From 1930 to 2006, Pluto was considered to be the ninth planet of our solar system. In 2006 it was reclassified as a dwarf planet, though many still consider it the ninth planet for historical reasons. One reason that it lost its planetary status is because it is so small. It has less than one percent of Earth's mass. There are numerous other bodies in that part of the solar system (called the Kuiper belt) with similar mass so the decision was made to make the classification of dwarf planet.
Another reason that Pluto lost its planetary status is because its largest moon, Charon, is not that much smaller than Pluto itself. As a result, many consider the Pluto-Charon system to be binary system rather than a classic planet-moon type system, though it is not technically classified as such.
ON 24THAugust 2007 october about 3000 astrnomers forming the International Astronomical Union ( IAU )met in Prague in the Czech Republic to consider why pluto should be regarded as a planet or not.
It was agreed at the meeting that for a heavenly boody to qualify as a planet,it must have the following features:
- It must be in an orbit around a star and not be the star itself.
- It must be massive enough to be a sphere by it's gravitational force.
- It must have cleared the neighbourhood around it's orbit.
They excluded Pluto because pluto failed to meet all these features.So it was considered as a ''TRANS-NEPTUNIAN Object''or minor planet,showing we have 8 planets in the solar system.
Basically everyone used to count Pluto as a planet and scientists were happy having 9 planets. That was until they found many other forms in our Solar System which were relatively the same size as Pluto- other dwaf planets. And these scientests/astronomers were like 'we can't declare all of them as planets, we'd have too many!' because they knew that they'd find possibly hundreds more. So everyone demaded for a new definition of these dwarf planets (that's how they got their name).
So they declared Pluto as a dwarf planet but not an actual planet and here we are today with 8 planets and several if not 100+ dwarf planets in out Solar System