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Space Topics: Trans-Neptunian ObjectsDwarf Planet 136199 ErisFormerly Known As 2003 UB313
Named after the goddess of strife, 136199 Eris (provisional designation 2003
UB313) is the object that brought the heated debate about the definition of
the planet to a crisis. Larger than Pluto in both mass and diameter, Eris'
discovery precipitated the decision
by the International Astronomical Union on August 24, 2006 to define a
new class of solar system objects called "dwarf planets." Dwarf
planets are objects large enough to be round, but too small to dominate their
region of the solar system. At the time it was discovered, Eris was the most
distant solar system object discovery at nearly 97 astronomical units (over
14.5 billion kilometers or 9 billion miles) away. Basic FactsDiameter: approximately 2400 kilometers (about 18% of Earth) The discovery of Eris was announced in July 2005 by Mike Brown, Chad Trujillo, and David Rabinowitz. Though the discovery images were taken in 2003, a reanalysis of the data in early 2005 revealed an object moving very slowly with respect to the stars. The amount of motion is directly related to the distance, so Eris' unprecedentedly slow motion indicates a distance almost three times as far from Earth as Pluto. At the time of its discovery, Eris was referred to provisionally as 2003 UB313.
For an object that far away to be visible, even to modern telescopes, it must be either highly reflective (large albedo) or very large. To distinguish between these two possibilities required more detailed studies, but within a year the diameter of Eris was directly measured by the Hubble Space Telescope to be 2,400 +/- 100 kilometers (1,490 +/- 60 miles), or 5% larger than Pluto. Other measurements based on the amount of heat given off (in the form of infrared or sub-millimeter radiation) also indicate that Eris is larger than Pluto. Eris is so bright, despite its great distance from Earth, because it reflects about 88% of the light it receives from the Sun back to our telescopes. If Mars (which only reflects about 15% of the light that hits it) were transported to Eris' distance from the Sun, it would be barely any brighter a point in the sky than Eris is, despite having a diameter almost three times as large. Like Pluto and many other Kuiper belt objects, Eris has a moon, named Dysnomia (the mythological daughter of Eris). Dysnomia is quite small compared to Eris and orbits its primary every 15.8 days. The presence of a moon allow astronomers to calculate the mass of the body it orbits. According to Kepler's third law of planetary motion, the size and period of an orbit are directly related to the mass of the objects. By tracking the motion of Dysnomia, Mike Brown's team found that Eris has a mass of 1.66 x 1022 kilograms, which makes it 27% more massive than Pluto. This mass indicates that Eris has a slightly larger density (2.3 grams per cubic centimeter) than Pluto, implying that it contains a higher proportion of rock relative to ice.
Eris is classified as a Scattered disk object because it has been thrown by Neptune into a very eccentric and highly inclined orbit. The plane of its orbit is tilted by almost 45 degrees from the orbits of the other planets (not including Pluto). Eris' orbit is also highly elliptical; its 557-year orbit takes it as close as 37 and as far as 97 Astronomical Units (AU) from the Sun. The reflective nature of the surface of Eris is thought to be due to frozen methane (natural gas or CH4) frost. A near-infrared spectrum shows deep methane absorption features, similar to those found on Pluto. In fact, Pluto and Eris probably have very similar surfaces. When they are closest to the Sun (perihelion), the temperature is warm enough (a frigid 43 Kelvin, -230 Celsius, or -382 Fahrenheit for Eris) for the methane and other gases on the surface to form a weak atmosphere. Pluto is currently near perihelion, and does have a tenuous atmosphere. Eris is currently near its aphelion, and its surface temperature is now 27 Kelvin (-246 Celsius or -411 Fahrenheit). At this much colder temperature, the atmosphere has condensed into a frost onto the surface. Like freshly fallen snow, the frost is highly reflective. A deeper understanding of the surfaces of these icy dwarf planets will be revealed from detailed images and data taken by New Horizons during its flyby of Pluto in 2015. Briefly considered the "tenth planet" and nicknamed "Xena", Eris will always hold a prominent spot in the history of planetary science. |
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