubble data and introduces a new density estimate based on satellite orbit analysis.

## Key findings

- Orbit's eccentricity is approximately 0.3.
- Size of the primary is about 1230 km.
- Bulk density of the primary is approximately 1.75 g/cm³.

## Abstract

The satellite of (225088) 2007 OR10 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$0.3, may be a consequence of an intrinsically eccentric orbit and slow tidal evolution, but may also be caused by the Kozai mechanism. Dynamical considerations also suggest that the moon is small, D$_{eff}$ $<$ 100 km. Based on the newly determined system mass of 1.75x10$^{21}$ kg, 2007 OR10 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$^{+0.46}_{-0.14}$ g cm$^{-3}$, unexpectedly low for a 1000 km-sized dwarf planet.