After reclassification in 2005, pluto is no longer considered a planet because it

Pluto, it’s believed that it formed near the Solar System and was trapped by the Sun's gravity. Currently, because of its small size, it’s discovered that it invades the orbit of Neptune, and have an orbit inclined by 17 degrees, in difference with other planets: was evaluated by the International Astronomical Union, on August 24, 2006, and was changed to Dwarf Planet. The characteristics that Pluto complies: To have enough mass to have a spherical shape, and maintain an orbit around the sun, besides not being a satellite of any other body, and not having ''cleaned up'' the vicinity of its orbit. This did not agree with some astronomers, because the dwarf planets would not really be planets, being somewhat confusing.