For the first time in 2006, astronomers specifically defined a planet and the oddball Pluto just did not fit into that definition. There was always something mysterious about it, which set it apart from other planets. It was much smaller, made of solid ice instead of rock or gas, which with an orbit that was highly elliptical and highly inclined at 17⁰ compared to the orbits of other planets. (See the diagram below.)
A number of questions arose to which the astronomers had no satisfactory answers. If the 2,900 kilometers wide Pluto formed along with the other planets, why did it have such a different orbit? If it did not form with the others, where did it come from? Was it an alien body captured by the Sun’s gravitation? If so, could there be other similar bodies with orbits unlike those of the planets?
A few years ago, such questions gained importance. Astronomers discovered some icy bodies orbiting the Sun beyond Neptune. One of them was found to be larger than Pluto itself. Therefore, if Pluto is considered a planet, should not some of these newly-found bodies be accorded the same status? In that case, our solar system might actually have two or there dozen planets instead of just nine.
Bracketing these comparatively small bodies with heavyweights like Earth, Jupiter, Saturn and Uranus seemed to be an illogical mix-up. The only way out was to change Pluto’s status to something other than a planet. So, in 2006, the International Astronomical Union dethroned Pluto and classified it as a dwarf planet.
What is a dwarf planet?
A dwarf planet is defined in terms of four parameters. A dwarf planet is a body that 1) orbits the Sun, 2) has pulled itself into a round shape, 3) is not a satellite of another body and 4) which has not cleared the rest of its neighborhood of celestial debris with its gravitational force.
On the basis of these parameters, Pluto was shown the door, while the largest asteroid Ceres was elevated to the position of dwarf planet.
Dwarf planet (Wikipedia)