Haumea is exceptional among the known trans-Neptunian objects because of its extreme elongation. Although its shape has not been directly observed, calculations from its light curve suggest it is an ellipsoid twice as long along its greatest axis as its shortest. Nonetheless, its gravity is believed sufficient for it to have relaxed into hydrostatic equilibrium, so it meets the definition of a dwarf planet. This elongation, along with other characteristics such as its unusually rapid rotation, high density, and high albedo—thought to be due to a layer of water ice on the surface—are thought to be the results of a giant collision, which left Haumea the largest member of a collisional family that includes its two known moons.
Following guidelines established by the IAU that classical KBOs be given names of mythological beings associated with creation, in September 2006 the Caltech team submitted formal names from Hawaiian mythology to the IAU for both and its moons, in order "to pay homage to the place where the satellites were discovered". The names were proposed by David Rabinowitz of the Caltech team. Haumea is the patron goddess of the island of Hawaii, where the Mauna Kea Observatory is located. In addition, she is identified with Pāpā, the goddess of the earth and wife of Wākea (space), which is appropriate because is thought to be composed almost entirely solid rock, without the thick ice mantle over a small rocky core typical of other known Kuiper belt objects. Lastly, Haumea is the goddess of fertility and childbirth, with many children who sprang from different parts of her body; this corresponds to the swarm of icy bodies thought to have broken off during an ancient collision. The two known moons, also believed to have been born in this manner, are thus named after two of Haumea's daughters, Hiiaka and Namaka.
Brown came to suspect the Spanish team of fraud upon learning that his observation logs were accessed from the Spanish observatory the day before the discovery announcement—logs which included enough information to allow the Ortiz team to precover Haumea in their 2003 images,—and then were accessed again just before Ortiz scheduled telescope time to obtain confirmation images for a second announcement to the MPC on July 29. Ortiz later admitted he had accessed the Caltech observation logs but denied any wrongdoing, stating he was merely verifying whether they had discovered a new object.
IAU protocol is that discovery credit for a minor planet goes to whoever first submits a report to the MPC with enough positional data for a decent determination of its orbit, and that the credited discoverer has priority in choosing a name. However, the IAU announcement on September 17, 2008, that Haumea had been accepted as a dwarf planet, made no mention of a discoverer. The location of discovery was listed as the Sierra Nevada Observatory of the Spanish team, but the chosen name, Haumea, was the Caltech proposal.
Haumea has a typical orbit for a classical trans-Neptunian object, with an orbital period of 285 Earth years, a perihelion of 35 AU, and an orbital inclination of 28°. The diagram at right shows the orbital position of Haumea in yellow, compared to Pluto in red and Neptune in grey, as of April 2006. Haumea passed aphelion in early 1992, and is currently more than 50 AU from the Sun.
Haumea's orbit lies at a slightly higher inclination than the other members of its collisional family. This may be due to a possible 12:7 orbital resonance with Neptune. Such a resonance would have shifted its orbit over the course of the last billion years, through the Kozai effect, which allows the exchange of an orbit's eccentricity for increased inclination.
With a visual magnitude of 17.5, Haumea is the third brightest object in the Kuiper belt after Pluto and Makemake. However, since the planets and most of the small Solar System bodies still remain in a common orbital alignment, left after their formation in the primordial disk of the Solar System, most early surveys for distant objects focused on the projection on the sky of this common plane, the ecliptic. As the region of sky close to the ecliptic became well explored, successive sky surveys began looking for objects that had been dynamically excited into orbits with higher inclinations, and also objects that were more distant, with slower mean motions across the sky. This made possible the discovery of Haumea, with its high orbital inclination (65% greater than Pluto's 17°) and its current position far from the ecliptic.
Haumea rotates roughly once every four hours, faster than any other known equilibrium body in the Solar System and indeed faster than any known body larger than 100 km in diameter. Its short rotation period is likely to have been caused by the same giant impact which created its satellites and its collisional family.
rect 646 1714 2142 1994 The Earth
circle 1786 614 142 (136472) Makemake
circle 2438 616 155 (136108) Haumea
circle 342 1305 137 (90377) Sedna
circle 1088 1305 114 (90482) Orcus
circle 1784 1305 97 (50000) Quaoar
circle 2420 1305 58 (20000) Varuna
rect 0 0 2749 1994 desc none
The only way to estimate the size of a small, isolated trans-Neptunian object is to use the body's optical magnitude and location, and assuming a value for its albedo. For larger, brighter objects, their thermal emission can also be measured, which gives direct evidence for the albedo. The mass can only be crudely estimated by assuming a value for its density. However, the addition of a satellite allows the mass of the system to be calculated directly from the satellite's orbit using Kepler's third law. It the case of Haumea, the result is 4.2 × 1021 kg, or 28% the mass of the Plutonian system. Nearly all of this mass is in Haumea.
Haumea displays large fluctuations in brightness over a period of 4 hours, indicating that it rotates faster than any other large object known in the Solar system. The rotational physics of deformable bodies implies that over geological time, Haumea has been distorted into the equilibrium form of a scalene ellipsoid by this four-hour rotational period, and it is thought that the alternating display of side view–end view–side view causes most of the brightness fluctuation. These fluctuations could also be partially due to a mottled surface.
The rapid rotation and elongated shape, together with the well-defined mass provided by the existence of its moons, provide strong constraints on the composition of Haumea. Mass and volume are the requirements to calculate density — and the denser the object, the less elongate and more spherical it becomes, for a given rotational period. This constrains Haumea's density to around 2.6–3.3 g/cm³, a value typical of silicate minerals such as olivine and pyroxene, which from the element abundances in the solar nebula form the rock-dominated objects of the Solar System. For comparison, the Earth's moon, which is mostly rock, has a density of 3.3 g/cm³, while Pluto, a typical icy object in the Kuiper belt, has a density of 2.0 g/cm³. Models of Pluto's structure suggest that its lower mean density is due to a thick mantle of ice over a small rocky core. If Haumea had a density closer to that of Pluto, implying a Pluto-like composition, it would have an even greater elongate distortion. This suggests that Haumea has a substantial rocky content, with a thin ice veneer. The original thick ice mantle with which it is likely to have formed may have been removed during the massive collision that formed Haumea's collisional family.
The limits on mass and density place constrains on Haumea's possible dimensions. The best fit to the data as of 2008 is that Haumea is approximately the diameter of Pluto along its longest axis and about half the diameter of Pluto at its poles. This would make it one of the largest trans-Neptunian objects discovered, and possibly fourth after , Pluto, and Makemake. It would be larger than , , and .
Consistent with a surface of crystalline ice, Haumea is about as bright as snow, with an albedo greater than 0.6. However, this unusually high albedo does not appear to be unique among large TNOs. Recent measurements of Eris imply an even higher (inferred) albedo of 0.86. Surprisingly, 66% to 80% of the Haumean surface appears to be covered in pure crystalline ice, the remainder being material of unknown composition.
Best fit modeling of the surface composition that would produce the observed spectra suggests that one strong contributor to the high albedo may be hydrogen cyanide or phyllosilicate clays. Inorganic cyanide salts such as copper potassium cyanide may also be present. In strong contrast to Makemake, the absence of a measurement of methane in the spectra means that no more than 10% of Haumea's surface could be covered in methane.
One analysis of color variations in Haumea's light curve found shifts that could not be explained by its shape, suggesting that there is a region on the surface that differs both in color and albedo from the average. Such surface variations have been found on Pluto, but further light curve observations of Haumea would be needed to confirm if these also occur on Haumea.
Hiiaka, nicknamed "Rudolph" by the Caltech team, was the first to be discovered, on January 26, 2005. It is the outer and larger of the two (at around 310 km), and orbits Haumea every 49 days. Strong absorption features at 1.5 and 2 micrometres in the infrared spectrum are consistent with water ice; their strength, greater than that of any other body in the Solar System, suggests that water ice covers much of the surface. The unusual spectrum, along with similar absorption lines on Haumea, led Brown et. al. to conclude that capture was an unlikely model for the system's formation, and that the Haumean moons must be fragments of Haumea itself.
Namaka, nicknamed "Blitzen" by the Caltech team, is the smaller, inner satellite of Haumea. It orbits Haumea in roughly 34 days, assuming a circular orbit. Its discovery was announced on November 7, 2005. It is inclined approximately 40° from the larger moon. Assuming a similar surface composition to the larger moon, its brightness implies a diameter 12% that of Haumea, or some 170 km.
The presence of the collisional family could imply that Haumea and its "offspring" might have originated in the scattered disc. In today's sparsely populated Kuiper belt, the chance of such a collision occurring is less than 0.1 percent. The family could not have formed in the denser primordial Kuiper belt because such a close-knit group would have been disrupted by Neptune's migration into the belt—the believed cause of the belt's current low density. Therefore it appears likely that the dynamic scattered disc region, in which the possibility of such a collision is far higher, is the place of origin for the object that generated Haumea and its kin.
Because it would have taken at least a billion years for the group to have diffused as far as it has, the collision which created the Haumea family is believed to have occurred very early in the Solar System's history.