Dwarf planet is a planet classification 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 other planet classifications are :-
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. Some people voted because they didn't like the number of planets to increase.
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. A large planet is most likely able to clear the path in front of it.
Our Solar System despite us living on it is nothing special, it doesn't stand out against the other stars in the galaxy or Universe for that fact. Other stars in the Universe will have Dwarf Planets in orbit round them. As we can't tell if they match the conditions for normal or dwarf planet, they are only called Exoplanets at the moment.
For a planet to be fully recognised as a planet, it had to pass all the following rules. If it breaks any of those rules then it is a Dwarf Planet. If it is not spherical then it is classed as an Sun orbiting asteroid.
The first eight planets up to and including Neptune passed each of the rules. The following planet descriptions have no effect on whether it is a planet, dwarf planet or asteroid.
There are five recognised Dwarf planets in the solar system at the time of writing. This can increase in the future when one is discovered or the rules have changed. There are some candidates such as Ixion and Quaoar but they are awaiting confirmation.
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 from asteroid to Dwarf Planet status.
Ever since Pluto was discovered by Clyde Tombaugh, it has 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 area that Pluto orbits in is the Kuiper Belt, a large area of space in which asteroids orbit the Sun. As it crosses their other object paths, it is deemed not to be a planet. Pluto's orbit is irregular and during its orbit, it will be inside the orbit of Neptune, its nearest largest neighbour.
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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