Before, any form of extensive research was performed on the solar system we believed that, there are over nine planetary bodies, along with the satellites and the sun inside our solar system in a given sequence or a pattern. As, these celestial bodies are analyzed each time by the scientists, research workers, and astronomers. While, there are some standards set for each celestial body to be attributed as a planet, star, or any satellite etc. And for that reason, when the research workers analyzed the ninetieth and the far most planet from the sun which is also known as "Pluto", named after a Greek God.
We can say that its properties were far different from those that any planet had and thus it was analyzed for a very long time, as to know more about it and fortunately we were able to find out that it is completely one-of-a-kind the Kuiper Belt(of trans-Neptunian objects) inside the solar system.

And that is how the research workers decided that Pluto, which was once considered as planet is no more listed as one but it is a less planetary object present inside the Solar system.