(Above: An artist concept of the dwarf planet Eris. The sun is the small star in the distance. Courtesy NASA/JPL-Caltech)
(WHTM) — In 2005 the team of Mike Brown, Chad Trujillo and David Rabinowitz were studying photographs taken by the Samuel Oschin Schmidt telescope at Palomar Observatory, looking for large objects in the outer Solar System.
A set of three images taken in 2003 revealed a small object which had changed position against the stellar background. Follow-up observations determined the object orbited about three times further from the sun than the planet Pluto.
They announced the discovery on July 29th, 2005. In October 2005 more observations revealed the planet has a moon. This discovery helped scientists determine the mass of the planet-about 27% greater than Pluto’s.
But what to name them?
Amongst themselves, Brown, Trujillo and Rabinowitz called the object Xena, after the title character in the television series “Xena: Warrior Princess.” (They referred to the moon as Gabrielle, after Xena’s sidekick.) Word got out, and a lot of people were hoping those would be the permanent names.
But the final decision would be made by the International Astronomical Union (IAU), the non-governmental body tasked with naming objects in the solar system. There’s a long tradition of naming planets using Greek and Roman mythology.
With that in mind, the discovery team suggested naming the planet Eris, after the Greek goddess of strife and discord, and the moon Dysnomia, Eris’s daughter and Greek goddess of lawlessness. The IAU accepted these as the official names on September 13th, 2006.
The strife and discord began in earnest.
For years the IAU defined objects in the solar system as “planets” and “minor planets”. The minor planets designation was getting a bit unwieldy. (And it’s getting worse-as of July 2021, there are 567,132 numbered minor planets -most of them don’t have names- and a total of 1,092,846 observed bodies.) And the question of whether Eris should be considered the tenth planet was causing much disagreement and debate.
The organization gave a group of astronomers the task of developing a more precise definition of the word “planet.” The new definition was adopted on August 24th, 2006, just a few weeks before Eris was formally named. It says a planet’s a planet if is in orbit around the Sun, is massive enough to form into a roughly spherical shape (a state called “hydrostatic equilibrium”), and has “cleared the neighborhood” around its orbit-that is, no other object of similar size (except a planet’s moons) are in the planet’s orbit. A planet that did not meet that third criterium would be known as a “dwarf planet”.
Both Pluto and Eris were designated dwarf planets.
This did not go down well with many people. Many were furious that Pluto, which had been The Ninth Planet for decades, was “demoted”. A lot of scientists felt that the “clear the neighborhood” criteria would be difficult to prove or disprove. And many considered the new standards just another layer of complication-why have two classes of planets at all? Eris seems to have lived up to its name. The debate continues, but for now, the dwarf planet category is still in use.
Some quick factoids:
Eris is pronounced…ummm…it depends. The classical English pronunciation uses a long e-“EE-ris.” Nowadays, though, many English speakers pronounce it with a short e, and run the e and r together-“Air-is”.
Eris’ diameter is 1,445 miles (2,326 kilometers). This means it’s slightly smaller than Pluto at 1,473 miles (2,370 km), as measured by the New Horizons spacecraft in 2015. But even though it’s smaller, Eris is slightly more massive.
Eris takes about 559 years to go around the sun. Instead of an almost circular orbit like the inner planets, the orbit of Eris is a pronounced oval. Studies of the orbit suggest the planet reached its furthest point from the sun around 1977, and will be closest to the sun around December-of 2257. Mark your calendars.
The moon Dysnomia orbits Eris in 16 Earth days.
If you could gather up all the objects in the asteroid belt between Mars and Jupiter, they could fit inside Eris.
Eris is so far from the Sun and gets so little sunlight, that its atmosphere sometimes freezes. This may be why it’s very bright-96% of the light that hits reflects back.
Scientists believe that the surface of Eris is rocky, similar to the surface found on Pluto.