Ring Around Haumea: Pluto’s Peculiar Neighbor

Lisa R. Parker

Far from our fiery Star, and well-hidden from our view in a distant region of perpetual twilight, there lies a belt composed of icy, frozen objects. Located beyond the orbit of the deep-blue, banded, ice-giant planet, Neptune–the most distant known major planet from our Sun–this myriad of strange objects, composed of ice and rock, are dancing in the dark. This remote region–called the trans-Neptunian belt–is also the home of a mysterious quartet of dwarf planets: Pluto, Eris, Makemake, and Haumea. Of the four, Haumea stands out in the crowd as the most bewitching and bewildering small world, with its extremely elongated shape and rapid rate of rotation. In October 2017, an international team of scientists, led by astronomers from the Institute of Astrophysics of Andulusia (Spain), published a study in the journal Nature revealing that the oddly shaped object has something in common with the ringed, gas-giant planet Saturn. That is because there is also a ring around little Haumea.

This international observing campaign was able to establish the main physical characteristics of Haumea. That proved to be a difficult task, because trans-Neptunian-objects are notoriously hard to observe due to their small size, their low brightness, and the immense distances that separate Earth-based astronomers from them. However, there is one efficient but complex method that can overcome these difficulties. The study of stellar occultations–or the passing of these small objects in front of a star (creating something like a miniature eclipse)–proved to be a successful tool in the astronomers’ toolbox. Stellar occultations had previously enabled astronomers to determine the main physical characteristics of three other dwarf planets: Pluto, Eris, and Makemake.

“We predicted that Haumea would pass in front of a star on the 21st of January 2017, and twelve telescopes from ten different European observatories converged on the phenomenon. This deployment of technical means allowed us to reconstruct with a very high precision the shape and size of dwarf planet Haumea, and discover to our surprise that it is considerably bigger and less reflecting than was previously believed. It is also much less dense than previously thought, which answered questions that had been pending about the object,” explained Dr. Jose Luis Ortiz in an October 11, 2017 Institute of Astrophysics of Andulusia (IAA-CSIC) Press Release. Dr. Ortiz, a researcher at the IAA-CSIC, led the study.

Haumea is a bewitching object. This is because it rotates around our Sun in an elliptical orbit which takes years to complete. As of this writing, it is situated 50 Astronomical Units (AU) from our Sun. One AU is equivalent to the mean distance between Earth and Sun which is about 93,000,000 miles. Haumea also takes 3.9 hours to rotate around on its axis, which is considerably less than any other body, measuring more than a hundred kilometers long, in our entire Solar System This rotational speed causes it to flatten out–which accounts for its ellipsoid shape that has been compared to a rugby ball. The recently published data show that Haumea measures 2,320 kilometers in its largest axis–almost identical to that of Pluto–but it doesn’t have a global atmosphere like Pluto. Haumea’s mass is also about one-third that of Pluto, and only 1/1400 that of our own planet.

The Twilight Region Beyond Neptune

Two teams of astronomers claim credit for discovering Haumea. Dr. Michael Brown and his team at the California Institute of technology (Caltech) in Pasadena, California, discovered Haumea in December 2004 on images that they had obtained on May 6, 2004. On July 20, 2005, they published an online abstract of a report that they were planning to present at a meeting, held in September 2005, announcing this discovery. At almost the same time, Dr. Ortiz and his team, at the IAA-CSIC at the Sierra Nevada Observatory in Spain, found Haumea on images taken on March 7-10, 2003. Dr. Ortiz emailed their discovery to the Minor Planet Center of the International Astronomical Union (IAU) on the night of July 27, 2005.

IAU protocol is that discovery credit for a minor planet is bestowed upon whoever first submits a report to the Minor Planet Center with sufficient positional information for a credible determination of its orbit to be made. Also, 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. Haumea was the name proposed by the Caltech team, while Dr. Ortiz’s team proposed the name “Ataecina”, for the Iberian goddess of Spring.

Haumea is a plutoid. Plutoids are dwarf planets located beyond the orbit of Neptune. Haumea is classified as a dwarf planet because it is thought to be sufficiently massive to have been rounded by its own gravity into a shape that is in hydrostatic equilibrium, but not massive enough to have cleared its neighborhood of similarly sized objects. Haumea was at first listed as a classical Kuiper Belt Object (KBO) in 2006 by the Minor Planet Center, but its status was changed. The nominal trajectory indicates that Haumea is in a weak 7:12 orbital resonance with Neptune, which renders it a resonant object instead. Even though there are pre-discovery images of Haumea dating all the way back to March 22, 1955, obtained by the Palomar Mountain Digitized Sky Survey, additional observations of the orbit are needed to verify its dynamic status. The Palomar Observatory is located in California.

The Kuiper Belt was named after the Dutch-American astronomer Gerard Kuiper (1905-1973). In 1992, 1992 QB1 was discovered, and it became the very first Kuiper Belt Object (KBO) to be spotted since Pluto was discovered, by the American astronomer Clyde Tombaugh (1906-1997), back in 1930. Since the discovery of 1992 QB1, the number of known KBOs has skyrocketed to more than a thousand. In addition, over 100,000 KBOs, that are more than 62 miles in diameter, are hypothesized to exist. On September 17, 2008, Haumea was recognized as a dwarf planet by the IAU, and was then given its name in honor of Haumea, the Hawaiian goddess of childbirth. Its minor-planet designation is 136108 Haumea.

Haumea’s elongated shape, along with its high density, high albedo (resulting from a bright surface of crystalline water ice), and speedy rotation, are all considered to be the outcome of a giant, catastrophic collision that occurred long ago. This devastating collision resulted in Haumea becoming the largest member of a distinct collisional family of objects. This family includes several other large trans-Neptunian-objects (TNOs), as well as Haumea’s duo of known tiny moons, named Hi’iaka and Namaka.

Haumea has an orbital period of 284 Earth years, a perihelion (when it is closest to our Sun) of 35 AU, and an orbital inclination of 28 degrees. It passed aphelion (when it is the furthest from our Sun) in early 1992, and is currently over 50 AU from our Star. Haumea’s orbit has a slightly greater eccentricity than that of any other member of its collisional family, and this is thought to be the result of Haumea’s weak 7:12 orbital resonance with Neptune–which has been gradually modified from its original orbit over the passage of a billion years.

Haumea is also the third brightest denizen of the distant Kuiper Belt after Pluto and Makemake, and it can be easily seen with a large amateur telescope. It also shows large fluctuations in its brightness over a period of 3 to 9 hours, which can only be explained by a rotational period of this length of time. This is faster than any other known equilibrium object in our Solar System, and it is indeed faster than any other known body that is larger than 100 kilometers in diameter. While most rotating bodies in equilibrium are flattened into oblate spheroids, Haumea stands out in the crowd because it rotates so rapidly that it is distorted into a triaxial ellipsoid. If Haumea rotated much more rapidly, it would morph into a dumbbell shape and split into two distinct pieces. This rapid rate of rotation is believed to have resulted from the impact that formed its two tiny moons and the other members of its collisional family.

Haumea’s birthplace–the distant, frigid Kuiper Belt–displays something of a “family resemblance” to the Main Asteroid Belt. However, the Kuiper Belt is considerably more vast than the Main Asteroid Belt, which is located between Mars and Jupiter. Indeed, the Kuiper Belt is approximately 20 times as wide and 20 to 200 times as massive as the Main Asteroid Belt. The Kuiper Belt is the home of a multitude of relatively small frozen objects, that are tattle-tale primeval relics left over from our Solar System’s birth about 4.56 billion years ago. While asteroids are generally composed of rock and metal, most of the KBOs are composed of frozen volatiles (ices), such as water, methane, and ammonia. Some of the most mysterious moons inhabiting our Solar System–such as Phoebe of Saturn and Triton of Neptune–are believed to have originated in the Kuiper Belt, but wandered far from their place of birth very long ago. Triton orbits Neptune backwards, which is an indication that it is really a captured moon that was caught by its planet’s powerful gravity after it had escaped from the Kuiper Belt. So, now, Triton circles its adopted parent-planet in the wrong direction, and it is likely doomed to crash down one day into the swallowing thick blanket of clouds that cover Neptune–the adopted parent-planet that it has circled since it made its treacherous journey away from its frozen birthplace in the Kuiper Belt long ago.

After Pluto was discovered in 1930, many astronomers began to suspect that it did not live alone in our Solar System’s dark, deep freeze. For example, in 1930, the American astronomer Armin O. Leuschner (1868-1953) proposed that Pluto “may be one of many long-period planetary objects yet to be discovered.”

Ring Around Haumea

“One of the most interesting and unexpected findings was the discovery of a ring around Haumea. Until a few years ago we only knew of the existence of rings around the giant planets; then, recently, our team discovered that two small bodies situated between Jupiter and Neptune, belonging to a group called centaurs, have dense rings around them, which came as a big surprise. Now we have discovered that bodies even farther away from the centaurs, bigger and with very different general characteristics, can also have rings,” commented Dr. Pablo Saritos-Sariz, in the October 11, 2017 IAA-CSIC Press Release. Dr. Santos-Sanz is another member of the IAA-CSIC team that discovered the ring around Haumea.

According to the data obtained, using the stellar occultation technique, the ring is located on the equatorial plane of strange little Haumea. This is exactly the same as Haumea’s largest moon, Hi’laka, and it shows a 3:1 resonance with respect to the rotation of its dwarf planet. This means that the icy, frozen particles which make up the ring rotate three times more slowly around the small world than it rotates around on its own axis.

“There are different possible explanations for the formation of the ring; it may have originated in a collision with another object, or in the dispersal of surface material due to the planet’s high rotational speed,” Dr. Ortiz explained in the IAA-CSIC Press Release.

This discovery is important because it represents the first time a ring has been observed around a trans-Neptunian-object, and it also indicates that the presence of rings may be much more common than previously thought–both in our own Solar System, as well as in other distant stellar systems, where alien planets can be found circling stars beyond our own Sun.

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