1) Pluto,a dwarf planet as defined by the International Astronomical Union (IAU), is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but which has not cleared its neighbouring region of planetesimals and is not a satellite. Asteroids are rocky and metallic objects that orbit the Sun but are too small to be considered planets. They are known as minor planets.
2) In 2006 the International Astronomical Union (IAU) stripped Pluto of its planet-hood, demoting it to dwarf planet status.
3) Earth is a planet because it has all the requirements for a planet.The gravity,The Atmosphere,The apt distance form a energy source(sun )and All good chemical composition.