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Eris, Our Tenth Planet

In 2003, an object was discovered in the Kuiper Belt of the Solar System.  This was discovered by M. E. Brown, C.A. Trujillo, and D. L. Rabinowitz of the Palomar Observatory.

Of course, these people and other scientists have been studying it since, and it has been found to be bigger than the Planet Pluto, both in mass and in diameter.  This was briefly considered by some people to be our tenth planet.  The existence of Eris and other similar objects that had been discovered as well as the very strong probability of many others being found led to serous consideration of what is a planet.

No Definition

Before this time there had been no real scientific definition of a planet.  But if Pluto was a planet, then Eris was certainly entitled to the same status. 


The planets are named after the Gods of the ancient Greeks and Romans.  However, the names of most of these had been used up on asteroids. Eris was the Goddess of warfare and quarrelling.


Eris has a highly elliptical orbit that takes about 560 years to orbit the Sun.  At its furthest it is around 97 astronomical units (An astronomical unit it the distance of the Earth from the Sun) and at its closest it is only 38 astronomical units. This is even more elliptical than Pluto.  At its closest approach to the Sun, The orbit of Eris goes inside that of Pluto and goes nearly as close to the Sun as Neptune.

Light can take about half a day to get from Eris to the Earth, although this obviously varies with how far away it is.

Generally the dwarf planets have an orbit which is not in the same plane as the planets.  This plane in called the plane of the ecliptic. Eris takes this to extremes.  Its orbit is inclined 44 degrees to the ecliptic.


The surface of Eris is surprisingly bright.  This may be caused by a thin layer of frozen methane.  This is the atmosphere of Eris.

Temperature and Atmosphere

Although the atmosphere of Eris is frozen at the moment, when the dwarf planet comes closer to the Sun, the temperature will probably rise from its present level of about minus 230 degrees C.  It may get high enough for some of its atmosphere to change into gasses.  If this happens, the surface may get darker. I am looking forward to seeing if this happens, in about 250 years.

Eris and Dysnomia

Picture by NASA
Sizes of Dwarf Planets compared with The Earth
The Picture is from NASA, but the names of the Dwarf Planets are in Spanish.
This is only significant in relation to Pluto and its moons.
Orbit of Eris in relation to the Planets
By User: Orionist [GFDL (, CC-BY-SA-3.0 ( or CC-BY-SA-2.5-2.0-1.0 (], via Wikimedia Commons.
This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.