Haumea

• 1. Introduction

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  Haumea is a dwarf planet so strange that astronomers initially struggled to believe what they were seeing. Spinning faster than any other large body in the Solar System, its surface gleams as bright as fresh snow, and it sits in the frozen dark beyond Neptune's orbit. When a stellar occultation was observed on the 21st of January 2017, it revealed something no one had expected from a trans-Neptunian object: a ring. What series of events could leave a world shaped like a stretched football, circled by two moons and a band of debris, rotating once every 3.9 hours? The answer traces back to a catastrophic collision at least a billion years in the past. Who gets credit for finding it became an international controversy. And the name itself carries a story worth telling.

• 2. A Disputed Discovery

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  Mike Brown of Caltech, David Rabinowitz of Yale University, and Chad Trujillo of Gemini Observatory in Hawaii first captured Haumea on images taken on the 6th of May 2004, and confirmed it on the 28th of December 2004. They planned to announce the find at a conference in September 2005, publishing an online abstract on the 20th of July 2005.

  Before that announcement could take place, José Luis Ortiz Moreno and his team at the Instituto de Astrofísica de Andalucía at Sierra Nevada Observatory in Spain emailed the Minor Planet Center on the night of the 27th of July 2005. They had located Haumea in their own images taken between the 7th and the 10th of March 2003. The email arrived days before Brown's planned conference presentation.

  Brown initially conceded discovery credit, but suspicion followed. Records showed the Spanish observatory had accessed Brown's online observation logs the day before Ortiz filed the discovery claim, a fact the Spanish team had not disclosed. Those logs contained enough positional data to help locate Haumea in the earlier images. The logs were accessed again just before Ortiz scheduled telescope time for a second confirmation report to the Minor Planet Center on the 29th of July.

  Ortiz later admitted accessing the Caltech logs but denied any wrongdoing, saying he was only checking whether the object was already known. Under IAU protocol, discovery credit goes to whoever first files a report with enough orbital data for the Minor Planet Center to work with. The IAU's announcement on the 17th of September 2008 listed the location of discovery as the Sierra Nevada Observatory, yet chose the name proposed by the Caltech team. Ortiz's team had proposed "Ataecina", an ancient Iberian goddess of spring, but the name was ruled ineligible because chthonic deities were reserved for objects in a 3:2 orbital resonance with Neptune, and Haumea did not qualify.

• 3. The Name and Its Meaning

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  Inside the Caltech team, Haumea traveled under the informal nickname "Santa", because the 28th of December 2004 discovery fell just after Christmas. Rudolph and Blitzen served as working names for the two moons before formal designations were assigned.

  When the time came to propose permanent names, the guidelines in place at the IAU called for classical Kuiper belt objects to carry names from mythology associated with creation. The Caltech team chose Hawaiian mythology specifically, as David Rabinowitz later explained, to pay homage to the place where the satellites were discovered, since observations were made at observatories on Mauna Kea.

  Haumea is the matron goddess of the island of Hawaii. She is identified with Papa, goddess of the earth and wife of Wākea, who represents space. At the time of naming, astronomers believed Haumea was composed almost entirely of solid rock, a body unlike the icy Kuiper belt objects more commonly encountered. Haumea in mythology is also the goddess of fertility and childbirth, with many children said to have sprung from different parts of her body. This matched the swarm of icy fragments believed to have broken away from the main body in an ancient collision.

  The two moons were named Hiiaka and Namaka after two of Haumea's daughters. A planetary symbol for Haumea, encoded in Unicode at U+1F77B, was designed by Denis Moskowitz, a software engineer in Massachusetts; it combines and simplifies Hawaiian petroglyphs representing woman and childbirth. The IAU did not clarify the naming rules for chthonic figures until late 2019, when it decided such names were to be reserved specifically for plutinos.

• 4. A Shape Unlike Any Other

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  Haumea rotates once every 3.9 hours, faster than any other known body in the Solar System that has settled into equilibrium. No other body larger than 100 km in diameter spins as fast. That speed has consequences: instead of the oblate spheroid shape that most rotating worlds adopt, Haumea has been stretched into a triaxial ellipsoid, with its longest axis roughly twice the length of its shortest.

  Calculations based on light-curve data constrained Haumea's density to a range of 2.6-3.3 grams per cubic centimeter. Pluto, a typical icy Kuiper belt object, has a density of 1.86 grams per cubic centimeter, while the Moon, which is rocky, reaches 3.3. Haumea's density pointed toward silicate minerals such as olivine and pyroxene rather than thick ice.

  A stellar occultation in January 2017 revised the picture. Haumea appeared considerably larger than earlier estimates suggested, roughly the diameter of Pluto along its longest axis and about half that at the poles. The density implied by this shape, around 1.8 grams per cubic centimeter, fell closer to the values of other large trans-Neptunian objects.

  A 2019 study used numerical modeling to reconcile the conflicting measurements, finding best-fit dimensions of approximately 2,100 by 1,680 by 1,074 km. The model placed a core of roughly 1,626 by 1,446 by 940 km at the center, with a composition largely of hydrated silicates such as kaolinite, and a surrounding icy mantle ranging from about 70 km thick at the poles to 170 km along the longest axis. Haumea's mass works out to 28% that of the entire Plutonian system and about 6% that of the Moon.

• 5. An Enigmatic Surface

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  In 2005, the Gemini and Keck telescopes recorded spectra from Haumea's surface that showed strong crystalline water ice features, similar to those seen on Pluto's moon Charon. Crystalline ice is not expected to persist at Haumea's surface temperature, which falls below 50 K. At that temperature, water ice forms in an amorphous state, and cosmic rays and solar energetic particles should convert any crystalline ice back to amorphous ice within roughly ten million years. Haumea has been sitting in the cold outer Solar System for billions of years, making the presence of crystalline ice difficult to explain.

  The bright, fresh appearance of the ice suggests recent resurfacing, though no plausible mechanism for that resurfacing has been identified. Haumea's albedo falls in the range of 0.6-0.8, as bright as snow. Best-fit modeling indicates that somewhere between 66% and 80% of the surface appears to be nearly pure crystalline water ice, with hydrogen cyanide or phyllosilicate clays possibly contributing to the high reflectivity.

  A large dark red area was observed on Haumea's surface in September 2009. It appears to be a region richer in minerals and organic, carbon-rich compounds than the surrounding icy terrain, possibly marking an impact site. Later spectral studies pointed to an intimate mixture of amorphous and crystalline ice across the surface, with no more than 8% organics. The absence of ammonia hydrate rules out cryovolcanism as an explanation. The absence of measurable methane is consistent with a warm collision history that would have driven off volatile compounds, a contrast with other large Kuiper belt objects where methane persists.

• 6. A Ring in the Outer Solar System

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  On the 21st of January 2017, astronomers watched Haumea pass in front of a distant star and recorded the event from multiple sites. The analysis, published in Nature in October 2017, announced the first ring system ever found around a trans-Neptunian object and the first around any known dwarf planet.

  The ring sits at a radius of about 2,287 km from Haumea's center, with a width of roughly 70 km and an opacity of 0.5. It lies well within Haumea's Roche limit, which would be at a radius of about 4,400 km if the body were spherical, though Haumea's non-spherical shape pushes that limit outward. The ring plane is inclined 3.2 degrees with respect to Haumea's equatorial plane and approximately follows the orbital plane of the larger outer moon, Hiiaka.

  The ring sits close to the 1:3 orbit-spin resonance with Haumea's rotation, which places it at a radius of 2,285 plus or minus 8 km. A 2019 study by Othon Cabo Winter and colleagues showed that the resonance itself is dynamically unstable, but that a stable region in phase space exists consistent with the ring's location. The ring particles appear to follow circular, periodic orbits near but not inside that resonance. The ring contributes an estimated 2.5% to Haumea's total brightness as seen from Earth.

• 7. Two Moons and a Family

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  Darin Ragozzine and Michael Brown discovered both of Haumea's known moons in 2005 using the W. M. Keck Observatory. Hiiaka, nicknamed "Rudolph" by the Caltech team, was found on the 26th of January 2005. It is the outer and larger of the two, roughly 310 km in diameter, and traces a nearly circular orbit around Haumea every 49 days. Its infrared spectrum shows strong absorption features at 1.5 and 2 micrometres, consistent with nearly pure crystalline water ice across most of its surface.

  Namaka, the smaller inner moon nicknamed "Blitzen", was discovered on the 30th of June 2005. It carries about one tenth the mass of Hiiaka, completes an orbit every 18 days, and follows a highly elliptical, non-Keplerian path inclined 13 degrees from Hiiaka's orbital plane. Hiiaka's gravitational pull perturbs Namaka's orbit continuously. Between roughly 2008 and 2011, both moons appeared nearly edge-on from Earth, and Namaka periodically passed in front of Haumea. One such occultation was observed on the 19th of June 2009, from the Pico dos Dias Observatory in Brazil. Hiiaka last crossed in front of Haumea in 1999, before the system was known to exist, and will not do so again for about 130 years.

  Haumea is the largest member of a collisional family, a group of trans-Neptunian objects sharing similar physical and orbital characteristics, the first such family identified among TNOs. The family includes several other bodies: one approximately 364 km across, another around 252 km, others around 230, 200, and 174 km. Because the family members have spread out over time, the collision that formed them is estimated to have occurred at least a billion years ago. The chance of such an impact happening in today's sparse Kuiper belt is less than 0.1 percent, which suggests Haumea likely originated in the denser scattered disc, where collisions were far more probable before Neptune's migration thinned the belt.

• 8. Orbit, Future, and Exploration

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  Haumea orbits the Sun every 284 Earth years, with a perihelion of 35 AU and an orbital inclination of 28 degrees. It passed aphelion in early 1992 and currently sits more than 50 AU from the Sun. It will not reach perihelion again until 2133. With a visual magnitude of 17.3, it is the third-brightest object in the Kuiper belt after Pluto and one other body, and can be observed with a large amateur telescope.

  Haumea's orbital future is uncertain. A simulation of 34 trans-Neptunian objects ranked Haumea as the most statistically likely among them to be ejected from the Solar System or flung into the inner Solar System over the next billion years. Its weak 7:12 resonance with Neptune gradually shifts its orbit through the Kozai effect, which trades orbital inclination for increased eccentricity over time. The resonance breaks twice per precession cycle, or about every 2.3 million years, before re-establishing itself a hundred thousand years or so later.

  New Horizons observed Haumea three times from afar, in October 2007, January 2017, and May 2020, at distances of 49, 59, and 63 AU. The spacecraft's outbound trajectory allowed observations at high phase angles not achievable from Earth, helping characterize how Haumea's surface scatters light. Trajectory studies suggest a dedicated flyby mission could reach Haumea in 16.45 years if launched on the 1st of November 2026, or on similar trajectories available on the 23rd of September 2037 and the 29th of October 2038.