Eris is the most massive and second-largest known dwarf planet, orbiting the Sun far beyond Neptune. This Trans-Neptunian Object (TNO) gained notoriety because its discovery triggered a significant debate that led to the formal redefinition of a planet. Finding Eris involved years of systematic searching, specialized imaging techniques, and a careful re-analysis of previously collected data.
Setting the Stage: The Hunt for Trans-Neptunian Objects
The discovery of Eris was the culmination of a focused astronomical search for objects residing in the outer solar system, a region populated by Trans-Neptunian Objects (TNOs). Beyond the orbit of Neptune lay a vast population of icy, rocky bodies, remnants from the solar system’s formation. This region includes the Kuiper Belt, a doughnut-shaped zone of stable orbits, and the more distant, highly eccentric Scattered Disc. Eris would ultimately be classified as a member of the Scattered Disc.
The search was systematically conducted by a team based at Caltech, led by astronomer Mike Brown, along with colleagues Chad Trujillo and David Rabinowitz. Their goal was to map this distant territory and find the largest objects residing there, some expected to rival or exceed the size of Pluto. This ambition required a dedicated, large-scale sky survey, and the team utilized the 1.2-meter Samuel Oschin Telescope at Palomar Observatory for their imaging work. The team had already found several other large TNOs, including Quaoar and Sedna, underscoring the success of their methodical approach.
The Technical Method of Detection
The method used to find Eris relied on a process similar to the classic “blink comparison” technique, adapted for digital astronomy. This involved taking multiple digital images of the same sky area over a period of hours or days. When comparing these sequential images, distant background stars appear fixed, while any object orbiting the Sun will show a slight movement.
The massive dataset generated by the Palomar survey was first processed by automated software designed to flag objects exhibiting motion. Eris’s immense distance meant its apparent motion against the background stars was incredibly slow. Due to this slow speed, the initial automated search in 2003 actually missed the object, as the software was configured to filter out anything moving slower than a certain threshold to avoid false detections.
The crucial moment came during a re-analysis of the data, prompted by the discovery of other distant, slow-moving objects like Sedna. On January 5, 2005, the team manually re-examined the images that had been initially excluded, finding the tiny, slow-moving spot that was Eris. Although the raw data had been collected earlier, on October 21, 2003, the actual discovery date is considered the time of its identification. The object was immediately given the provisional designation 2003 UB313.
Confirmation and Defining the Solar System
Following the initial detection, the team performed follow-up observations to confirm the object’s existence and calculate its orbit. These observations involved using larger telescopes, like the Gemini North Telescope, to obtain spectroscopic data, which revealed the presence of methane ice on its surface, similar to Pluto. A later key observation in 2005 confirmed the presence of a small moon, which was eventually named Dysnomia.
The discovery of Dysnomia was significant because its orbit allowed astronomers to accurately determine Eris’s mass. This calculation revealed Eris to be approximately 27% more massive than Pluto. The existence of an object demonstrably larger and more massive than Pluto challenged the existing, informal definition of a planet.
This finding immediately sparked an intense debate within the scientific community, forcing the International Astronomical Union (IAU) to address the lack of a formal definition. In August 2006, the IAU adopted a new, formal definition that required a planet to have “cleared the neighborhood around its orbit.” Because Eris, like Pluto, resides in a zone with many other similar objects, neither met this new requirement. The resolution created the new category of “dwarf planet,” into which both Eris and Pluto were placed. Eris, named after the Greek goddess of strife and discord, officially received its permanent name in September 2006, a nod to the controversy its discovery had caused.