Eris (2003 UB313)

On July 29, 2005, a team of astronomers (including Mike Brown, Chad Trujillo, and David Rabinowitz) announced the discovery of a planetary body (2003 UB313) that appeared to be larger than Pluto (NASA press release; latest news; and Brown et al, 2005, in pdf). A little over a year later on August 24, 2006, the International Astronomical Union (IAU) voted to classify 2003 UB313 as a "dwarf planet," because it is not massive enough to clear its orbital neighborhood of similarly sized objects. Nearly a month later on September 13, 2006, the IAU announced its official approval of the names of Eris and Dysnomia for 2003 UB313 and its satellite that were submitted by its discovery team. In Greek mythology, Eris (known as the goddess of discord, strife, and chaos) caused a quarrel among the goddesses that sparked the Trojan War, while her daughter Dysnomia was known as the goddess or spirit of lawlessness. Not surprisingly, the discovery of Eris and the finding that it is larger than Pluto have also caused strife in the astronomical community, forcing some astronomers to produce a strict definition of the term "planet" which eventually led to Pluto losing its status as the "ninth planet" that it had held since its discovery in 1930.

Initally nicknamed Xena, the object is currently located at around 98 AUs, which appears to be the point of its farthest orbital distance from our Sun, Sol -- a distance that is up to more than three times farther out than that of Pluto or Neptune. However, Eris will eventually move in as close as Pluto and Neptune (around 38 AUs) in a 557-year orbit around Sol that has a semi-major axis of 67.7 AUs. It has an elliptical orbit (e= 0.442) that is more eccentric than Pluto's. The dwarf planet is also tilted almost 44.2 degrees from the ecliptic where most of the planets in the Solar System orbit, and so many astronomers assume that gravitational encounters with a more massive object moved it into its current, highly inclined orbit.

Bertoldi et al,
Universität Bonn, MPG



Larger illustration.



Eris (formerly "Xena"
or 2003 UB313) is
now estimated to be
around 27 percent
more massive than
Pluto, and may
eventually be defined
as a planet, as well
as an Edgeworth-Kuiper
object (more).

On February 1, 2006, a team of astronomers (including Frank Bertoldi, Wilhelm Altenhoff, Axel Weiss, Karl M. Menten, and Clemens Thum) tentatively confirmed that Eris appeared is indeed around 30 percent larger than Pluto (joint press release with abstract). The largest object found in the Solar System since Neptune's discovery in 1846, Eris was estimated to have a diameter of around 3,000 +/- 400 kilometers (1,860 +/- 250 miles), based on infrared light at a wavelength of 1.2 millimeters. This new measurement used radiation emitted by Eris in response to being warmed by sunlight ("thermal emission"), regardless of the shininess of its surface. The infrared findings indicated that UB313 has a reflectivity, or albedo, of about 60 percent, which is similar to Pluto's and suggested that the two bodies have surfaces are made of very similar materials such as frozen methane and nitrogen snow at a temperature of -248 °C or -418 °F. By April, Eris' original discovery team hoped to announce an even more accurate estimate of its size -- to the nearest 50 km or 25 miles -- determined using new images from the Hubble Space Telescope (more discussion and an illustration from news@Nature and Astronomy Picture of the Day; and a simulation of its current location using a Java applet from NASA).

On April 11, 2006, Eris' discovery team announced that they had used the Hubble Space Telescope to determine that the dwarf planet is actually smaller at 2,400 +/- 100 kilometers (or 1,490 +/- 60 miles) wide, which is only around three percent larger than Pluto's 2,320 km (1,440 miles). However, while Pluto has a mottled-looking surface that reflects around 60 percent of the sunlight falling on it, Eris is relatively uniform, white, and unusually bright, reflecting around 86 +/- 7 percent of the light that hits it (more from astronomer Mike Brown and the hubblesite.org).

Brown et al (2005, in pdf)



Larger illustration



Although Eris has a very large
orbit around the Sun, it appears
to move as close as Pluto and
Neptune at its closest approach
to our Sun (more).

Based on analysis of infrared sunlight reflected off of Eris' surface, the object appears to be almost white in appearance, while Pluto is "moderately" red. Because Eris is currently so far away from the Sun, methane and nitrogen may have frozen out of the atmosphere as a coating of frost on its surface like Pluto and its interior is likely to be composed of rock and ice like Pluto and other Edgeworth-Kuiper objects (EKOs). During the period of Eris' discovery and astronomical analysis, two other EKOs (2003 EL61 and 2005 FY9) with around two-thirds to three-fourths or more of Pluto's diameter were also located at around 52 AUs from Sol. (Updates and images on all three objects are available from astronomer Mike Brown's discovery page. More information and links can be found at Astronomy Picture of the Day's web pages for July 31 and August 1, 2005.)