The discovery of Pluto by careful telescopic observation in 1930 added a ninth planet to the solar system. Then in 1978 better technology revealed that what was seen in 1930 was not a single planet, but two smaller objects orbiting each other. The larger object retained the name Pluto, the smaller object was name Charon. This discovery meant that Pluto was significantly smaller than any other planet, and was actually smaller than some moons of other planets.
Beginning in 2000 further improvements in astronomical technologies led to the discovery of many more Pluto and Charon sized objects beyond Pluto. By 2005 eleven other objects were discovered in orbits beyond Pluto, an area of the solar system called the Kuiper belt. A debate broke out among astronomers. Should astronomy add eleven more planets to the official list of planets? The Kuiper belt contains thousands of small objects thought to be composed primarily of ices of water, nitrogen, methane, along with rocky minerals such as silicates, olivines, and pyroxenes. How big is big enough to be a planet? If the eleven new Kuiper belt objects were not planets, then neither was Pluto. Ultimately astronomers decided to refer to Pluto and the other Kuiper belt objects as planetoids, not planets. Thus the solar system has only eight planets starting with Mercury near the sun and ending with Neptune as the outermost planet.