A dwarf planet is a classification of planet that arose out of the 2006 International Astronomical Union (I.A.U.) meeting. During the meeting, it was discussed what is a planet as there was not a defined description of a planet.
The discovery of the kuiper belt object Eris had prompted the discussion. Eris was believed to be bigger than Pluto which at that time was the ninth planet in the Solar System. A number of requirement was drawn up as to what defines a planet. At the end of the meeting, three rules had been created and for a planet to be called a planet, it had to satisfy a number of rules.
The vote came at the end of the meeting when a number of scientists had already left. A lot of people disagree with the decision but there is little appetite from the I.A.U. to change it. The most controversial one is the neighbourhood rule.
The neighbourhood rule could go and be replaced with say one that requires an atmosphere. A dwarf planet is not based on size even though the name implies. All the dwarf planets just do happen to be small. The larger a planet is, the more influence it would have on the neighbourhood.
For a planet to be fully recognised as a planet, it had to follow the following rules.
The first eight planets up to and including Neptune passed each of the rules.
Ceres was discovered in the 19th century and was immediately recognised as a planet. It was then later downgraded to that of an asteroid. It is the only spherical asteroid in the asteroid belt between Mars and Jupiter. After the IAU meeting, the planet was promoted again.
Ever since Pluto was discovered by Clyde Tombaugh, its been referred to as a planet. When the rules for the planets were drawn up, it failed the third rule which is not having cleared its neighbourhood around its neighbourhood. The planet was demoted to Dwarf Planet status. The downgrading was highly controversial amongst many scientists.
The New Horizons space probe had already taken off and was heading towards the planet when news came in that the planet was downgraded. Professor Alan Stern, lead scientist on the New Horizons project was not pleased with the downgrading. EarthSky
The dwarf planet that started it was discovered in 2003 by Professor Mike Brown and others. Mike relishes in the fact that it was his discovery that caused Pluto to be downgraded. Mike's twitter account is PlutoKiller. Eris has a highly elliptical orbit, one takes it out as far as 97 A.U., that's 97 times the distance between Earth and the Sun and back into 38 A.U.
The final two objects that make up the dwarf planets both reside in the Kuiper Belt. Oddly enough, Haumea isn't a spheroid but more a smooth egg but its been let into the dwarf planet group. Haumea is roughly about the same size as Pluto but its shape is highly irregular. N.A.S.A
A sub-section of dwarf planets was created by the I.A.U. in August 2006. All the dwarf planets except Ceres are member of this sub-group. For them to be a member of the group, they must be at the end of the Solar System, orbiting out farther than Neptune. I.A.U.
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