Eris (2003 UB313)

On July 14, 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.

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 June 11, 2008, the International Astronomical Union (IAU) voted at the meeting of its Executive Committee to establish bright "dwarf planets beyond the orbit of Neptune as a new class of substellar objects in the Solar System called "plutoids" (IAU press release). According to the IAU, these plutoids "are celestial bodies in orbit around the Sun at a distance greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighbourhood around their orbit. Satellites of plutoids are not plutoids themselves, even if they are massive enough that their shape is dictated by self-gravity. As of July 14, 2008, the three known and named plutoids are Pluto, Eris, and Make-make. In addition, "it was decided that, for naming purposes, any Solar System body having (a) a semimajor axis greater than that of Neptune, and (b) an absolute magnitude brighter than H = +1 will, for the purpose of naming, be considered to be a plutoid," so that many large and relatively round Edgeworth-Kuiper ice bodies may eventually be designated as plutoids.

Initally nicknamed Xena, the dwarf planet and plutoid 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.

Adolf Schaller, STScI, ESA, NASA,



Larger and jumbo illustrations.



Eris has its own satellite,
now named Dysnomia (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 initially 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), but this estimate was subsequently revised again.

In June 2007, the discovery team that discovered Eris and its satellite Dysnomia announced that recently completed orbit calculations of the two bodies indicated that Eris is 27 (+/- 2) percent more massive than Pluto. Based on a series of seven observations using the Keck Telescope and the Hubble Space Telescope, Dysnomia's around Eris was found to take almost 16 days. Dysnomia's nearly circular orbit supports the theory that Dysnomia was created from the collision of Eris with another Edgeworth-Kuiper object (EKO), as a gravitationally captured object would be expected have a more elliptical orbit around Eris. Moreover, unlike Pluto, no more moons around Eris were discovered. Lastly, a comparison of Eris' mass and diameter indicates a density 2.3 grams per cubic centimeter, which is very similar to the density of Pluto, the large EKO 2003 EL61, and Neptune's moon Triton (likely to be a captured EKO). Such densities imply that these bodies have a significant rocky composition (more from Hubblesite.org and the Eris discovery team).

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).

Eris is relatively uniform, white, and unusually bright, reflecting around 86 +/- 7 percent of the light that hits it, while Pluto has a mottled-looking surface that reflects only around 60 percent of the sunlight falling on it (more from astronomer Mike Brown and the hubblesite.org). 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.)

On September 10, 2005, astronomers and engineers at the Keck Observatory (including Marcos van Dam, David Le Mignant, and Antonin Bouchez) on Mauna Kea in Hawaii discovered that Eris has a satellite or "moon," using a new instrument which a resolution approaching that of the Hubble Space Telescope (press release). The moon has an orbital period of about 14 days and is only about 1/60th as bright as Eris, which suggests that the moon only has around an eighth of its diameter.

Robert Brown,
CalTech, ESA, NASA



larger image.



Dysnomia has a highly
circular orbit Eris that
takes nearly 16 days to
complete (more).