The Dense, Rocky Core of Haumea Revealed by Satellite Dynamics

Abstract

The icy dwarf planets in the outer solar system are among the most enigmatic bodies known, showing activity ranging from atmospheric resurfacing to possible cryovolcanism. Yet the study of their interiors remains largely theoretical, with few empirical measurements to guide our understanding. The orbits of their moons offer a way to break this deadlock, providing a direct probe of dwarf planet interiors. Here we present a detailed investigation of Haumea's interior derived from the motion of its satellite system. Fitting a new dynamical model to two decades of precise Hubble Space Telescope astrometry, we simultaneously measure Haumea's dynamical oblateness, $J_2$, and the masses of its satellites. Our analysis of the $J_2$ measurement combined with occultation-derived shape models conclusively shows that Haumea has a dense rocky core overlaid with an ice-rich mantle. However, the current dataset is not constraining enough to fully determine the detailed internal structure (core/mantle density, two-layer vs. three-layer, etc.). We do find a slight preference for models where Haumea retains a frozen-in fossil figure, but more data are needed to statistically confirm this. Future observations of the satellite system, as well as observations of stellar occultations, are needed to fully test this hypothesis and should substantially tighten constraints on Haumea's interior. Taken together with other recent evidence for geophysical and geochemical evolution of dwarf planets, our work shows that dense rocky cores and subsurface oceans are likely to be ubiquitous among the trans-Neptunian dwarf planets.