Eris and Dysnomia

The scattered disk object Eris is the largest dwarf planet and the most distant solar-system object astronomers have identified. Observation of Eris so far has revealed only one satellite, Dysnomia. Eris was initially hailed as a possible tenth planet when discovered in 2005, but debate within the astronomical community regarding what constitutes a planet led the International Astronomical Union to categorize it as a dwarf planet in 2006.

Overview

Eris, formally designated 136199 Eris, is the largest of the known trans-Neptunian objects (TRO). Eris's moon, Dysnomia, orbits Eris and is classified as a scattered disk object (SDO). Despite its remoteness, it can be viewed with powerful amateur equipment. At aphelion (the point in the orbit when it is farthest from the Sun), Eris lies beyond the outermost region of the Kuiper Belt; at perihelion (the point in the orbit when it is closest to the Sun), it passes within the range of Neptune’s influence. Although Eris is roughly the same size as Pluto, it is much denser than Pluto, its orbital period is more than twice Pluto’s, and at aphelion, it is roughly three times farther from the Sun than Pluto.

Once Eris's discovery was confirmed in 2005, the bright, slow-moving object was easy to spot in images going back to 1989. Eris takes approximately 557 years to travel around the Sun, following a highly elliptical path with a semimajor axis, or mean orbital radius, of 67.9 astronomical units (AU), or more than ten billion kilometers. When Eris was discovered, it was near its aphelion of 97.5 AU (more than 14.5 billion kilometers) from the Sun. It will reach perihelion at 38.2 AU (more than 5.7 billion kilometers) in the mid-twenty-third century. For the most part, the orbit of Eris is typical of an SDO. However, Eris’s orbital inclination is an atypically high 44 degrees from the solar system’s orbital plane.

In 2008, images from the Hubble Space Telescope allowed scientists to estimate Eris's diameter to be roughly 2,400±100 kilometers, making it slightly larger than Pluto. In November 2010, however, astronomers used a technique called stellar occultation in which the light from a star, which is stationary, is blocked by a moving body, which in this case was Eris. The occultation allowed astronomers to discover more about Eris and accurately fine-tune its measurements. (Notably, Eris, for a time, was the most distant object in the solar system to be observed successfully using stellar occultation. In 2018, Farfarout was discovered, making it the furthest known object at 140 astronomical units (AU) from the Sun.)

Using stellar occultation, researchers worldwide determined Eris's diameter to be 2,326±12 kilometers, which is smaller and with a significantly smaller margin of error than the previous measurement. Calculations based on information from the Keck Observatory in Hawaii and the Hubble Space Telescope observations of Dysnomia’s orbit around the dwarf planet indicated that Eris has a mass of 1.66 10²² kilograms, or 3.66 10²² pounds (27 percent greater than Pluto’s) and a bulk density of 2.52 grams per cubic centimeter.

Using Eris’s density, researchers have determined that the dwarf planet’s interior is comprised mainly of rocky materials with a relatively thin mantle of ice. Pluto’s surface appears reddish and partly rocky. However, Eris has a uniform, highly reflective surface that reflects over 90 percent of the light that falls on it, making Eris one of the most reflective objects in the solar system. Scientists believe that the surface of Eris is a combination of nitrogen-rich ice mixed with methane, which coats the dwarf planet's surface with a one-millimeter-thick layer of ice. In Pluto’s case, darker surface hues are attributed to tholins, reddish-brown breakdown products formed when methane and similar organic compounds are subjected to solar ultraviolet irradiation. Where tholin deposits make Pluto’s surface darker, the albedo is lower, and the temperature higher. Methane ice melts away from these comparatively warm patches. By contrast, methane ice appears to envelop Eris in a bright, near-uniform coating. This suggests Eris remains cold enough that its methane stays frozen and that a subsurface source of methane may replenish Eris’s surface coating of methane ice and cover up whatever tholins are deposited.

As Eris moves from aphelion to perihelion, its temperature increases from -405° Fahrenheit (-243° Celsius) to -359° Fahrenheit (-217° Celsius). It is possible that when Eris approaches the Sun, some of its surface ice becomes warm enough to sublimate and form a thin atmosphere. Whatever gases do not escape the atmosphere freeze once again as the dwarf planet moves toward aphelion.

Dysnomia, the only known satellite of Eris, is known technically as Eris I. Data indicate that Dysnomia is roughly 150 kilometers in diameter, and it takes almost sixteen days for Dysnomia to complete its near-circular orbit around Eris at a distance of approximately 37,000 kilometers. The small satellite is believed to be composed largely of frozen water.

Astronomers Mike E. Brown of the California Institute of Technology, Chad A. Trujillo of Gemini Observatory, and David Rabinowitz of Yale University discovered Eris through an ongoing survey conducted at Palomar Observatory in southern California using the Samuel Oschin telescope. Images taken on the night of October 21, 2003, showed the large, bright object traveling slowly across the sky. Its movement was slow enough, in fact, that Eris went undetected when the images were first analyzed. The researchers’ discovery in November 2003 of Sedna, another large and slow-moving TNO, led them to adjust their detection scheme and reanalyze their survey data. On January 5, 2005, they identified the planet-sized scattered disk object that would later be known as Eris, designating it 2003 UB313.

Brown and his team intended to follow standard scientific protocols by verifying their discovery, studying it, documenting it thoroughly, and making it known through a scientific paper published in a reputable journal. However, in July 2005, they learned that detailed records of their telescope use had inadvertently been made accessible to anyone with Internet access, and an abstract they had recently published unwittingly contained clues about where in the sky to look for their recent—and unannounced—trans-Neptunian discoveries. When, five days after the abstract was issued, researchers in Spain announced the discovery of 2003 E161—a trans-Neptunian object Brown and his colleagues had also found—it appeared the team had to lay claim to 2003 UB313 or risk having someone else take the credit for finding it.

The California Institute of Technology, the Jet Propulsion Laboratory, and the National Aeronautics and Space Administration (NASA) announced the discovery of 2003 UB313 on July 29, 2005, in press releases that referred to the object as the tenth planet. The media was quick to adopt the team’s nickname for the newly discovered member of the solar system: Xena, so-called for the heroine of the television series Xena: Warrior Princess. (Some overenthusiastic journalists, seeing “planetlila” in Brown’s web address, pounced upon Lila as the new planet’s name, only to learn that the URL was a whimsical tribute to Brown’s newborn daughter.)

That same year, Brown, Trujillo, and Rabinowitz collaborated with the engineering team at the Keck Observatory on Mauna Kea, Hawaii, to search 2003 UB313 and three more of the brightest trans-Neptunian objects for satellites. Using the Keck’s new Laser Guide Star Adaptive Optics system, which enabled the researchers to view details as precise as those seen from the Hubble Space Telescope, they found S/2005 (2003 UB313) 1—the faint satellite that would come to be called Dysnomia—on September 10, 2005. The team dubbed the satellite Gabrielle after Xena’s television-show sidekick.

In the summer of 2006, at a meeting of the General Assembly of the International Astronomical Union (IAU), criteria were developed regarding what constitutes a planet, and a new category, dwarf planet, was established. Under the IAU’s new definitions, 2003 UB313 was not a planet but rather a dwarf planet. On September 6, 2006, the discovery team proposed to the IAU that dwarf planet 2003 UB313 be named Eris and its moon be named Dysnomia. The IAU accepted and announced the names one week later, on September 13, 2006.

Knowledge Gained

The 2005 discovery of 2003 UB313 added fuel to a long-standing and heated debate in the astronomical community over Pluto’s status as a planet. The newly discovered object was believed to have a much greater diameter than Pluto and was more massive than its distant neighbor. If Pluto’s size and mass were sufficient to qualify it for planethood, then the new object should be similarly classified.

The controversy surfaced again in August 2006 at IAU’s General Assembly in Prague. At the unusually contentious meeting, members reached the non-unanimous conclusion that a celestial body should be considered a planet if it (1) orbits the Sun (satellites are not planets), (2) is massive enough for self-gravity to shape it into a sphere; and (3) has accreted or scattered other bodies in its neighborhood to clear its orbit. Objects meeting the first two criteria but not the third would be classified as dwarf planets.

Both Eris and Pluto orbit the Sun, and both are spherical, yet neither has cleared the vicinity around its orbit. With the IAU’s official acceptance of the new definitions on August 24, 2006, 2003 UB313 ceased to be a possible tenth planet and became the largest known dwarf planet. Likewise, Pluto was demoted to dwarf planet status after more than seven decades of being regarded as a planet. Ceres was determined to be a dwarf planet that might also be an asteroid. 2003 UB313 received its official name, Eris, on September 13, 2006. In Greek mythology, Eris was the goddess of discord and strife. Given the uproar the object’s discovery caused among astronomers—and a public accustomed to a nine-planet solar system—it was aptly named. The dwarf planet’s satellite was named after the mythological figure Dysnomia, daughter of Eris and the spirit of lawlessness.

Almost two years later, Eris was recategorized as a dwarf planet. On June 11, 2008, the IAU announced that dwarf planets orbiting the Sun at a semimajor axis more significant than that of Neptune would be known as “plutoids.” Eris and Pluto qualified; Ceres, located in the asteroid belt, did not. In July 2008, the Kuiper Belt object known as 2005 FY9 received its official name, Makemake (pronounced mah-kee mah-kee), and became the third largest dwarf planet and plutoid. Though exploration of the dwarf planet would require a nearly twenty-five-year journey, the New Horizons spacecraft observed Eris from a distance in 2020 following its flyby of Pluto. Scientists could see new angles of the planet and gain foundational knowledge about Eris's makeup.

Context

The discovery of small objects beyond Neptune in the early 1990s inspired researchers like Brown to investigate the Kuiper Belt and beyond for larger celestial bodies. Brown and Trujillo made their first significant trans-Neptunian find in 2002 with Quaoar. In 2003, they discovered a larger object, Sedna, the remotest solar-system object known at the time. Sedna’s slower motion across the heavens led the team to look for objects moving at even lower rates, which revealed Eris.

Theoretical models suggest that Eris and other comparatively large trans-Neptunian objects in high-inclination orbits were originally near the inner edge of the Kuiper Belt. When they were subsequently scattered into the outer belt and beyond, they achieved orbits with higher inclinations than objects originating in the outer belt. More massive objects that had their origins in the inner belt may occupy remote, high-inclination orbits. Researchers continue to look for as-yet-undiscovered large objects orbiting at these high inclinations.

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