Last planet of the Solar System - Pluto, it was found by the astronomer Tombo on February 18, 1930. Strictly speaking, Pluto cannot be considered as the planet any more, in 2006 the decision to rank Pluto as dwarf planets, such as largest asteroid Ceres or the satellite of Pluto Charon was made.
The reason decisions on reckoning of Pluto to dwarf planets became made on the same assembly of 2006 criteria by which belonging of a space body to a class of planets is determined. One of them is that other object cannot cross an orbit of the planet, and the orbit of Pluto is crossed by the Neptune.
Dwarf planets
Pluto belongs to number of those planets which existence at first was confirmed by means of calculations and only then it was recorded by the telescope. For determination of the sizes of far-out planets and distance to them Kepler and Newton's laws are used. By means of Kepler's laws it is proved that orbits of planets have no form of the correct circle. Newton's laws define interaction of two planets on the basis of their weight and their distance from each other. The more the mass of planets, the stronger they are attracted, the it is less than a distance between them, the attraction force more operating on them. On the basis of these laws the scientists calculated an estimated orbit of the movement of Uranium which then was considered as the last planet of the solar system, however observations of its movement revealed that its real orbit will not coincide with settlement. Then some scientists expressed opinion that behind Uranium there is a planet which is not discovered still which the attraction influences the Uranium orbit. The Neptune who was found by the Berlin observatory was this planet.
However the Neptune's attraction not up to the end explained strangenesses in the movement of Uranium. In 1915 American Percival Lowell made a hypothesis that for the Neptune there is one more unknown planet which also influences the Uranium orbit, and specified in what site of the sky it is necessary to look for it, in 15 years, in 1930 the new planet was found by means of studying photos of the star sky, in that area of the sky specified by Lowell.