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 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.
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.
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.
Owen Gingerich, historian and astronomer emeritus at Harvard who led the committee that proposed the initial definition, called the new definition "confusing and unfortunate" and said he was "not at all pleased" with the language about clearing the neighborhood.
Gingerich also did not like the term "dwarf" planet.
"I thought that it made a curious linguistic contradiction," Gingerich said during a telephone interview from Boston (where he could not vote). "A dwarf planet is not a planet. I thought that was very awkward."
Gingerich added: "In the future, one would hope the IAU could do electronic balloting."
Years of debate
Astronomers have argued since the late 1990s 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 Thursday's vote.
Astronomers at the IAU meeting debated the proposals right up to the moment of the vote.
Caltech's Mike Brown loses out in one sense. The Pluto-sized object his team found, called 2003 UB313, will now be termed a dwarf planet.
"As of today I have no longer discovered a planet," he said. But 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."
Textbooks and classroom charts will, of course, have to be revised.
"For astronomers, this doesn't matter one bit. We'll go out and do exactly what we did," Brown said. "For teaching this is a very interesting moment. I think you can describe science much better now" by explaining why Pluto was once thought to be a planet and why it isn't now. "I'm actually very excited."