What happened? The story begins 76 years ago:

In those days astronomers thought something was wrong with the orbits of Uranus and Neptune. The two gas giants didn't go around the sun as circularly as they should, which meant another planet must be out there, disrupting their orbits. To find the troublemaker, 24-year old Clyde Tombaugh made a careful survey of the sky from the Lowell Observatory in Arizona.

On Jan. 23, 1930, he spotted Pluto, a dim speck moving among the stars. It was quickly hailed as the 9th planet from the Sun--not to mention the first planet ever discovered by an American. In the USA, patriotic feelings ran high.

Something didn't add up, however. Pluto was too small to disturb the orbits of mighty Uranus and Neptune. Years later astronomers realized there was nothing wrong with the orbits of Uranus and Neptune. It was all a big mistake, but a lucky one for Tombaugh.

Tombaugh considered many names for the new planet. His favorite ("Pluto") was suggested by 11-year old Venetia Burney of Oxford, England. Still living in England today as a retired teacher, Venetia denies that she named Pluto after Mickey Mouse's dog, which also debuted in 1930. "It has now been satisfactorily proven that the dog was named after the planet, rather than the other way round," she says: full story.

From the beginning, astronomers knew Pluto was a misfit:

First, Pluto is tiny. It is six times smaller than Earth, and even smaller than seven of the solar system's moons: the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton.

Second, Pluto has a weird orbit. While most planets go around the sun in near-circles, Pluto's orbit looks more like an egg. It actually crosses inside the orbit of Neptune, making Pluto the 8th planet when it is not the 9th. Furthermore, Pluto's orbit is tilted by 17 degrees with respect to the plane of the solar system.

Despite these oddities, Pluto's planethood was never seriously questioned until 1992. That's when astronomers starting finding other things "out there." Pluto's neighborhood, it turns out, is cluttered with icy bodies about the size of asteroids. They orbit the sun in a busy belt, a bit like the asteroid belt, all beyond the orbit of Neptune. Back in 1951, the great astronomer Gerard Kuiper predicted such a belt to explain where certain comets came from, and today it bears his name: the Kuiper Belt.

The discovery of the Kuiper Belt (with Pluto inside it) troubled some astronomers. "Is Pluto a planet?" they asked. "Or is it just another Kuiper Belt Object (KBO)?" Other astronomers insisted Pluto was both—a planet and a KBO. So the arguments began…

The real trouble started in 2005 when Caltech astronomer Mike Brown and colleagues found something in the Kuiper Belt even larger than Pluto. Its name is 2003 UB313. (Brown calls it "Xena.") If Pluto is a planet, they reasoned, then 2003 UB313, being larger than Pluto, must be a planet, too.

That sounds perfectly reasonable, except for one thing: There could be dozens of worlds larger than Pluto hiding in dark recesses of the Kuiper Belt. Are they all planets? Does the Solar System really need dozens of planets?

Some astronomers said "Why not?" The more the merrier. With each new discovery, the Solar System becomes a livelier place, with new planets to chart and study. What better way to rouse the interest of young scientists and explorers? Other astronomers disagreed. They felt that mixing KBOs with real planets would be unscientific and confusing--like mixing apples and oranges.

Who is right?

To decide the question once and for all, the International Astronomical Union (IAU) formed a "Planet Definition Committee" consisting of historians, science writers and professional astronomers. Their job was to craft an official definition of planet, which all astronomers could agree on and use.

The Committee met, argued and debated, and finally settled on a definition, which they presented to the IAU General Assembly on August 16, 2006:

Translation: If it is round and it orbits the sun, then it is a planet.

If this simple definition had been accepted, then Pluto would have been a planet, as would Ceres (a giant asteroid the size of Texas) and 2003 UB313. The total number of planets in the solar system would have been twelve: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto and Charon (a double planet), and 2003 UB313.

But it was rejected. Astronomers at the General Assembly voted against it.

However, no one wanted to end the meeting without figuring out what a planet was. So the astronomers continued their debate for another six days. On August 24, 2006, a modified definition was proposed:

Translation: To be a planet, a world must have enough mass and gravity to gather itself into a ball; it must orbit the sun; and it must reign supreme in its own orbit, having "cleared the neighborhood" of other competing bodies.

This definition was approved and, in a stroke, Pluto was no longer a planet. According to the IAU, the Solar System now has eight and only eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

Why not Pluto? It has not "cleared the neighborhood around its orbit." A planet does this using gravity to gobble up or fling aside competing bodies. Pluto's gravity is too weak to clear out its part of the Kuiper Belt. The neighborhood is a mess and, thus, Pluto is not a planet. (Note: Ceres is not a planet for the same reason. It has failed to clear out its neighborhood, the asteroid belt.)

So, the trouble with Pluto is finally over. Right?

Wrong. Many people, including many astronomers and other scientists, do not accept the IAU's decision.

For one thing, the IAU's definition of planet is not entirely self-consistent. Consider the following: Pluto itself crosses the orbit of Neptune, which means Neptune has failed to clear its own neighborhood. Is Neptune, therefore, not a planet? (Taken to extremes, this "logic" would add up to zero planets in the solar system, because no planet has completely cleared its neighborhood of asteroids and other debris--just ask any dinosaur.)

Astronomers aren't the only ones with an interest in this: The word "planet" is thousands of years old. People use the word on a daily basis and know what it means. It's Plain Language.

The IAU, meanwhile, has been around for less than a century. It was founded in 1919 and now, in 2006, has a membership of about 9,000. Less than 4% of the IAU's members actually participated in the vote to demote Pluto.

Perhaps a larger and more varied group of people should be allowed to decide. To that end, we have established a web page where anyone can vote:

PlutoPetition.com

When one million votes (or some similarly-impressive number) have been collected, we plan to present the tally—pro vs. con—to the IAU, and ask the General Assembly to consider the results.