Definitions

dwarf planet

dwarf planet

Body, other than a natural satellite (moon), that orbits the Sun and that is, for practical purposes, smaller than the planet Mercury yet large enough for its own gravity to have rounded its shape substantially. The International Astronomical Union adopted this category of solar system bodies in August 2006, designating Pluto, the even more-remote object Eris, and the asteroid Ceres as the first members of the category. Unlike major planets, these three bodies are not massive enough and are in orbits too elliptical, too inclined, or both to have swept up most smaller nearby bodies by gravitational attraction; they thus failed to grow larger.

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Haumea (), formal designation (136108) Haumea, is a dwarf planet in the Kuiper belt, one-third the mass of Pluto. It was discovered in 2004 by a team headed by Mike Brown at Caltech and the Mauna Kea Observatory in the United States, and in 2005 by a team headed by J. L. Ortiz at the Sierra Nevada Observatory in Spain, though the latter claim has been contested. On September 17, 2008, it was classified as a dwarf planet by the International Astronomical Union and named after Haumea, the Hawaiian goddess of childbirth.

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.

Classification

Haumea is classified as a dwarf planet, meaning that it is believed to be massive enough to have reached a state of hydrostatic equilibrium, but not enough to have cleared its neighborhood of similar objects. It orbits beyond Neptune, which together with being a dwarf planet defines it as a plutoid. Since it is not proven to be in resonance with Neptune, nor likely capable of being significantly perturbed by it, Haumea was provisionally listed as a classical Kuiper belt object, the most numerous population of trans-Neptunian objects observed to date. Although Haumea is thought to be far from spherical, it is considered a dwarf planet because its suspected ellipsoidal shape is the equilibrium state resulting from its rapid rotation—in much the same way that a water balloon stretches out when tossed with a spin—and is not due to Haumea having insufficient gravity to overcome the tensile strength of its material. Indeed, Haumea is currently the best illustration that a body need not be spherical to be a planet or dwarf planet.

Name

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. On September 7, 2006, after the Spanish team announced the discovery to the Minor Planet Center (MPC) in July 2005, Haumea was given its first official label, the temporary designation , with the "2003" based on the date of the Spanish discovery image.

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.

Discovery controversy

Two teams claim credit for the discovery of Haumea. Mike Brown and his team at Caltech discovered Haumea in December 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. Around this time, José Luis Ortiz Moreno and his team at at the Instituto de Astrofísica de Andalucía at Sierra Nevada Observatory in Spain, the found Haumea on images taken on March 7–10, 2003. Ortiz emailed the MPC with their discovery on the night of July 27, 2005, giving a discovery date of March 7, 2003.

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.

Orbit and rotation

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.

Physical characteristics

Size and composition

Image:EightTNOs.png|thumb|250px|left|Haumea compared to Eris, Pluto, Makemake, Sedna, Orcus, Quaoar, Varuna, and Earth (all to scale)

  1. Earth

rect 646 1714 2142 1994 The Earth

  1. Eris and Dysnomia

circle 226 412 16 Dysnomia circle 350 626 197 (136199) Eris

  1. Pluto and Charon

circle 1252 684 86 Charon circle 1038 632 188 (134340) Pluto

  1. Makemake

circle 1786 614 142 (136472) Makemake

  1. Haumea

circle 2438 616 155 (136108) Haumea

  1. Sedna

circle 342 1305 137 (90377) Sedna

  1. Orcus

circle 1088 1305 114 (90482) Orcus

  1. Quaoar

circle 1784 1305 97 (50000) Quaoar

  1. Varuna

circle 2420 1305 58 (20000) Varuna

  1. link to image (under all other links)

rect 0 0 2749 1994 desc none

  1. - setting this to "bottom-right" will display a (rather large) icon linking to the graphic, if desired
  2. Notes:
  3. Details on the new coding for clickable images is here: #While it may look strange, it's important to keep the codes for a particular system in order. The clickable coding treats the first object created in an area as the one on top.
  4. Moons should be placed on "top" so that their smaller circles won't disappear "under" their respective primaries.

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 .

Surface

In 2005, 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, and the surface temperature of Haumea is below 50 K, and at this temperature the energetically preferred form of ice is an amorphous structure. In addition, the lattice structure of crystalline ice is unstable under the constant rain of energetic particles from the Sun and cosmic rays from other stars 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, and trans-Neptunian objects have been in their present distant, cold-temperature locations for timescales close to the multi-billion year age of the Solar System. 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. Therefore, the spectra and colour suggest Haumea and its family members have undergone resurfacing that produced fresh ice. However, no plausible resurfacing mechanism has been found to account for their apparent youth.

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.

Moons

Two small satellites have been discovered orbiting Haumea, (136108) Haumea I Hiiaka and (136108) Haumea II Namaka. They were both discovered in 2005, through observations with the W.M. Keck Observatory by a team headed by Mike Brown at Caltech, only a few years after occultations of Hiiaka with Haumea in 1999. Hiiakan occultations will not happen again until 2138. Namaka went through five occultations between May and June 2008. Mike Brown's team has calculated a better orbital solution for Namaka and think that the occultations might occur for a few more years.

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.

Collisional family

Haumea is the largest member of a TNO collisional family, similar to asteroid families: a group of objects with similar orbital parameters and common physical characteristics, presumably with a common origin in a disruptive impact of the progenitor object of Haumea. This family is the first to be identified among TNOs and includes—beside Haumea and its moons— (~600 km), (< 700 km), (~500 km), (< 700 km), and (< 700 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.

References

External links

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