Astronomers announced the discovery of a distant solar-system body provisionally designated 2003 UB313 and later named Eris. The announcement carried unusual historical significance because the object appeared comparable to, and possibly larger than, Pluto. Its discovery forced astronomers to confront a question that had remained unresolved for decades: what physical characteristics distinguish a planet from the many smaller bodies orbiting the Sun? Eris did more than enlarge the known solar system. It prompted the International Astronomical Union to establish the first formal scientific definition of a planet and changed Pluto’s official classification.
The discovery resulted from a systematic survey conducted by Michael E. Brown of the California Institute of Technology, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University. The team used the 48-inch Samuel Oschin Schmidt Telescope at Palomar Observatory in California to photograph broad regions beyond Neptune. The telescope recorded the object on October 21, 2003, but its slow apparent movement caused it to escape immediate detection. Brown’s team identified it on January 5, 2005, while examining earlier images with computer-assisted methods intended to reveal distant moving objects.
The researchers did not announce the discovery immediately. They wanted additional observations to establish the object’s orbit and physical properties. Events accelerated in July 2005 after information related to another trans-Neptunian discovery became publicly accessible. Concerned that independent observers might identify 2003 UB313 in archived data, Brown’s team disclosed the object on July 29. NASA’s Jet Propulsion Laboratory announced it as a possible “tenth planet,” reflecting the scientific uncertainty surrounding its classification.
Early measurements indicated that 2003 UB313 was at least as large as Pluto. Its brightness, distance, and estimated reflectivity initially suggested a diameter greater than Pluto’s. Later observations refined its diameter to approximately 2,326 kilometers, making it slightly smaller than Pluto by current measurements. Eris nevertheless has about 27 percent more mass than Pluto. The discovery of its moon, Dysnomia, allowed astronomers to calculate Eris’s mass by measuring the moon’s orbital motion.
Eris follows a highly elongated and inclined orbit. It completes one revolution around the Sun in approximately 557 Earth years and travels between roughly 38 and 98 astronomical units from the Sun. One astronomical unit represents the average distance between Earth and the Sun. Eris therefore travels far beyond Pluto during much of its orbit. Its extreme path placed it within the scattered disk, a distant population of icy bodies whose present orbits were strongly affected by the gravitational influence of Neptune.
The announcement intensified an existing problem in planetary science. Pluto had been classified as the ninth planet since its discovery in 1930, yet surveys during the late twentieth and early twenty-first centuries revealed that it belonged to a broad population of icy trans-Neptunian objects. Astronomers discovered increasingly large bodies in this region, including Quaoar, Sedna, Haumea, and Makemake. Eris presented the decisive challenge. If Pluto remained a planet solely because of its size and spherical shape, then the more massive Eris appeared entitled to the same status. Similar future discoveries could have expanded the planetary roster repeatedly.
The International Astronomical Union addressed the issue during its General Assembly in Prague in August 2006. On August 24, members approved a definition requiring a planet to orbit the Sun, possess enough mass for gravity to produce a nearly round shape, and clear the neighborhood surrounding its orbit. A dwarf planet met the first two conditions but not the third. Under this definition, the solar system contained eight planets. Pluto, Ceres, and 2003 UB313 became the first officially recognized dwarf planets.
In September 2006, the International Astronomical Union approved the name Eris, drawn from the Greek goddess associated with discord and strife. The name referred to the disputes that the discovery had generated within astronomy and among the public. Its moon received the name Dysnomia, after the mythological daughter of Eris associated with lawlessness.
The historical importance of the July 29 announcement rests on its effect upon scientific classification. Eris exposed the weakness of inherited planetary categories and compelled astronomers to replace custom with stated physical criteria. The resulting decision remained controversial, especially because it removed Pluto from the planetary roster, but it created a consistent framework for classifying newly discovered solar-system bodies. Eris therefore became significant less as a proposed tenth planet than as the object that caused scientists to redefine what a planet is.
References / More Knowledge:
Brown, Michael E. “The Discovery of Eris, the Largest Known Dwarf Planet.” California Institute of Technology. https://web.gps.caltech.edu/~mbrown/planetlila/
California Institute of Technology. “The Dwarf Planet Formerly Known as Xena Has Officially Been Named Eris, the IAU Announces.” September 14, 2006. https://www.caltech.edu/about/news/dwarf-planet-formerly-known-xena-has-officially-been-named-eris-iau-announces-1187
International Astronomical Union. “IAU 2006 General Assembly: Result of the IAU Resolution Votes.” August 24, 2006. https://www.iau.org/IAU/News/PR2006/iau-2006-general-assembly-resolution-votes.aspx
NASA. “Eris.” NASA Science. https://science.nasa.gov/dwarf-planets/eris/
NASA. “Hubble Finds ‘Tenth Planet’ Is Slightly Larger Than Pluto.” April 11, 2006. https://science.nasa.gov/missions/hubble/hubble-finds-tenth-planet-is-slightly-larger-than-pluto/
NASA. “Astronomers Measure Mass of Largest Dwarf Planet.” June 14, 2007. https://science.nasa.gov/missions/hubble/astronomers-measure-mass-of-largest-dwarf-planet/
NASA Jet Propulsion Laboratory. “NASA-Funded Scientists Discover Tenth Planet.” July 29, 2005. https://www.jpl.nasa.gov/news/nasa-funded-scientists-discover-tenth-planet/
Palomar Observatory. “Palomar Observatory Chronology.” California Institute of Technology. https://sites.astro.caltech.edu/palomar/about/chronology.html