The solar system initially comprised 9 planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune and Pluto. However, in 2006, at the end of its 26th General Assembly, the International Astronomical Union (IAU), approved another classification of stellar objects in the solar system. According to their new classification, three types of stellar objects existed namely; planets, dwarf planets and solar system bodies.
Under the planet category, come celestial bodies that orbit the Sun, have sufficient mass to overcome the rigid forces and develop a nearly round shape and which have managed to clear the neighborhood around their orbits. The next category, is that of dwarf planets and includes those celestial objects that orbit the Sun; have sufficient mass to overcome the rigid forces and develop a nearly round shape; which have not managed to clear the neighborhood around their orbits and which are not satellites of any planets. Planets suck other smaller objects into themselves or fling them out, thus, clearing the orbit around them. However, dwarf planets with weaker gravitational pull, are unable to clear their orbits. The third category of solar system bodies, comprises all other celestial objects that orbit the Sun, such as asteroids and comets.
IAU decided to demote Pluto and reclassified it under the dwarf planet category. As of today, we have five known dwarf planets: Eris, Pluto, Haumea, Makemake and Ceres. Eris, is the largest of the dwarf planets and Ceres is the smallest. Eris was initially known as the tenth planet. However, its discovery prompted astronomers and scientists to reclassify celestial objects and thus Pluto and Eris were demoted from being planets to dwarf planets.
The Dwarf Planet: Eris
Eris, the largest known dwarf planet today, was spotted in January 2005 (first seen in 2003, however important factors were confirmed in 2005) by Michael Brown, of the California Institute of Technology and his team. This celestial object was the largest object spotted after Neptune's discovery and its moon in 1846 and outdid Pluto in size. However, beyond the Kuiper Belt (the outer fringe or belt of the solar system), Eris is the most distant object to be discovered in the orbit of the Sun.
Initially, this dwarf planet was named 2003 UB313. Later it was nicknamed Xena (a temporary name while awaiting IAU's decision) which became quite popular as well. However, the dwarf planet was officially named Eris on September 16, 2006 by the IAU members. The name Eris comes from Greek mythology, wherein, Eris happens to be the Goddess of warfare and strife. According to the mythology, Eris stirred up hatred, jealousy and anger among men and caused them to fight each other. Since this dwarf planet had stirred up a lot of controversies regarding its classification, the IUA members decided that the name Eris suited this object perfectly.
Eris' diameter is 2400 kilometers, which happens to be slightly larger than Pluto at 2320 kilometers. It is approximately 97 AU (Astronomical Units) away from the Sun, and is three times farther than Pluto, taking twice as much time (577 years) to orbit the Sun, as compared to Pluto. Eris has an unusually eccentric orbit. At its closest, Eris is only 38 AU from the Sun, which means that at times, Eris is closer to us than Pluto. Eris encounters temperatures ranging from -217º Celsius to -243º Celsius.
With the help of several spectral data, scientists today presume that Eris is covered by an outer layer of frozen methane which has actually seeped its way from the interior regions. In Pluto's case, it has been observed that methane on its surface undergoes chemical transformations, which causes the methane to redden. The faint solar radiations reaching this planet from the Sun can be the reason behind this. However, since Eris is farther than Pluto, the solar radiations reaching Eris are even fainter, which is why Eris' surface is more yellowish than the reddish-yellow surface of Pluto.
Currently Eris has one known moon which orbits it and is called Dysnomia. The name Dysnomia comes from the daughter of the Goddess Eris, who was known to be the cause of lawlessness. Dysnomia takes two weeks to orbit this dwarf planet. Unlike other planets like Mars or Jupiter, Eris cannot be observed by the naked eye or even through binoculars. To view Eris, one needs to have very powerful telescopes with plenty of amateur astronomical experience under his belt.
The discovery of Eris has been a landmark in astronomy, because it led to the reclassification of celestial objects and formation of a new category known as dwarf planets. From this, we can draw that the universe and our solar system is full of hidden secrets, and what we know today is only like a drop inside an ocean!