Predictions of Stellar Occultations by Haumea and the Event of 4 May 2026

Abstract

Haumea is the third-largest of the five officially recognized dwarf planets and one of the four that reside in the trans-Neptunian region. It is among the most exotic bodies in the Solar System, with an exceptionally rapid rotation, a highly elongated triaxial shape, and a ring that orbits about three times more slowly than Haumea itself. Because of its large heliocentric distance, direct exploration by dedicated space missions is not feasible in the short term, so progress must rely on ground- and near-Earth facilities. Stellar occultations are among the most powerful tools to investigate trans-Neptunian objects. We present new predictions of stellar occultations by Haumea and its ring for stars down to Gaia G = 21, and assess their scientific potential, with special emphasis on the 4 May 2026 event. We computed occultation opportunities for the coming years and evaluated the 4 May 2026 geometry in detail, including Haumea's rotation phase, known 3D shape, pole orientation, and sky-plane motion, to estimate the expected shadow-path width. Because the target star has a very large Gaia RUWE, we also carried out a dedicated reliability analysis, including speckle observations. We identify eleven valuable events through 2030. For 4 May 2026, we derive an expected sky-plane shadow width of $2224 \pm 30$ km, substantially larger than conservative nominal assumptions and therefore highly favorable for observations. Speckle imaging reveals a companion at $\sim 0.12$ arcsec and $Δm \sim 3.1$; this companion is also expected to be occulted and shifts the nominal main-star path prediction on Earth by about 8 mas. These results confirm the strong scientific return expected from coordinated observations of upcoming Haumea occultations, especially the 4 May 2026 event, and provide an updated framework to improve constraints on Haumea's shape, density, ring properties, and environment.

Figures (14)

• Figure 1: Plot of the contrast in magnitudes for the occultation star versus angular separation. The inserts show the speckle combined images at the two observing wavelengths. A companion of the main star at 0.12 arcsec with a brightness of 3.1 mag fainter than the primary is clearly seen. The background contrast limits achieved tend to be shallow close to the star, only reaching their full contrast depth near separations of 0.1 and beyond (see Howell & Furlan 2022). Speckle interferometric Fourier reconstructions contain a 180° ambiguity (a ghost) for detected close companions, which is the case here.
• Figure 2: LCO photometric monitoring of star UCAC4 524-056397. Top panel: photometry versus time obtained with LCO 0.35m telescopes. Bottom panel: light curve folded to a period of 6.52 hours. The diamonds represent the observed brightness as a function of phase, using the first observation as the reference for zero phase. The continuous line is the fitted curve, with a peak-to-valley oscillation of 0.013 mag.
• Figure 3: Predicted geometry of the 2026 May 4 event showing the broader ring-occultation shadow region relative to the main-body shadow. A zoomable version of the map is available at https://opop.obspm.fr/media/data/chords/156331/Haumea_4th_May_2026_occultation_map_IAA-CSIC.html. Observations of this event can be reported through the occultation portal at https://opop.obspm.fr/create_report/2518/.
• Figure 4: Stellar occultation prediction map and geometry for Haumea event 1 of 11. Time markers (first/middle/last): 20h 20m 40s / 20h 16m 30s / 20h 12m 30s UTC (50 markers). Note that the correct path prediction for this event is shown in Fig. 3 of the main paper
• Figure 5: Stellar occultation prediction map and geometry for Haumea event 2 of 11. Time markers (first/middle/last): 2h 11m 40s / 2h 18m 20s / 2h 24m 50s UTC (80 markers).