Last updated: August 3, 2025

Haumea and its Moons: A Singularity of the Solar System

Haumea and its moons in the Kuiper Belt — *Haumea with its two moons, Namaka (above Haumea) and Hiʻiaka (below), photographed on June 30, 2015 by the Hubble Space Telescope. Image source: NASA*

A Celestial Body Out of the Ordinary

Haumea is one of the most intriguing objects in the Kuiper Belt. Located about 43 AU from the Sun (Neptune ~30 AU), this ellipsoidal dwarf planet is distinguished by an extremely rapid rotation (about 3.9 hours) that has elongated it into a rugby ball shape. This rapid rotation suggests a violent history, probably linked to a giant impact, and complicates precise measurements of its mass and volume. With an average density of about 2.6 g/cm³, Haumea is probably composed of a rocky core surrounded by a layer of water ice. Its high albedo (0.7 to 0.8) reinforces this hypothesis, as crystalline ice strongly reflects solar light.

A Stable Ring System in the Kuiper Belt

Discovered in 2017 thanks to the occultation of a star by Haumea, the equatorial ring extends about 2,287 km from the center of the dwarf planet, with a width of about 70 km. This disk of ice particles follows the equator, consistent with the rapid rotation of the central body. The ring is particularly stable, orbits within the 3:1 resonance with Haumea's rotation, and is probably maintained by gravitational resonance effects or viscous dissipation. Its presence is all the more remarkable as only two other small bodies in the Solar System (Chariklo and Chiron, between Jupiter and Neptune) are known to possess one.

Two Moons from a Giant Collision

Hiʻiaka and Namaka are the two natural satellites of Haumea, discovered in 2005 with the Keck telescope. Hiʻiaka, the largest and most distant (~49,500 km), is probably a block of almost pure ice, while Namaka, closer (~25,700 km), has a strongly perturbed orbit. The orbital configuration of the two moons, which are not on the same plane, indicates a common origin in a collisional event, reinforcing the hypothesis of a massive impact that would have ejected debris, forming both the moons and the collisional family around Haumea.

Summary Table of Haumea's, its Moons', and its Ring's Characteristics

Physical Characteristics of Haumea, Hiʻiaka, Namaka, and the Ring
Object	Dimensions (km)	Average Distance to Center (km)	Orbital Period	Albedo
Haumea	2,322 × 1,704 × 1,138	—	Rotation: 3.9154 h	0.7–0.8
Ring	~70 (width)	2,287	3:1 Resonance	High (ice)
Hiʻiaka	≈ 310	49,500	49.1 days	~0.8
Namaka	≈ 170	25,700	18.3 days	~0.8

Sources: Ortiz et al. (2017), AJ • Ragozzine & Brown (2009) • Brown et al. (2007)

A Collisional Family Identified by Dynamics and Spectroscopy

The presence of about ten objects around Haumea, called members of its collisional family, is based on several pieces of evidence from observation and physical modeling.

Common Orbital Elements

The objects associated with Haumea share very similar orbital characteristics: a semi-major axis around 43 AU, an inclination of about 28°, and moderate eccentricity. They form a coherent group in the space of orbital elements, analogous to asteroid families in the main belt. This coherence suggests that they come from the same cataclysmic event, likely a collision.

Unique Spectral Signature

All these objects have infrared spectra dominated by crystalline water ice, with clear absorption bands at 1.5 µm and 2.0 µm. This characteristic is rare in the Kuiper Belt, where objects generally have dark surfaces, enriched in irradiated organic compounds (tholins). The high albedo and spectral purity of the members of Haumea's family indicate an origin by ejection of superficial icy material, resulting from an impact.

Retroactive Dynamic Modeling

Researchers have simulated the velocities and expected orbits for fragments resulting from an impact on Haumea. The results show that these fragments, ejected at low velocity (~150 m/s), remain confined in a stable region of the Kuiper Belt. By tracing back the orbital time of these objects, we obtain a convergence towards a common event about 4 billion years ago, reinforcing the hypothesis of an ancient fragmentation.

Partial List of Associated Objects

To date, about a dozen trans-Neptunian objects are identified as very probable members of this family:

2002 TX₃₀₀
1995 SM₅₅
2003 OP₃₂
2003 UZ₁₁₇
2005 RR₄₃
2003 SQ₃₁₇
And other objects sharing the same dynamic and spectral characteristics

Their census is based on the catalogs of the Minor Planet Center and the spectroscopic studies conducted notably by Brown et al. (2007) and Ragozzine & Brown (2009).

A Major Observational Bias

The low intrinsic luminosity of these objects greatly limits their detection. It is likely that Haumea's family includes several dozen, or even hundreds of members, but only the largest (from 100 to 400 km) have been detected with current instruments. Statistical modeling of sizes suggests a much larger population, largely hidden in the sky background.

Haumea's Collisional Family: A Fragment of Solar System History

Around Haumea orbits a collisional family, a rarity in the Kuiper Belt. This dozen or so objects sharing similar orbital parameters also have a spectral composition dominated by crystalline water ice. This homogeneity suggests that they all come from the same initial event. These objects are like witnesses to a cataclysmic fragmentation, and they constitute a valuable population for understanding the early moments of the outer solar system.

Table of Objects Associated with Haumea's Collisional Family

Known Members of Haumea's Collisional Family
Object	Absolute Magnitude (H)	Estimated Diameter (km)	Albedo	Remarks
2002 TX₃₀₀	3.2	286	0.88	Dominant crystalline ice
1995 SM₅₅	4.8	160	0.8 (estimated)	Spectrum similar to Haumea
2003 OP₃₂	4.0	230	0.8	Fast rotation, high reflectance
2005 RR₄₃	4.3	220	0.8	Near-infrared spectroscopy
2003 UZ₁₁₇	4.7	170	0.8 (estimated)	Close dynamic group
2003 SQ₃₁₇	5.0	150	0.7–0.8	Probable fragment of icy mantle
2009 YE₇	4.4	190	0.75	Orbitally linked to Haumea
2003 HX₅₆	5.1	140	0.75 (estimated)	Possible distant member
2002 GH₃₂	4.6	180	0.8	Compatible characteristics
2003 EL₆₁ (Haumea)	0.2	~1,620 (spherized)	0.75	Parent object of the family