After a heated debate, 2,500 scientists and astronomers voted at the International Astronomical Union (IAU) on Thursday that Pluto, which has been called a planet since being discovered in 1930, would be put into a category called "dwarf planets".
The IAU resolution said: "The eight planets are Mercury, Earth, Venus, Mars, Jupiter, Saturn, Uranus and Neptune."
Pluto's status had been contested for many years by astronomers who said that its tiny size and highly eccentric orbit precluded it from joining the other acknowledged planets.
The need to define what it takes to be a planet stems from technological advances that enable astronomers to look farther into space and to measure more precisely the size of celestial bodies in our solar system.
The scientists agreed that to be called a planet, a celestial body must be in orbit around a star while not itself being a star.
It must also have sufficient mass for its own gravity to pull it into a nearly spherical shape and have cleared the neighbourhood around its orbit.
Pluto was disqualified because its orbit overlaps that of Neptune.
In addition to the categories of "planet" and "dwarf planet", the definition creates a third category to encompass all other objects, except satellites, to be known as small solar system bodies.
Discovery of Xena
The anti-Pluto movement gained ground after the discovery of a distant object beyond Pluto's orbit called 2003 UB313, also known unofficially as Xena.
Its discoverer said UB313 was as big as Pluto and thus could lay claim to being a planet.
Pluto was discovered on February 18, 1930, by an American astronomer, Clyde Tombaugh, then aged 24.
Named after the god of the underworld in classical mythology, it orbits the Sun at an average distance of 5,906,380,000 kilometres, taking 247.9 Earth years to complete a single circuit.
An unmanned US spacecraft, New Horizons, is due to fly by Pluto and the Kuiper Belt in 2015.