After heated debate among members of a 2500-strong meeting of the International Astronomical Union (IAU) in Prague, Pluto has been redefined as a 'dwarf planet'.
It is now considered a distinct object from the other eight traditional planets of the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
Australian astronomer Dr Simon Johnston, from the Parkes Observatory in New South Wales is shocked at the decision.
"Up to a week ago I thought we had 12 planets," he says, referring to a definition released by the IAU last week that would have expanded rather than reduced the number of planets.
The overseer of science investigations on NASA's New Horizons mission to Pluto calls the reclassification rash and illogical.
"I think people are going to consider Pluto a planet regardless," says Dr Alan Stern of the Southwest Research Institute in Boulder, Colorado.
"What they did was scientifically ludicrous and publicly embarrassing," he says. "Pluto is a planet with clouds, weather and geology."
But other astronomers are more philosophical, including Dr Mike Brown of the California Institute of Technology.
"Pluto is dead," says Brown, who in 2003 discovered an object larger than Pluto that many claimed was the 10th planet
Brown now accepts the icy rock, called 2003 UB313 and nicknamed Xena, will also be categorised as a mere dwarf planet under the new definition.
"Pluto would never be considered a planet if it were discovered today, and I think the fact that we've now found one Kuiper-belt object bigger than Pluto underscores its shaky status," says Brown.
Several Johns Hopkins University astronomers describe the decision as a "muddled" ruling that is unlikely to settle ongoing debates over how to define a planet and whether the term should apply to Pluto.
In an informal poll, only one astronomer was pleased to hear about Pluto's new status.
But at least one astronomer appears indifferent to the decision.
"The classification doesn't matter," says Dr Louis Friedman, executive director of The Planetary Society. "Pluto, and all solar system objects, are mysterious and exciting new worlds that need to be explored and better understood."
Pluto fails the test
Pluto's status has been contested for many years by astronomers who say its tiny size and highly eccentric orbit precludes it from joining the other acknowledged planets.
This discovery of Xena prompted questions as to whether Pluto and other largish objects could be considered planets or just simply rocks – rubble left over from the formation of the solar system.
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.
This criterion disqualified Pluto whose egg-shaped orbit overlaps Neptune's.
The large asteroid Ceres, along with Pluto and Xena, will also be defined as a dwarf planet. This is, according to the IAU, an important new class of trans-Neptunian objects.
Around a dozen other objects are already dwarf candidates and all other objects, except satellites, will be known as "small solar system bodies" under the new definition.
Hate mail from angry children
Pluto was discovered in 1930 by a 24-year-old American astronomer, Clyde Tombaugh who died in January 1997, convinced he had achieved eternal recognition as the discoverer of the solar system's ninth planet.
Tombaugh's announcement smashed the perceived boundaries of the solar system, established 84 years earlier with the discovery of Neptune.
This week's Prague vote will have similarly widespread public implications with models and textbooks of the solar system becoming instantly obsolete.
Officials at the American Museum of Natural History in New York, who have been at the vanguard of the movement to demote Pluto, say they have already received irate calls from the public.
"We had enormous numbers of telephone calls and I would say things that verged on hate mail from second-graders - very angry children who said, 'What have you done? This is the cutest, most Disney-esque of the planets. How could you possibly demote it?'" says Dr Michael Shara, the museum's astrophysics curator.
"It's going to be a difficult thing to accept at first, but we will accept it eventually, and that's the right scientific and cultural thing to do," says Brown.
But at least one young visitor to the Smithsonian Institution's National Air and Space Museum, 15-year-old Quinn Huebner, is happy.
"It's one less planet to memorise," he says.