Pluto was no longer classified as a planet in solar system since 2006. But why this happens? There are serveral reasons for this.

First of all, Pluto is so small that it is so much smaller than any other planets in the solar system. Since it is so small and far away from the Earth, even the Hubble Space Telescope can't see much details on it, except the largest feature on its surface. We basically just know it has a moon called "Charon", which is about half the size of Pluto. Compared to other planets, the moons on the other planets are much smaller. The relatively large size of "Charon" makes Pluto unable to dominate its neighborhood, and so by the defintion of planet, Pluto should be kicked out.

Second of all, there are many other objects that have similar size with pluto nearby. Now all of these objects including pluto is classified as dwarf planet. We now currently have 44 dwarf planets discovered including Pluto. Imagine that if all of these are called planets, and it is possible that we will get more those small size objects discovered, what will happen? We will all get confused, and that is why we need a defintion for planet, which is  an object that orbits the sun and is large enough to have become round due to the force of its own gravity and is able to dominate its neighborhood.