"The eight planets are Mercury, Earth, Venus, Mars, Jupiter, Saturn, Uranus and Neptune," the International Astronomical Union (IAU) declared in a resolution made during their meeting in the Czech Republic capital of Prague.
The decision to downgrade Pluton to a dwarf planet was approved by raised hands after what, by the quiet traditions of the sky gazing community, was a stormy debate.
Pluto's status had been contested for many years by astronomers who said its tiny size and highly eccentric orbit precluded it from joining the other acknowledged planets.
Dwarf planet
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 2003 UB313 was as big as Pluto and therefore could lay claim to being a planet.
After spelling out the eight names in the Solar System's A-list of planets, the 2,500 IAU delegates assigned Pluto to a new category -- "dwarf planet".
The long and sometimes heated debate underlined a dilemma that arose several years ago with the emergence of more powerful telescopes and computer-assisted scanning of the heavens.
The telescopes began to suggest that Pluto, far from being a solitary wanderer on the outer fringes of the Solar System, was only one of dozens of scattered objects at this distance from the Sun.
This realisation in turn prompted questions as to whether Pluto and other largish objects could be considered planets or - simply and sadly - rocks.
The IAU defined the core difference between a planet and a dwarf planet as whether the celestial object has "cleared the neighbourhood around its orbit".
In other words, whether the object is massive enough to wield a gravity that draws in rocks and other debris that may clutter its orbital path.
While casting Pluto into, literally, the outer darkness, the IAU also tried to sugar the pill somewhat, saying Pluto was "an important prototype of a new class of trans-Neptunian objects."
Around a dozen other objects, such as 2003 UB313 and the large asteroid Ceres, are already candidates for the "dwarf" definition.
Three moons
British representative Michael Rowan-Robinson acknowledged that the rearrangement of the planetary hierarchy might cause deep confusion among the public.
And, he admitted, astronomers might be taken for "total idiots" because of what seemed to be an arcane debate. But, he said, it was vital to move in line with knowledge.
"The glass (of knowledge) is filling," he said. "In a few years, there may be 40 of those new dwarves. The fact that Pluto has been demoted is not so important."
Pluto was discovered on February 18, 1930 by a 24-year-old American astronomer, Clyde Tombaugh.
His announcement had an enormous impact. It smashed the perceived boundaries of the Solar System, established 84 years earlier with the discovery of Neptune.
Pluto is named after the god of the underworld in classical mythology.
It is an average distance of 5,906,380,000 kilometres from the Sun and takes 247.9 Earth years to complete an orbit.
Pluto has three moons: Charon, discovered in 1978, and the tiny Nix and Hydra, spotted in 2005 by US astronomers using the orbiting Hubble Telescope.
Charon has a diameter of around 1,212 kilometres, making it about half the size of Pluto's 2,300 kilometres.
Hydra and Nix measure between 48 kilometres and 165 kilometres across. There is not enough data yet to gauge their size exactly.
On January 19 this year, the United States launched an unmanned spacecraft, New Horizons, which is due to fly by Pluto and the Kuiper Belt in 2015.