When is a planet not a planet?
When is a planet not a planet? This question has troubled scientists and students for decades, and the recent demotion of Pluto from planet to plutoid marked a significant change in how scientists classify planets. In 2006 the International Astromical Union identified three key criteria to classify a body as a planet:
- The body is in orbit around the Sun,
- The body has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
- The body has “cleared the neighbourhood” around its orbit.
By this definition Pluto was excluded, but the discovery of a transneptunian object on January 5, 2005 added very briefly a planet to the solar system.
Many teams were racing to map the outer solar system, specifically the belt of asteroids and debris in the scattered disc known as transneptunian objects, and the discovery of Eris and Dysnomia took several years of searching, two years of analysis with both computers and good old fashioned legwork. The team of Mike Brown, Chad Trujillo, and David Rabinowitz had programmed their computers to rule out objects moving too slowly to count and missed seeing Eris when it was first photographed. Manually pouring over the photos almost a year and a half later, they found the largest (to date) plutoid or dwarf planet.
Wanting to name more sky objects after women, internally the team referred to the dwarf planet as Xena, after the popular television series. After a period of several years (and much consensus building) the name Eris was settled on. Eris was the goddess of strife in Greek mythology, the meddling troublemaker who doomed the marriage of Peleus and Thetis, the parents of Achilles, which led to the disastrous war that destroyed both Troy and much of the invading Greek armies. Eris’s own son was named Dysnomia, meaning lawlessness, and when the Hubble Space Telescope discovered that Eris had a moon, well, the name was inevitable. Since the discovery of Eris and Dysnomia, several more plutoids have been discovered, but none yet as large. Eris was considered a planet for less than a year, before both Eris and Pluto were reclassified as plutoids.
Images of Eris and Dysnomia courtesy NASA/Hubble.