Haumea is a plutoid, a technical term used to describe dwarf planets beyond Neptune's orbit.
Haumea's extreme elongation makes it unique among known dwarf planets. Although its shape has not been directly observed, calculations from its light curve suggest it is an ellipsoid, with its major axis twice as long as its minor. Nonetheless, its gravity is believed sufficient for it to have relaxed into hydrostatic equilibrium, thereby meeting the definition of a dwarf planet.
This elongation, along with its unusually rapid rotation, high density, and high albedo (from a surface of crystalline water ice), are thought to be the results of a giant collision, which left Haumea the largest member of a collisional family that includes several large trans-Neptunian objects (TNOs) and its two known moons.
Discovery
Two teams claim credit for the discovery of Haumea. Mike Brown and his team at Caltech discovered Haumea in December 28, 2004 on images they had taken on May 6, 2004. On July 20, 2005, they published an online abstract of a report intended to announce the discovery at a conference in September 2005.
At around this time, José Luis Ortiz Moreno and his team at the Instituto de Astrofísica de Andalucía at Sierra Nevada Observatory in Spain found Haumea on images taken on March 7–10, 2003. Ortiz emailed the Minor Planet Center with their discovery on the night of July 27, 2005.
IAU (International Astronomical Union) protocol is that discovery credit for a minor planet goes to whoever first submits a report to the MPC (Minor Planet Center) 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, did not mention 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; Ortiz's team had proposed "Ataecina", named for the ancient Iberian goddess of Spring.
Naming
Until it was given a permanent name, the Caltech discovery team used the nickname "Santa" among themselves, as they had discovered Haumea on December 28, 2004, just after Christmas. The Spanish team proposed a separate discovery to the Minor Planet Center (MPC) in July 2005.
On July 29, 2005, Haumea was given its first official label, the temporary designation 2003 EL61, with the "2003" based on the date of the Spanish discovery image. On September 7, 2006, it was numbered and admitted into the official minor planet catalogue as (136108) 2003 EL61.
In September 2006 the Caltech team submitted formal names from Hawaiian mythology to the IAU for both (136108) 2003 EL61 and its moons, in order "to pay homage to the place where the satellites were discovered".
In Hawaiian mythology, Haumea is the Hawaiian goddess of fertility and childbirth. She is the mother of Pele, Kanemilohai, Kā-moho-aliʻi, Nāmakaokaha'i, Kapo and HiʻiakaikapolioPele. She was a powerful being, and gave birth to many creatures, some after turning herself into a young woman to marry her children and grandchildren. She was finally killed by Kaulu.
Stats
Estimated Diameter: 1500 km (1150 - 1500 km)
Aphelion: 51.54 AU
Perihelion: 34.72 AU
Semi-major axis: 43.13 AU
Orbital Period: 283.28 years
Rotation period: 3.92 hrs
Date discovered: 2004.12.28
Satellite: 2
Classification: TNO, KBO - fifth-order 7:12 resonance with Neptune
Orbit
Haumea has a typical orbit for a classical Kuiper-belt object, with an orbital period of about 283 Earth years, a perihelion of 35 AU, and an orbital inclination of 28°.
Haumea's orbit has a slightly greater eccentricity than the other members of its collisional family. This is thought to be due to Haumea's weak fifth-order 7:12 orbital resonance with Neptune gradually modifying its initial orbit over the course of a billion years, through the Kozai effect, which allows the exchange of an orbit's inclination for increased eccentricity.
Haumea displays large fluctuations in brightness over a period of 3.92 hours, which can only be explained by a rotational period of this length. This is faster than any other known equilibrium body in the Solar System, and indeed faster than any other known body larger than 100 km in diameter. This rapid rotation is thought to have been caused by the impact that created its satellites and collisional family.
With a visual magnitude of 17.3, Haumea is the third-brightest object in the Kuiper belt after Pluto and Makemake, and easily observable with a large amateur telescope.
Most early surveys for distant objects focused on the ecliptic - the apparent path that the Sun and planets follows through the sky over the course of the year . As the region of sky close to the ecliptic became well explored, later sky surveys began looking for objects that had been dynamically excited into orbits with higher inclinations, as well as more distant objects, with slower mean motions across the sky. These surveys eventually covered the location of Haumea, with its high orbital inclination and current position far from the ecliptic.
Physical characteristics
The rotation and amplitude of Haumea's light curve place strong constraints on its composition. If Haumea had a low density like Pluto, with a thick mantle of ice over a small rocky core, its rapid rotation would have elongated it to a greater extent than the fluctuations in its brightness allow. Such considerations constrain its density to a range of 2.6–3.3 g/cm3.
This range covers the values for silicate minerals such as olivine and pyroxene, which make up many of the rocky objects in the Solar System. This suggests that the bulk of Haumea is rock covered with a relatively thin layer of ice. A thick ice mantle more typical of Kuiper belt objects may have been blasted off during the impact that formed the Haumean collisional family.
Since no observations of occultations of stars by Haumea or occultations of the dwarf planet with its moons have yet been made, direct, precise measurements of its dimensions, like those that have been made for Pluto, do not yet exist.
Surface
The Gemini and Keck telescopes obtained spectra of Haumea which showed strong crystalline water ice features similar to the surface of Pluto's moon Charon. This is peculiar, because crystalline ice forms at temperatures above 110 K, while the surface temperature of Haumea is below 50 K, a temperature at which amorphous ice is formed.
In addition, the structure of crystalline ice is unstable under the constant rain of cosmic rays and energetic particles from the Sun that strike trans-Neptunian objects. The timescale for the crystalline ice to revert to amorphous ice under this bombardment is on the order of ten million years, while trans-Neptunian objects have been in their present cold-temperature locations for timescales of thousands of millions of years.
Radiation damage should also redden and darken the surface of trans-Neptunian objects where the common surface materials of organic ices and tholin-like compounds are present, as is the case with Pluto. Therefore, the spectra and colour suggest Haumea and its family members have undergone recent resurfacing that produced fresh ice. However, no plausible resurfacing mechanism has been suggested.
Haumea is as bright as snow, with an albedo in the range of 0.6–0.8, consistent with crystalline ice. Other large TNOs such as Eris appear to have albedos as high or higher. Best-fit modeling of the surface spectra suggested that 66% to 80% of the Haumean surface appears to be pure crystalline water ice, with one contributor to the high albedo possibly hydrogen cyanide or phyllosilicate clays. Inorganic cyanide salts such as copper potassium cyanide may also be present.
However, further studies of the visible and near infrared spectra suggest a homomorphous surface covered by an intimate 1:1 mixture of amorphous and crystalline ice, together with no more than 8% organics.
The absence of ammonia hydrate excludes cryovolcanism and the observations confirm that the collisional event must have happened more than 100 million years ago, in agreement with the dynamic studies. The absence of measurable methane in the spectra of Haumea is consistent with a warm collisional history that would have removed such volatiles.
Moons
Two small satellites have been discovered orbiting Haumea, (136108) Haumea I Hiʻiaka and (136108) Haumea II Namaka.
Haumea family
The Haumea family is the only identified trans-Neptunian collisional family; that is, the only group of trans-Neptunian objects (TNOs) with similar orbital parameters and spectra (nearly pure water-ice) that suggest they originated in the disruptive impact of a progenitor body.
Currently, there are 12 confirmed members of the Haumea family.
How big is it?
Haumea is so far away in the outer solar system that we don't know for sure how large it is. Because all we see is a dot of light, which is sunlight reflected off the surface of the dwarf planet. But we don't know if the object is bright because it is large or if it is bright because it is highly reflective or both.
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