Non-ellipsoidal hydrostatic shape of Haumea

With a rotation period of 3.915341 ± 0.000005 h [1], (136108) Haumea is the fastest‑spinning known body larger than 100 km in the Solar System [2]. Photometric observations reveal a large‑amplitude, nearly sinusoidal light curve, which strongly suggests that Haumea has a triaxial shape [3]. A multi-chord stellar occultation in 2017 provided an elliptical limb with semi-axes lengths of 852 km and 569 km [4], which has been shown to be coherent with a nearly ellipsoidal, hydrostatic body made of an ice shell on top of a hydrated silicate core [5]. A new stellar occultation by Haumea happened on May 4, 2026 [6], which may refine the observational constraints on its shape. This motivates a new study of the range of hydrostatic shapes that Haumea can possibly have and the investigation of whether Haumea’s hydrostatic equilibrium figure must be close to an ellipsoid, or more complex geometries are likely.

We model Haumea as a body made of two or three homogeneous layers, which is justified because self-compression is negligible under the body’s relatively low pressures (<1 GPa). To compute the equilibrium figures, we use the BALEINES code [7], which solves iteratively for the shape of the layers’ boundaries so that they are equipotential surfaces.

Figure 1: Projections of the hydrostatic shape of a critical rotator in the sky plane at the time of the 2017 occultation (left) and the 2026 occultation (right). The ellipsoids obtained in Refs. [4] (O17) and [5] (D19) are shown in white dotted line for comparison.

Our models show that hydrostatic solutions compatible with the 2017 occultation can depart greatly from an ellipsoid, with deviations up to 110 km. The most extreme configurations even exhibit a pinch at the tip of the equatorial major axis, which indicates a state of critical rotation, i.e. the centrifugal and gravitational forces balance exactly each other thereat. Projections of pinched hydrostatic figures onto the sky plane at the epochs of the 2017 and 2026 occultations (Figure 1) show that while the earlier event would mask the pinch, the equatorial major axis being aligned with the line of sight, the latter could reveal substantial departures from an ellipse. This suggests that the recent 2026 occultation has the potential to discriminate between nearly ellipsoidal and strongly non‑ellipsoidal hydrostatic solutions. The modelled internal structures are physically plausible and consistent with current understanding of Haumea’s composition and thermal evolution, with a silicate core and a shell, whose density (830-980 kg/m³) can be interpreted as pure ice or a porous mixture of ice and rock. A third layer can either be a small high-density inner core (~4500 kg/m³), which could correspond to a metal-rich silicate layer, or a partially differentiated region between the core and the shell.

Overall, our results show that Haumea could be significantly different from an ellipsoid and could even be a critical rotator. The 2026 occultation is therefore an important opportunity to constrain the internal structure of the dwarf planet from which formation and evolution scenarios can be derived, with implications for other dwarf planets.

Acknowledgments: This work has been supported by the French Agence Nationale de la Recherche, project Roche, number ANR-23- 240 CE49-0012

References: [1] Lellouch et al. 2010, A&A, 518, L147. [2] Rabinowitz et al. 2006, ApJ, 639, 1238. [3] Lockwood et al. 2014, EM&P, 111, 127. [4] Ortiz et al. 2017, Nature, 550, 219. [5] Dunham et al. 2019, ApJ, 877, 41. [6] Ortiz et al. 2026, MNRAS, 548, stag692. [7] Staelen & Huré 2026, A&A, 708, A316.