On February 18, 1930 Pluto was discovered at the Lowell Observatory in Flagstaff, Arizona, by astronomer Clyde W. Tombaugh.
The existence of an unknown ninth planet was first proposed by Percival Lowell in 1906, who theorized that wobbles in the orbits of Uranus and Neptune were caused by the gravitational pull of an unknown planetary body. Lowell calculated the approximate location of the hypothesized ninth planet and searched for more than a decade without success.
However, in 1929, using the calculations of Lowell and W.H. Pickering as a guide, the search for Pluto was resumed. On February 18, 1930, Tombaugh discovered the tiny, distant planet by the use of a new astronomic technique of photographic plates combined with a blink microscope.
In 2005, Mike Brown discovered Eris, an icy object thought to be about the same size as Pluto, out beyond its orbit. Were we then to have 10 planets? The International Astronomical Union met in 2006 to decide. What they did, however, was change the definition of a planet.
According to the new definition not only was Eris not a planet, Pluto was not a planet either. It was demoted to dwarf planet, along with Eris, Ceres, Haumea, and Makemake. The Solar System now has eight planets and five dwarf planets.