I actually no clue that Pluto was in trouble or questioned of its size thus got kicked out from the 9 planets of the solar system until I read this at the Carnegie Science Center. originally I learned since grade school that there are 9 Planets in our Solar System namely; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto (which is now named one of the Dwarf Planet). So, what happened to Pluto?
#1. The object must be in orbit around the Sun, not another or other body. Pluto does this.
#2 To be considered a "planet", the object has to be big enough that gravity pulls in into a roundish shape - like an orange. There are a lot of things in the solar system, like asteroids or the moons of Mars, which have an odd shape, like a potato, because they don't have enough gravity to mold them otherwise. Pluto, like Earth, is mostly round.
#3 An object must be big enough that its gravity removes other objects in the same orbit. They would either be pulled into it, or knocked out of the same orbit. But Pluto orbits in an area filled with other small objects, and does not have enough gravity to clear the area of these other objects. In fact, not only Pluto not large enough to clear its own orbit, its orbital period is actually tied to that of its giant neighbor Neptune.
In 2006, the International Astronomical Union [the governing body of astronomers having the authority to label and name celestial objects] determined that Pluto should be 'demoted' to dwarf planet status - one of five such bodies known today.
Pluto doesn't seem to mind, and keeps orbiting without complaint.
Discovered in 1930, Pluto was originally classified as the ninth planet from the Sun. However, its status as a major planet fell into question following further study of it and the outer Solar System over the ensuing 75 years. Starting in 1977 with discovery of minor planet 2060 Chiron, numerous icy objects similar to Pluto with eccentric orbits were found. A number of scientists hold that Pluto should have remained classified as a planet, and that other dwarf planets should be added to the roster of planets along with Pluto
Pluto, minor-planet designation 134340 Pluto, is the largest object in the Kuiper belt, and the tenth-most-massive body observed directly orbiting the Sun. It is the second-most-massive known dwarf planet, after Eris. Pluto is composed primarily of rock and ice and is relatively small, approximately one-sixth the mass of the Moon and one-third its volume. In 2015, the Pluto system is due to be visited by spacecraft for the first time. The New Horizons probe will perform a flyby during which it will attempt to take detailed measurements and images of the plutoid and its moons. Read more about Pluto from article source:wikipedia.org/wiki/Pluto