Masses and densities of dwarf planet satellites measured with ALMA

TL;DR

Using ALMA, the study precisely measured the positions and wobble of dwarf planets Orcus and Eris to determine satellite masses and densities, revealing insights into their formation and evolution.

Contribution

First direct astrometric measurements of dwarf planet satellites with ALMA, providing new mass ratios and density estimates that inform formation scenarios.

Findings

Vanth-Orcus mass ratio of 0.16±0.02

Dysnomia's density <1.2 g/cm³

Eris's satellite likely formed from a giant impact

Abstract

We have used the Atacama Large Millimeter Array (ALMA) to measure precise absolute astrometric positions and detect the astrometric wobble of dwarf planet Orcus and its satellite Vanth over a complete orbit. We also place upper limits to the astrometric wobble induced by Dysnomia on dwarf planet Eris around its orbit. From the Vanth-Orcus barycentric motion, we find a Vanth-Orcus mass ratio of 0.16$\pm$0.02 -- the highest of any known planet or dwarf planet. This large ratio is consistent with the hypothesis that Vanth is a largely-intact impactor from a giant collision in the system, and that the system has likely evolved to a double synchronous state. We find only an upper limit of the barycenter motion of Eris, which implies a one sigma upper limit to the Dysnomia-Eris mass ratio of 0.0085, close to the modeled transition region between giant impact generated satellites which are largely intact remnants of the original impactor and those which form out of reaccreted disk material left over post-impact. The low albedo of Dysnomia leads us to marginally favor the intact impactor scenario. We find that Dysnomia has density of <1.2 g cm$^{-3}$, significantly lower than the 2.4 g cm$^{-3}$ of Eris.