Haumea
Overview
Haumea has a formal designation of 136108 Haumea, and it had the original designation 2003 EL61. It was the fifth dwarf planet classified by the IAU, named in September 2008. It is the most elongated dwarf planet known, with its primary axis being approximately twice as long as its minor axis. Despite this elongation, its gravity is calculated to be enough to have reached "hydrostatic equilibrium," capable of pulling itself into a sphere if it were not completely solid.
Little is known about Haumea because it is so far away, spending most of its time beyond Pluto. Because its average distance from the sun is past Neptune, it is known as a "trans-Neptunian object," which makes it a sub-class of dwarf planets known as "plutiods."
Haumea has two known moons, Hi'iaka and Namaka, that are believed to be the product of a violent collision early in Haumea's history.
History
The discovery of Haumea is a sordid tale of competition and accusations of fraud, theft, and intellectual dishonesty. Mike Brown, the head of a team from Caltech, discovered Haumea on December 28, 2004, in two images they had taken on May 6 of the same year. They announced their findings on July 20, 2005, in an online abstract of a report that they intended to announce the discovery at a conference in September.
At this time, José Luis Ortiz Moreno and his team at the Instituto de Astrofísica de Andalucía in Spain announced that they had found Haumea on images they took on March 7-10, 2003. Ortiz told the Minor Planet Center (the reporting body for these discoveries) on July 27, 2005, of this, and they placed the discovery date as March 7, 2003.
Brown suspected that the Spanish team had committed fraud when he learned that his observation logs - enough information to find the object - were accessed by the team the day before their discovery announcement. The logs were also accessed by Ortiz's team just before Ortiz scheduled telescope time to obtain confirmation images for a second report to the Minor Planet Center on July 29, 2005. Ortiz later admitted that he accessed the logs, but he claimed he just was trying to verify whether they had discovered a new object.
The International Astronomical Union (IAU) protocol says that credit for the discovery of a minor planet goes to whomever first submits a report to the Minor Planet Center with enough data for a decent determination of its orbit. The discoverer is also allowed to name it. Because, officially, the Spanish team was the first to report it to the Minor Planet Center, it would seem that they should receive the credit. Confusingly, the IAU lists the discovery location as the Sierra Nevada Observatory that the Spanish team used, but the official name is the one proposed by the Caltech team.
What Is Known
One of the first pieces of data discovered about Haumea (Rabinowitz et al., 2006) is that it has the fastest known rotation period (day) of any object larger than 100 km; its day is approximately 3.92 hrs long (confirmed by Lacerda, Jewitt, & Peixinho (2008)). In the same paper, the albedo is estimated to be greater than 0.6.
Due to the rapid rotation rate and supported by the observed brightness variations with time, Rabinowitz et al. (2006) think the dwarf planet has elongated over time due to the forces acting on such a large, rotating body. By assuming a density and constrained by observations of the major axis, they were able to mathematically extrapolate the other two dimensions of the ellipsoid (shown in the table below).
Spectroscopic data show that the surface of Haumea is unlike many of the other dwarf planets discovered to-date, as it is mostly covered in crystalline water ice (Trujillo et al., 2007). Crystalline water ice is energetically less favored than an amorphous water ice structure at low temperatures. This suggests that a resurfacing process may be taking place on the object. Trujillo et al. (2007) find a best-fit to their data of a 65-80% pure crystalline water ice with 20-35% being a material that reflects near-infrared blue light, such as hydrogen cyanide or kaolinite. It is unlikely to be covered in a carbonaceous black material.
Asymmetries in different color brightnesses can be explained by albedo variations across the surface, much as has been observed on Pluto (Lacerda, Jewitt, & Peixinho, 2008).
Its Moons, Hi'iaka and Namaka
On January 26, 2005, the moon Hi'iaka was discovered and originally named "Rudolph." On June 30, 2005, a second moon, Namaka, was discovered and originally named "Blitzen." Both of these provisional names come from the childrens' story of the names of Santa Claus' reindeer.
Little is known about these moons except for the data for the following table, from primary sources (see References below) and Johnston's Archive for the Haumea family.
| Hi'iaka | Namaka | |
|---|---|---|
| Semi-Major Axis (103 km) | 49.5±0.4 | 39.3 |
| Orbital Inclination (from Haumea's Equator) | 234.8°±0.3° | 39°±6° relative to Hi'iaka |
| Orbital Eccentricity | 0.050±0.003 | >0.2 |
| Major Axis Radius (km) | 310? | 170? |
| Average Axis Radius (km) | ||
| Minor Axis Radius (km) | ||
| Mass (1015 kg) | ||
| Density (water=1 g/cm3) | ||
| Sidereal Orbital Period (hours) | 1179±1 | 432±2 |
| Geometric Albedo | ||
| Visual Magnitude | +20 | +21 |
| Discoverer | M.E. Brown et al. | M.E. Brown et al. |
| Discovery Date | Jan. 26, 2005 | Jun. 30, 2005 |
Spectroscopic data show a strong signal from water ice present on the surface of Hi'iaka (Barkume, Brown, & Schaller, 2006). It is reasonable to assume that these observations would also hold true for Namaka.
The two known moons are likely to be the results of a collision early in the dwarf planet's history (a collision is also the most likely reason for the very rapid rotation rate of Haumea). Ragozzine & Brown (2007) estimate that it would take at least 1 Gyr for Hi'iaka and Namaka to reach their present-day orbits from a collisional event, indicating that the system is likely very old.
Besides the two known moons, Haumea is the largest member of a "collisional family," a group of objects with similar physical and orbital characteristics that are thought to have formed when a large progenitor was shattered by an impact. Less than a dozen members have been identified, but that number may grow with further observations (Levison et al., 2008).
Name Mythology
The codename for the object when it was originally discovered by a team at Caltech was "Santa" because it had been discovered on December 28, 2004, just after Christmas. A Spanish team had discovered the object a year earlier, so once it was confirmed in 2006, the object was given the name (136108) 2003 EL61.
Once Haumea was reclassified as a dwarf planet by the IAU in 2008, it was given a name based on a creator god as is the custom for dwarf planets today. The name comes from Hawaiian mythology, and they were proposed by David Rabinowitz from the Caltech team.
Haumea is the patron goddess of Hawai'i. She is also identified with Pāpā, the goddess of the earth, and the wife of Wākea, space (much like in Greek mythology with Gaea, the earth, being the wife of Uranus, the heavens). This is appropriate because Haumea is thought to be almost entirely solid rock. In addition, Haumea is the goddess of fertility and childbirth, which corresponds to a swarm of icy bodies thought to have broken from the dwarf planet during a collision.
This collision is believed to have formed Haumea's two known moons, Hi'iaka and Namaka, which are appropriately two of Haumea's daughters in the corresponding Hawaiian mythology.
Data
The following table lists data for Pluto and the other dwarf planets for comparison purposes. Data is compiled from NASA's planetary factsheet for Pluto, and from primary sources for the other dwarf planets; see the individual pages on the dwarf planets for references.
| Ceres | Pluto | Haumea | Makemake | Eris | |
|---|---|---|---|---|---|
| Perihelion (109 km) | 0.381 | 4.436 | 5.260 | 5.761 | 5.65 |
| Semi-Major Axis (106 km) | 0.415 | 5.906 | 6.484 | 6.850 | 10.12 |
| Aphelion (106 km) | 0.449 | 7.376 | 7.708 | 7.940 | 14.60 |
| Average Orbital Velocity (km/s) | 10.587° | 17.16° | 4.484 | 4.419 | 3.436 |
| Orbital Inclination (from Earth's Orbit) | 10.587° | 17.16° | 28.19° | 28.96° | 44.187° |
| Orbital Eccentricity | 0.080 | 0.2488 | 0.18874 | 0.159 | 0.44177 |
| Equatorial Radius (km) | 975 | 1195 | ~996 x 1518 x 1960 | ~2000 | 1300+200−100 |
| Polar Radius (km) | 909 | 1195 | ~2000 | 1300+200−100 | |
| Mass (1021 kg) | 0.95 | 12.5 | 4.2±0.1 | ~40 | 16.7±0.2 |
| Density (water=1 g/cm3) | 2.08 | 1.75 | 2.6-3.3 | ~2 | 2.3±0.3 |
| Sidereal Rotation Period (hours) | 9.074 | 153.2820 | 3.915 | ? | ~25.9 |
| Sidereal Orbital Period (days) | 1679.819 | 90,588 | 104,234 | 113,183 | 203,600 |
| Apparent Magnitude | +6.7 to +9.3 | +15.1 | +16.7 | +17.3 | +18.7 |
| Absolute Magnitude | +3.36±0.02 | -0.7 | -0.48 | +0.17 | -1.15 |
| Number of Moons | 0 | 3 | 2 | 0 | 1 |
| Discoverer | G. Piazzi | C. Tombaugh | M.E. Brown, C.A. Trujillo, & D.L. Rabinowitz | M.E. Brown, C.A. Trujillo, & D.L. Rabinowitz | M.E. Brown, C.A. Trujillo, & D.L. Rabinowitz |
| Discovery Date | Jan. 1, 1801 | Feb. 18, 1930 | Dec. 28, 2004 | Mar. 31, 2005 | Oct. 21, 2003 |