Exploring Eris: The Distant Dwarf Planet of Strife and Discovery

Eris, officially designated as 136199 Eris, holds a significant position in our Solar System as the most massive and second-largest known dwarf planet. This distant celestial body is a fascinating subject of study, particularly due to its role in reshaping our understanding of planetary classification.

As a trans-Neptunian object (TNO), Eris resides far beyond the orbit of Neptune, inhabiting the vast, sparsely populated region known as the scattered disk. Its journey around the Sun is characterized by a highly eccentric orbit, meaning its path is significantly elongated, taking it through some of the most frigid and remote reaches of our Solar System.

The Discovery and Naming of Eris

The discovery of Eris marked a pivotal moment in modern astronomy. It was first identified in January 2005 by a dedicated team based at Palomar Observatory in California, led by the renowned astronomer Mike Brown. This discovery was subsequently verified later that same year, confirming the existence of a major new object in the outer Solar System.

In September 2006, Eris received its formal name, a decision that resonated deeply with the scientific developments it had sparked. It was named after Eris, the Greco-Roman goddess personifying strife and discord. This name proved remarkably apt, as the very existence and apparent size of this dwarf planet ignited a significant debate within the astronomical community that ultimately led to a redefinition of what constitutes a "planet."

Eris's Astronomical Significance and Physical Characteristics

Eris is currently recognized as the ninth-most massive known object directly orbiting the Sun. When considering all celestial bodies within the Solar System, including moons, Eris ranks as the sixteenth-most massive overall. Notably, it also holds the distinction of being the largest object in our Solar System that has yet to be visited by a robotic spacecraft, making it a prime candidate for future exploratory missions.

Precise measurements indicate Eris has a diameter of 2,326 ± 12 kilometers (approximately 1,445 ± 7 miles). Its mass is immense for a dwarf planet, equating to about 0.28% of Earth's mass and notably 27% greater than that of Pluto. This mass difference is significant, even though Pluto has been measured to be slightly larger by volume. This intriguing contrast highlights differences in density and internal composition between the two dwarf planets.

Regarding its rotation, Eris is thought to complete one full turn on its axis approximately every 25.9 hours, meaning its day length is remarkably close to that of Earth. However, due to its immense distance and the inherent challenges of observing such a small, faint object, sources sometimes present differing values for its exact rotation period.

Dysnomia: Eris's Moon

Eris is not alone in its distant orbital dance; it is accompanied by one known moon, Dysnomia. This large satellite was discovered in September 2005. Fittingly, Dysnomia is named after the goddess of lawlessness, the daughter of Eris in Greek mythology. The discovery of Dysnomia was crucial as it allowed astronomers to accurately determine Eris's mass by observing the moon's orbit around it.

Extreme Distance and Solar System Records

The vast distance of Eris from the Sun is truly staggering. In February 2016, Eris was situated approximately 96.3 astronomical units (AU) from the Sun, which translates to an incredible 1.441 × 10¹⁰ kilometers (or 8.95 × 10⁹ miles). To put this into perspective, this distance is more than three times that of Neptune or Pluto at their respective average distances from the Sun.

For a period, Eris and its moon Dysnomia held the record for being the most distant known natural objects in the Solar System, with the exception of certain long-period comets that temporarily venture much farther out. This record was surpassed in 2018 with the discovery of 2018 VG18, informally nicknamed "Farout," which was found to be even more remote.

The "Tenth Planet" Debate and the Redefinition of "Planet"

Upon its initial discovery, Eris appeared to be larger than Pluto, leading NASA and many in the scientific community to initially describe it as the Solar System's "tenth planet." This perception, coupled with the increasing likelihood of discovering other similarly sized or even larger objects in the far reaches of the Solar System, prompted a crucial re-evaluation of what defines a "planet."

This pressing need motivated the International Astronomical Union (IAU), the global authority for naming celestial objects, to formally define the term "planet" for the very first time. On August 24, 2006, the IAU approved a new definition that fundamentally altered the classification of many Solar System bodies. Under this landmark definition, an object is considered a "planet" only if it:

• Orbits the Sun.
• Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
• Has "cleared the neighbourhood" around its orbit, meaning it has become gravitationally dominant in its orbital path.

Eris, along with Pluto, Ceres, Haumea, and Makemake, met the first two criteria but failed the third, as they share their orbital regions with many other similar-sized objects. Consequently, Eris, Pluto, and others were reclassified as "dwarf planets." This decision effectively reduced the number of officially recognized planets in the Solar System from nine back to eight, matching the count prior to Pluto's discovery in 1930.

Refined Size Measurements: Eris vs. Pluto

Further scientific observations continued to refine our understanding of Eris's exact size. In 2010, astronomers utilized a technique called a stellar occultation—observing Eris as it passed directly in front of a distant star—to precisely measure its diameter. These highly accurate measurements revealed that Eris was in fact very slightly smaller than Pluto.

This finding was later corroborated and further clarified by the data from NASA's New Horizons mission. In July 2015, New Horizons provided highly accurate measurements of Pluto, confirming its diameter to be 2,377 ± 4 kilometers (approximately 1,477 ± 2 miles), definitively establishing Pluto as slightly larger in volume than Eris, despite Eris's greater mass.

Frequently Asked Questions about Eris

What is Eris?

Eris (minor-planet designation 136199 Eris) is the most massive and second-largest known dwarf planet in our Solar System. It is a trans-Neptunian object (TNO) located in the scattered disk, known for its highly eccentric orbit.

When was Eris discovered?

Eris was discovered in January 2005 by a team led by Mike Brown at Palomar Observatory and formally verified later that year.

Why is Eris named after the goddess of strife?

Eris was named after the Greco-Roman goddess of strife and discord in September 2006. This name was chosen because the discovery and initial classification of Eris sparked a significant debate among astronomers that ultimately led to the redefinition of what constitutes a "planet," causing considerable "strife" in the astronomical community.

Is Eris larger than Pluto?

While Eris is more massive than Pluto (by about 27%), it is slightly smaller in terms of volume. Early observations suggested Eris was larger, but precise measurements from a 2010 stellar occultation and New Horizons' 2015 flyby of Pluto confirmed Pluto's diameter to be slightly greater.

Why is Eris classified as a dwarf planet?

Eris is classified as a dwarf planet under the International Astronomical Union's (IAU) 2006 definition. Although it orbits the Sun and is massive enough to be nearly round, it has not "cleared the neighbourhood" around its orbit, meaning its orbital path is shared with many other objects, which is a key criterion for a full-fledged planet.

Does Eris have any moons?

Yes, Eris has one known moon named Dysnomia. Its discovery was crucial for accurately determining Eris's mass.

How far away is Eris from the Sun?

Eris's distance from the Sun varies due to its eccentric orbit, but in February 2016, it was approximately 96.3 astronomical units (AU) away, which is over three times the average distance of Neptune or Pluto from the Sun.