On January 23rd, 1930 Clyde Tombaugh, making a careful survey of the sky from the Lowell Observatory in Arizona, spotted a dim speck moving among the stars. It was quickly hailed as the 9th planet from the Sun.
Venetia Phair (née Burney), then an 11-yr old schoolgirl in Oxfordshire suggested the title Pluto for the newly discovered ninth planet.
Named after the Roman god of the Underworld [not the Disney dog who was named after the planet, rather than the other way round as some have mistakenly thought], on 1st May, 1930, the name Pluto was formally adopted.
As Dr. Tony Phillips and Amelia Phillips relate in Much Ado about Pluto, Pluto was a misfit from the beginning, but its planethood was never seriously questioned until 1992.
Pluto's neighbourhood is cluttered with icy bodies about the size of asteroids. They orbit the sun in a busy belt beyond the orbit of Neptune. Gerard Kuiper in 1951 had predicted such a belt to explain where certain comets came from, and today it is called The Kuiper Belt. Its discovery (with Pluto inside it) troubled some astronomers. "Is Pluto a planet?" they asked. "Or is it just another Kuiper Belt Object (KBO)?"
Then on 29th July 2005 it was reported that a 10th planet had been discovered.
The planet was found by Dr. Mike Brown and colleagues using the Samuel Oschin Telescope at Palomar Observatory near San Diego. Definitely bigger than Pluto, the new planet, officially known as 2003 UB313 and unofficially called Xena was placed more or less in the Kuiper Belt.
If Pluto is a planet, it was argued, then 2003 UB313, being larger than Pluto, must be a planet too. However, there could be dozens of worlds larger than Pluto hiding in dark recesses of the Kuiper Belt. Are they all planets?
To decide the question once and for all, the International Astronomical Union (IAU) formed a "Planet Definition Committee" consisting of historians, science writers and professional astronomers. Their job was to craft an official definition of the term planet, which all astronomers could agree on and use. The Committee met, argued and debated, and finally settled on a definition, which they presented to the IAU General Assembly in August 2006.
If their first draft definition of a planet had been accepted then Pluto would have been a planet, as would Ceres (a giant asteroid orbiting between Mars & Jupiter), 2003 UB313 (Xena) and Pluto's companion Charon. If other Kuiper Belt Objects such as Pallas, Vesta, and/or Hygeia were found to be in hydrostatic equilibrium, they would also be planets.
However a modified definition became the The Final IAU Resolution
Under this the eight planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, are defined as Classical Planets
Pluto and the other aforementioned bodies are defined as Dwarf Planets or Plutonian objects
Not everyone agrees of course and the Phillips have established a web page where anyone can vote on the matter at PlutoPetition.com