Capping years of intense debate, astronomers resolved today to demote Pluto in a wholesale redefinition of planethood that is a victory of scientific reasoning over historic and cultural influences.
Pluto is no longer a planet.
"Pluto is dead," said Caltech researcher Mike Brown, who spoke with reporters via a teleconference while monitoring the vote. The decision also means a Pluto-sized object that Brown discovered will not be called a planet. "Pluto is not a planet. There are finally, officially, eight planets in the solar system."
The decision establishes three main categories of objects in our solar system:
Planets: The eight worlds from Mercury to Neptune.
Dwarf Planets: Pluto and any other round object that "has not cleared the neighborhood around its orbit, and is not a satellite."
Small Solar System Bodies: All other objects orbiting the Sun.
Pluto and its moon Charon, which would both have been planets under the initial definition proposed Aug. 16, now get demoted because they are part of a sea of other objects that occupy the same region of space. Earth and the other eight large planets have, on the other hand, cleared broad swaths of space of any other large objects.
"Pluto is a dwarf planet by the ... definition and is recognized as the prototype of a new category of trans-Neptunian objects," states the approved resolution.
Dwarf planets are not planets under the definition, however.
"There will be hundreds of dwarf planets," Brown predicted. He has already found dozens that fit the category.
Contentious logic
The vote by members of the International Astronomical Union (IAU) came after eight days of contentious debate that involved four separate proposals at the group's meeting in Prague.
The initial proposal, hammered out by a group of seven astronomers, historians and authors, attempted to preserve Pluto as a planet but was widely criticized for diluting the meaning of the word.
The category of "dwarf planet" is expected to include dozens of round objects already discovered beyond Neptune. Ultimately, hundreds will probably be found, astronomers say.
The word "planet" originally described wanderers of the sky that moved against the relatively fixed background of star. Pluto, discovered in 1930, was at first thought to be larger than it is. It has an eccentric orbit that crosses the path of Neptune and also takes it well above and below the main plane of the solar system.
Recent discoveries of other round, icy object in Pluto's realm have led most astronomers to agree that the diminutive world should never have been termed a planet.
Astronomers have argued since the late 1990s, however, on whether to demote Pluto. Public support for Pluto has weighed heavily on the debate. Today's vote comes after a two-year effort by the IAU to develop a definition. An initial committee of astronomers failed for a year to do so, leading to the formation of the second committee whose proposed definition was then redefined for today's vote.
Astronomers at the IAU meeting debated the proposals right up to the moment of the vote. Brown called the result scientifically a good decision.
"The public is not going to be excited by the fact that Pluto has been kicked out," Brown said. "But it's the right thing to do."
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