The dwarf planet designated 136108 Haumea and its two satellites were discovered by Mike Brown et al. (Caltech) at Palomar Observatory in 2004. Haumea is the largest member of its collisional family, a group of astronomical objects with similar orbits and chemical compositions as a result of a progenitor object being destroyed during an impact with a massive object. Although Haumea’s shape and mass can only be inferred from light curve data, it is thought to be massive enough to be in hydrostatic equilibrium, and hence it was classified as a dwarf planet in 2008.
• Aphelion: 51.483 AU
• Perihelion: 34.952 AU
• Eccentricity: 0.19126
• Orbital period: 284.12 years (103,774 days)
• Equatorial rotation velocity: 3.9155 ± 0.0001 hours
• Dimensions: 2,322 × 1,704 × 1,138 km
• Volume: ≈3.5×109 cubic kilometres
• Mass: 4.006 ± 0.040 ×1021 kilograms (0.00066 Earths)
• Surface area: 8×106 square kilometres
• Mean density: 2.6 gram/cubic centimetre
• Escape velocity: 0.809 km/sec
• Apparent magnitude: 17.3 at opposition
• Satellites: 2 (Designated Namake and Hi’aka)
Since Haumea is the third brightest object in the Kuiper belt after Pluto and Makemake, it can be easily be found with a large (12-inch and bigger) amateur telescope.
In terms of its shape, Haumea is thought to be that of a triaxial ellipsoid, with its longest axis being twice as long as the shortest. Based on its mass, gravity, and rotation period, most investigators believe that Haumea is in a state of hydrostatic equilibrium, which is a condition in which an object’s own gravity has caused it to assume a rounded shape.
Although Haumea is the largest member of its collisional family, which is incidentally the first collisional family ever to be identified, there is some controversy about its origin. One theory holds that the Haumea collisional family came about as the result of a collision that produced Haumea itself (and removed its ice mantle in the process) and its two moons (designated Hi’aka and Namaka, respectively), as well as other large TNO’s (Trans Neptunian Objects).
However, a competing theory states that material ejected during the impact did not disperse, but instead coalesced to form Haumea’s largest moon, which was later destroyed in a subsequent collision that dispersed the remains of the moon outwards. From observational data, it appears that the second scenario is the more likely since observed trajectories and velocities of many family members closely match those produced in computer simulations of the destruction of a newly formed moon.
Observations of Haumea have shown that the object exhibits very large brightness fluctuations over a period of roughly 3.9 hours, which can only be explained by its high rate of rotation. While this rate of rotation is the fastest of all known bodies in the solar system that are larger than 100 km in diameter, most bodies that rotate at this speed assume the shape of an oblate spheroid. However, Haumea exhibits a somewhat stretched egg shape, and if it were to rotate any faster, it would likely assume the shape of a dumbbell, and split into two pieces.
The collision that had created the Haumea collisional family is assumed to have caused the object’s high rotation rate, but there remains some doubt about whether the object is in fact in hydrostatic equilibrium. At issue is the fact that observations of Haumea made during a stellar occultation in January 2017, suggest that the dwarf planet is significantly larger than was previously thought. In practice, the new data cast doubt as to whether its shape and high rotation rate would ever allow it to be in hydrostatic equilibrium.
Nonetheless, and despite the controversy about its true nature, Haumea is among the biggest Trans Neptunian Objects discovered to date; it is marginally smaller than Eris and Pluto, about the same size as Makemake and 2007 OR10, and bigger than Sedna, Quaoar, and Orcus.
Observations made with the Gemini and Keck telescopes show that Haumea is at least as bright as fresh snow, and its spectrum shows strong crystalline ice features that are similar to those of Pluto’s moon, Charon. However, crystalline ice only forms at temperatures above 110K, whereas Haumea’s’ surface temperature is known to be below 55K, the temperature at which amorphous ice (water ice that lacks a crystal lattice) forms.
Moreover, since the crystal lattice structure of crystalline ice is unstable under solar radiation and the time scales under which crystalline ice transforms to amorphous ice is only about 10 million years, it remains unclear how crystalline ice could be present on Haumea and other TNO’s that have been in their present locations for several billion years. One possible explanation is that Haumea and other TNO”s at its distance from the Sun could have undergone a recent episode of resurfacing, although the mechanisms that could have caused such a resurfacing remain unexplained.
Nonetheless, while some observations indicate that Haumea is covered by a homogeneous ice layer that consists of about 66% – 80% crystalline ice, infrared spectra suggest that the ice layer consists of equal parts of amorphous and crystalline ice, with organic material accounting for about 8% of the ice layer’s total volume. The observed absence of ammonia hydrate excludes cryovolcanism as a source of surface ice, while the absence of measurable amounts of methane in Haumea’s spectrum confirm that Haumea’s collisional history had been warm, which would have removed methane and other volatile material from the ice crust.
The same stellar occultation that showed Haumea to be bigger than was thought also revealed the existence of a ring of dust/ice crystals around the dwarf planet. This ring, which represents the first discovery of a ring around a TNO, is about 4,574 km in diameter, about 70 km wide, and has an opacity to visible light of 0.5, which means that it obscures 50% of the light that passes through it.
The plane of the ring roughly coincides with both Haumea’s equatorial plane, and the orbital plane of Haumea’s largest moon, Hi’iaka. The ring also accounts for about 5% of Haumea’s total brightness.