What is the '10th planet'?

The farther the heavenly objects are from us, the more difficult it is for us to say what they exactly are. Pluto can hardly be seen from the Earth. It was discovered in 1930 by pure chance, when the astronomers were trying to calculate the orbits of Neptune and Uranus. Something was disturbing the orbits, which had to be heavy, such as a planet. Thereafter, a careful survey of the sky found a heavenly body this was Pluto. But astronomers believed that Pluto was too small to be disturbing the orbit of Neptune, and thus the search for the 10th planet began. In 2003, a still farther object Sedna was discovered: Was this the 10th planet? Since Sedna and Pluto were too small to be clearly classified as planets, they were declared as 'dwarf planets' in 2006.