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Haumea (minor-planet designation: 136108 Haumea) is a dwarf planet located beyond Neptune's orbit. It was discovered in 2004 by a team headed by Mike Brown of Caltech at the Palomar Observatory, and formally announced in 2005 by a team headed by José Luis Ortiz Moreno at the Sierra Nevada Observatory in Spain, who had discovered it that year in precovery images taken by the team in 2003. From that announcement, it received the provisional designation 2003 EL61.
On 17 September 2008, it was named after Haumea, the Hawaiian goddess of childbirth and fertility, under the expectation by the International Astronomical Union (IAU) that it would prove to be a dwarf planet. Nominal estimates make it the third-largest known trans-Neptunian object, after Eris and Pluto, and approximately the size of Uranus's moon Titania. Precovery images of Haumea have been identified back to 22 March 1955.
Haumea's mass is about one-third that of Pluto and 1/1400 that of Earth. Although its shape has not been directly observed, calculations from its light curve are consistent with it being a Jacobi ellipsoid (the shape it would be if it were a dwarf planet), with its major axis twice as long as its minor. In October 2017, astronomers announced the discovery of a ring system around Haumea, representing the first ring system discovered for a trans-Neptunian object and a dwarf planet.
Haumea's gravity was until recently thought to be sufficient for it to have relaxed into hydrostatic equilibrium, though that is now unclear. Haumea's elongated shape together with its rapid rotation, rings, and high albedo (from a surface of crystalline water ice), are thought to be the consequences of a Impact event, which left Haumea the largest member of a collisional family (the Haumea family) that includes several large trans-Neptunian objects and Haumea's two known moons, Hiiaka and Namaka.
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 7–10 March 2003. Ortiz emailed the Minor Planet Center with their discovery on the night of 27 July 2005.
Brown initially conceded discovery credit to Ortiz,Michael E. Brown. How I Killed Pluto and Why It Had It Coming, chapter 9: "The Tenth Planet" but came to suspect the Spanish team of fraud upon learning that the Spanish observatory had accessed Brown's observation logs the day before the discovery announcement, a fact that they did not disclose in the announcement as would be customary. Those logs included enough information to allow the Ortiz team to precovery Haumea in their 2003 images, and they were accessed again just before Ortiz scheduled telescope time to obtain confirmation images for a second announcement to the MPC on 29 July. 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 (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 17 September 2008, that Haumea had been named by a dual committee established for bodies expected to be dwarf planets, 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", the ancient Iberian goddess of spring; as a chthonic deity, it would have been appropriate for a plutino, which Haumea was not.
Following guidelines established at the time by the IAU that classical Kuiper belt objects 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 (136108) 2003 EL61 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 matron goddess of the island of Hawaii, where Gemini and W. M. Keck Observatory are located on Mauna Kea. In addition, she is identified with Papa, the goddess of the earth and wife of Wākea (space), which, at the time, seemed appropriate because Haumea was thought to be composed almost entirely of 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 the main body during an ancient collision. The two known moons, also believed to have formed in this manner, are thus named after two of Haumea's daughters, Hiiaka and Nāmaka.
The proposal by the Ortiz team, Ataecina, did not meet IAU naming requirements, because the names of chthonic deities were reserved for stably resonant trans-Neptunian objects such as that resonate 3:2 with Neptune, whereas Haumea was in an intermittent 7:12 resonance and so by some definitions was not a resonant body. The naming criteria would be clarified in late 2019, when the IAU decided that chthonic figures were to be used specifically for plutinos.
A planetary symbol for Haumea, , is included in Unicode at U+1F77B. Planetary symbols are no longer much used in astronomy, and 🝻 is mostly used by astrologers, but has also been used by NASA. The symbol was designed by Denis Moskowitz, a software engineer in Massachusetts; it combines and simplifies Hawaiian petroglyphs meaning 'woman' and 'childbirth'.
With a visual magnitude of 17.3, Haumea is the third-brightest object in the Kuiper belt after Pluto and , and easily observable with a large amateur telescope. However, because the planets and most small Solar System bodies share a invariable plane from 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, called the ecliptic. 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 across the sky. These surveys eventually covered the location of Haumea, with its high orbital inclination and current position far from the ecliptic.
The plane of Haumea's equatorial bulge is oriented nearly edge-on from Earth at present and is also slightly offset to the orbital planes of its ring and its outermost moon Hiiaka. Although initially assumed to be coplanar to Hiiaka's orbital plane by Ragozzine and Brown in 2009, their models of the collisional formation of Haumea's satellites consistently suggested Haumea's equatorial plane to be at least aligned with Hiiaka's orbital plane by approximately 1°. This was supported with observations of a stellar occultation by Haumea in 2017, which revealed the presence of a ring approximately coincident with the plane of Hiiaka's orbit and Haumea's equator. A mathematical analysis of the occultation data by Kondratyev and Kornoukhov in 2018 placed constraints on the relative inclination angles of Haumea's equator to the orbital planes of its ring and Hiiaka, which were found to be inclined and relative to Haumea's equator, respectively.
The rotation and amplitude of Haumea's light curve were argued to place strong constraints on its composition. If Haumea were in hydrostatic equilibrium and had a low density like Pluto, with a thick mantle of ice over a small silicate core, its rapid rotation would have elongated it to a greater extent than the fluctuations in its brightness allow. Such considerations constrained its density to a range of 2.6–3.3 g/cm3. By comparison, the Moon, which is rocky, has a density of 3.3 g/cm3, whereas Pluto, which is typical of icy objects in the Kuiper belt, has a density of 1.86 g/cm3. Haumea's possible high density covered the values for silicate minerals such as olivine and pyroxene, which make up many of the rocky objects in the Solar System. This also suggested that the bulk of Haumea was 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.
Because Haumea has moons, the mass of the system can be calculated from their orbits using Kepler's third law. The result is , 28% the mass of the Plutonian system and 6% that of the Moon. Nearly all of this mass is in Haumea. Several ellipsoid-model calculations of Haumea's dimensions have been made. The first model produced after Haumea's discovery was calculated from ground-based observations of Haumea's light curve at visible spectrum wavelengths: it provided a total length of 1,960 to 2,500 km and a visible spectrum albedo (pv) greater than 0.6. The most likely shape is a triaxial ellipsoid with approximate dimensions of 2,000 × 1,500 × 1,000 km, with an albedo of 0.71. Observations by the Spitzer Space Telescope gave a diameter of and an albedo of , from photometry at infrared wavelengths of 70 μm. Subsequent light-curve analyses have suggested an equivalent circular diameter of 1,450 km. In 2010 an analysis of measurements taken by Herschel Space Telescope together with the older Spitzer Telescope measurements yielded a new estimate of the equivalent diameter of Haumea—about 1300 km. These independent size estimates overlap at an average geometric mean diameter of roughly 1,400 km. In 2013 the Herschel Space Telescope measured Haumea's equivalent circular diameter to be roughly .
However the observations of a stellar occultation in January 2017 cast a doubt on all those conclusions. The measured shape of Haumea, while elongated as presumed before, appeared to have significantly larger dimensions according to the data obtained from the occultation Haumea is approximately the diameter of Pluto along its longest axis and about half that at its poles. The resulting density calculated from the observed shape of Haumea was about more in line with densities of other large TNOs. This resulting shape appeared to be inconsistent with a homogenous body in hydrostatic equilibrium, though Haumea appears to be one of the largest trans-Neptunian objects discovered nonetheless, smaller than , , similar to , and possibly , and larger than , , and .
A 2019 study attempted to resolve the conflicting measurements of Haumea's shape and density using numerical modeling of Haumea as a differentiated body. It found that dimensions of ≈ 2,100 × 1,680 × 1,074 km (modeling the long axis at intervals of 25 km) were a best-fit match to the observed shape of Haumea during the 2017 occultation, while also being consistent with both surface and core scalene ellipsoid shapes in hydrostatic equilibrium. The revised solution for Haumea's shape implies that it has a core of approximately 1,626 × 1,446 × 940 km, with a relatively high density of ≈ , indicative of a composition largely of hydrated silicates such as kaolinite. The core is surrounded by an icy mantle that ranges in thickness from about 70 km at the poles to 170 km along its longest axis, comprising up to 17% of Haumea's mass. Haumea's mean density is estimated at ≈ , with an albedo of ≈ 0.66.
Radiation damage should also redden and darken the surface of trans-Neptunian objects where the common surface materials of organic molecule ices and tholin compounds are present, as is the case with Pluto. Therefore, the spectra and colour index 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 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 homogeneous 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 Impact crater that would have removed such volatiles, in contrast to .
In addition to the large fluctuations in Haumea's light curve due to the body's shape, which affect all Colour index equally, smaller independent colour variations seen in both visible and near-infrared wavelengths show a region on the surface that differs both in colour and in albedo. More specifically, a large dark red area on Haumea's bright white surface was seen in September 2009, possibly an impact feature, which indicates an area rich in minerals and organic (carbon-rich) compounds, or possibly a higher proportion of crystalline ice. Thus Haumea may have a mottled surface reminiscent of Pluto, if not as extreme.
The ring plane is inclined with respect to Haumea's equatorial plane and approximately coincides with the orbital plane of its larger, outer moon Hiiaka. The ring is also close to the 1:3 orbit-spin resonance with Haumea's rotation (which is at a radius of 2,285 ± 8 km from Haumea's center). The ring is estimated to contribute 5% to the total brightness of Haumea.
In a study about the orbital dynamics of ring particles published in 2019, Othon Cabo Winter and colleagues have shown that the 1:3 resonance with Haumea's rotation is dynamically unstable, but that there is a stable region in the phase space consistent with the location of Haumea's ring. This indicates that the ring particles originate on circular, periodic orbits that are close to, but not inside, the resonance.
Hiiaka, at first nicknamed "Rudolph" by the Caltech team, was discovered 26 January 2005. It is the outer and, at roughly 310 km in diameter, the larger and brighter of the two, and orbits Haumea in a nearly circular path every 49 days. Strong absorption features at 1.5 and 2 in the infrared spectrum are consistent with nearly pure crystalline water ice covering much of the surface. The unusual spectrum, along with similar absorption lines on Haumea, led Brown and colleagues 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, the smaller, inner satellite of Haumea, was discovered on 30 June 2005, and nicknamed "Blitzen". It is a tenth the mass of Hiiaka, orbits Haumea in 18 days in a highly elliptical, Osculating orbit orbit, and is inclined 13° from the larger moon, which perturbs its orbit. The relatively large eccentricities together with the mutual inclination of the orbits of the satellites are unexpected as they should have been damped by the tidal effects. A relatively recent passage by a 3:1 resonance with Hiiaka might explain the current excited orbits of the Haumean moons.
From around 2008 to 2011, the orbits of the Haumean moons appeared almost exactly edge-on from Earth, with Namaka periodically occultation Haumea. Observation of such transits would have provided precise information on the size and shape of Haumea and its moons, as happened in the late 1980s with Pluto and Charon. The tiny change in brightness of the system during these occultations would have required at least a medium-aperture professional telescope for detection. Hiiaka last occulted Haumea in 1999, a few years before discovery, and will not do so again for some 130 years. However, in a situation unique among regular moon, Namaka's orbit was being greatly torqued by Hiiaka, which preserved the viewing angle of Namaka–Haumea transits for several more years. One occultation event was observed on 19 June 2009, from the Pico dos Dias Observatory in Brazil.
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 over the age of the Solar System 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 Nice Model 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.
Joel Poncy and colleagues calculated that a flyby mission to Haumea could take 14.25 years using a gravity assist from Jupiter, based on a launch date of 25 September 2025. Haumea would be 48.18 AU from the Sun when the spacecraft arrives. A flight time of 16.45 years can be achieved with launch dates on 1 November 2026, 23 September 2037, and 29 October 2038. Haumea could become a target for an exploration mission, and an example of this work is a preliminary study on a probe to Haumea and its moons (at 35–51 AU).Paul Gilster: Fast Orbiter to Haumea . Centauri Dreams—The News of the Tau Zero Foundation. 14 July 2009, retrieved 15 January 2011 Probe mass, power source, and propulsion systems are key technology areas for this type of mission.
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