This lineup shows the 12 planets that were proposed last week, with a wedge of the sun at far left. Ceres, Pluto, Charon and 2003 UB313 are barely visible. Now Charon will continue to be considered Pluto's satellite, and the three other worlds will be dubbed "dwarf planets" rather than full-fledged planets. The planets are drawn to scale, but without correct relative distances.
View related photos
By Robert Roy Britt
Capping years of intense debate, astronomers resolved Thursday to demote Pluto in a wholesale redefinition of planethood that is being billed as a victory of scientific reasoning over historic and cultural influences. But the decision is already being hotly debated.
Officially, Pluto is no longer a planet.
"Pluto is dead," said Mike Brown, a planetary scientist at the California Institute of Technology 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," Brown said. "There are finally, officially, eight planets in the solar system."
The vote involved just 424 astronomers who remained for the last day of a meeting of the International Astronomical Union in Prague.
"I'm embarrassed for astronomy. Less than 5 percent of the world's astronomers voted," said Alan Stern, leader of NASA's New Horizons mission to Pluto and a scientist at the Southwest Research Institute.
"This definition stinks, for technical reasons," Stern told Space.com. He expects the astronomy community to overturn the decision. Other astronomers criticized the definition as ambiguous.
The resolution
The decision establishes three main categories of objects in our solar system.
Planets: The eight worlds starting with Mercury and moving out to Venus, Earth, Mars, Jupiter, Saturn, Uranus and 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 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. It would also have made planets out of the asteroid Ceres and Pluto's moon Charon. But not now.
"Ceres is a dwarf planet. it's the only dwarf planet in the asteroid belt," Brown said. "Charon is a satellite."
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.
'A farce'
Stern, in charge of the robotic probe on its way to Pluto, said the language of the resolution is flawed. It requires that a planet "has cleared the neighborhood around its orbit." But Earth, Mars, Jupiter and Neptune all have asteroids as neighbors.
"It's patently clear that Earth's zone is not cleared," Stern told Space.com. "Jupiter has 50,000 Trojan asteroids," which orbit in lockstep with the planet.
Stern called it "absurd" that only 424 astronomers were allowed to vote, out of about 10,000 professional astronomers around the globe.
"It won't stand," he said. "It's a farce."
Stern said astronomers are already circulating a petition that would try to overturn the IAU decision.
