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.

"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.

PLUTO IN PERSPECTIVE

Nightly News/NASA
Even though Pluto may have lost its official status as a planet, the frozen world and its kin are still worthy of study, says Louis Friedman, executive director of the nonprofit Planetary Society. “Anytime we visit a new world — planet, moon, asteroid, comet, whatever — we make exciting and surprising new discoveries about the evolution of our solar system and about our own planet,” he said.

NASA's New Horizons probe is in the midst of a 9½-year journey to study Pluto and other icy mini-worlds, and NASA's Paul Hertz said Thursday's decision would have no effect on the mission. “We will continue pursuing exploration of the most scientifically interesting objects in the solar system, regardless of how they are categorized,” he said.

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."

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA!!!! That sucks nuts.

That doesn't really make sense. What are they going to do, recall all of the books and articles mentioning Pluto and change them? I'd say leave it as it is, and just... leave it. Demoting it is more trouble than it's worth. :/

This doesn't make any sense. Though it is incredibly small, it acts just like a planet. The whole dwarf planet thing is kind of stupid, as well. Meh, I have no control over it.

Heard about this whole thing awhile ago. If the rest of their plan goes well, we'll have like 53 new planets by scientific definition. Pluto is and will be a planet.

Eh. The whole issue's pointless.

Then again, according to half the online quizzes I've taken, I'm emo so everything is supposed to be pointless for me. :P

Scientists are sometimes the biggest idiots on the planet... it's funny how this kind of thing turns out.

Do recall the naming of the newest planet: Xena, and its moon, Gabrielle. >.< I found it stupid that a satellite the same size (or maybe a tiny bit bigger...) would be given the rights of planethood, while Pluto is thrown out again. x.x

Ahahahahahahhahahaha, you're kidding me, right? How ironic. ^_^

I used to be completely in love in Astronomy when I was younger. Still find it interesting, though.

I will still always consider Pluto a planet. I don't see why it shouldn't be anymore. It's ridiculous.

Sara, I'm being completely serious. :o When I heard it, I thought it was a joke, too... but it isn't. >.<''

I completely agree with Pluto not being a planet. The reasons are completely valid. Pluto doesn't act any more like a planet than many of the rest of the Trans-Neptunian Objects. Are we supposed to let a mere 76 years of culture and supposition get in the way of scientific exploration and proper classification?

"Xena" was only ever a nickname for 2003 UB313, another Trans-Neptunian object. It is larger than pluto and has more of a right to planethood than it. From the first time I started following this issue, maybe a couple months ago, I thought, "if we have to give up Pluto, do it. Fine."

Think about it. Pluto doesn't follow the pattern. We have four solid planets, an asteroid belt, four gas giants, and a rock. One of a sea of rocks.

EDIT: I are smart. I messed up my math. I corrected it.

Truly? I thought it was a declared planet. :o

I do understand your point, but once a planet, always a planet, in my opinion. We can't say that George Washington was never a president, and so we shouldn't be able to revoke a planet's title, either. :/

Well, we haven't redefined "President" yet, but apparently we've done just that for planets.

I guess I don't really care too much right now... I'm going to college and getting nervous and tense...

Pluto as a planet or Pluto as a rock, I don't too much care.

It's not like we really care... It's a purpleish rock. Faaaaaaaaaaaar Away.

Its all a money game in reality. Think about it. Who has the ability to gain more money from the reprinting of countless books which label Pluto as a planet? No one in the scientific community I'm sure.

Textbooks, encyclopedias and dictionaries are all reprinted eventually anyway, and anything else can just be passed off as 20th century culture.

Verbal warning. Cut it out dude.

Yes, books repronted. But not everyone wil buy the new versions. Schools and such will need to buy new books to fix the innaccuracy. Money money money.

Well, if teachers aren't as stupid as a fucking rock they'll add that in during a lesson. Simple as that.

Its not up to the teachers to buy books. Its the school board. You're going to inevitably have dambass mothers in the PTA whine about how its not the right info. Then of course, the school board will give in pending some legal action, and the schools buy new books.