1 Technically, there was never a scientific definition of the term planet before 2006. When the Greeks observed the sky thousands of years ago, they discovered objects that acted differently than stars. These points of light seemed to wander around the sky throughout the year. We get the term planet from the Greek word planētēs, meaning "wanderer."

2 In the 1600s, scientists began to use telescopes to view our solar system. As technology got better, scientists discovered more planets orbiting our Sun, such as Uranus in 1781, Neptune in 1846, and Pluto in 1930. Then, in 1991, advances in telescope technology enabled scientists to discover many more objects in a disk-shaped cloud beyond Pluto called the Kuiper (KYE per) Belt. These objects were classified as Kuiper Belt Objects (KBOs), partly because they are smaller than Pluto. This classification of KBO seemed to work fine as long as objects weren't bigger than Pluto.

3 It was the recent discovery of an object larger than Pluto within the Kuiper Belt that changed everything. Is this object, now named Eris, our 10th planet, since it is larger than Pluto? This discovery and the naming of this new object prompted the International Astronomical Union (IAU) to discuss a scientific definition for the term planet. What if Eris is given planet status? Then our solar system could grow to dozens of planets as more and more Kuiper Belt Objects are discovered. Try remembering all those planet names. But if Eris is not a planet, then is Pluto still a planet?

4 Astronomers of the IAU voted on and passed the first scientific definition of a planet in August 2006. According to this new definition, an object must meet three criteria in order to be classified as a planet. First, it must orbit the Sun. Second, it must be big enough for gravity to squash it into a round ball. And third, it must have cleared other objects out of the way in its orbital neighborhood.

5 To clear an orbit, a planet must be big enough to pull neighboring objects into the planet itself or sling-shot them around the planet and shoot them off into outer space. According to the IAU, Pluto does not meet this third requirement but is now in a new class of objects called "dwarf planets." It is this third part of the definition that has sparked debate.

6 The problem for Pluto is the fact that its orbit is in the Kuiper Belt along with 43 other known KBOs. There are possibly billions of objects in the Kuiper Belt that have not been cataloged yet. Scientists have even found 8 KBOs between Neptune and Pluto. Some scientists view the new definition as unclear. Exactly how much does Pluto have to "clear" from its neighborhood to be considered a planet? And how much has Pluto already influenced its own neighborhood since the planet formed? These and other questions have been raised in response to the IAU's definition of a planet.

7 Consider this: Pluto crosses into Neptune's orbit, but Neptune is still classified as a planet. This is because of the orbits of Pluto and Neptune and the fact that they never get closer to each other than 17 AU. (1 AU = distance from Earth to the Sun.) Pluto may cross orbits with many other Kuiper Belt Objects, but how close do these objects get to Pluto? How close do objects have to get to Pluto to be considered "in" Pluto's neighborhood?

8 NASA's New Horizons spacecraft is speeding toward the edge of the solar system on its mission to Pluto. Launched in January 2006, it will not be until July 2015 that we will reach Pluto. It will swing past Jupiter for a gravity boost and scientific studies in February 2007, and reach Pluto and its moon, Charon, in July 2015. Then, as part of an extended mission, the spacecraft will head deeper into the Kuiper Belt to study one or more of the icy mini-worlds in that vast region, at least a billion miles beyond Neptune's orbit. Sending a spacecraft on this long journey will help us answer basic questions about the surface properties, geology, interior makeup, and atmospheres on these bodies.

Paragraphs 1-5 Summary: These paragraphs provide background information explaining:

• In 2005, we had 9 planets. By 2007, we only had 8.
• The IAU decided Pluto was a dwarf planet in August 2006.
• Many agreed with the change, but some fought to keep Pluto as a planet.
• Before 2006, there was no official definition—if it was round and orbited the sun, it was a planet.
• The discovery of Eris and other Kuiper Belt Objects created a dilemma: if Pluto stays a planet, we might have 50+ planets!

6 During the 2000s the IAU worked on developing a definition of a planet that its members could vote on. After years of work, on August 24, 2006, the IAU voted on and approved a definition. According to the definition, a planet must (1) orbit the sun, (2) be massive enough for its own gravity to have pulled it into a sphere, and (3) have cleared other objects from its orbital neighborhood. Pluto fails to meet the third criterion: It shares its neighborhood with far too many chunks of rock and ice. So, Pluto isn't a planet—it's a dwarf planet.

7 Right after the vote, many astronomers objected to the new definition. Alan Stern, head of the New Horizons mission to Pluto, called it "awful" and "sloppy science." The website Newscientist.com declared, "It appears likely that the definition will not be widely adopted by astronomers for everyday use." Within days of the vote, 304 astronomers had signed a petition protesting the IAU's definition. Those same astronomers also declared that the voting process had been deeply flawed.

8 The 304 astronomers who protested the definition focused on the third criterion—that a planet must have "cleared the neighborhood around its orbit." But what does "clear the neighborhood" mean? How big is the neighborhood? How clear must a planet make its orbit? Earth, Mars, Jupiter, and Neptune all share their orbits with thousands of asteroids, but no one claims they aren't planets. Also, asked protesters, why does it matter that Pluto's neighborhood is full of smaller objects? If a body goes around the sun and is round, why isn't that enough? As Alan Stern put it, planets "are what they are, independent of what they orbit near."

9 But the definition's supporters argue that the third criterion is accurate enough. True, Earth, Mars, Jupiter, and Neptune share their orbits with asteroids. But those planets have cleared their orbits far more successfully than Pluto has. What's more, they say, the orbits of Pluto and the similar Kuiper Belt Objects have more in common with each other than with the orbits of the eight planets. It just makes sense to split the solar system into two groups. Astronomer Mike Brown, who discovered many Kuiper Belt Objects, put it this way: "The precise definition . . . may be a tad unclear, but the concept is absolutely rock solid with absolutely no room for doubt about which objects do and do not belong."

10 Critics of the IAU's decision had another complaint: Not enough IAU members were allowed to vote on the new definition. In 2006 about 10,000 astronomers belonged to the IAU. Of those, fewer than 500 voted. Why so few? Because only members who are in the room when a vote is held can vote. If IAU members were allowed to vote by e-mail, say some astronomers, the definition of planet might not have passed. For many, the small number of voters is a problem. Astronomer David Weintraub, author of the book Is Pluto a Planet?, said, "I'm not convinced that the folks who were at the meeting represented well the larger community."

11 There was another objection to Pluto's reclassification. Pluto's identity as a planet wasn't just a matter of science. It was also a matter of history. After all, Pluto had been a planet for more than 70 years. People had grown used to the idea. Textbooks and museums would have to change what they said. One IAU member said that Pluto should stay a planet "partly for historical reasons," and others agreed. Members of the public, including angry schoolchildren, wrote the IAU demanding that Pluto stay a planet. New Mexico lawmakers even passed a bill saying that Pluto was still a planet. Even if other Kuiper Belt Objects are dwarf planets, they said, leave Pluto alone.

12 But supporters of the change argue that historical reasons shouldn't get in the way of science. After all, people believed for centuries that the planets and the sun orbit Earth, but scientists eventually discovered the truth: Earth and the other planets orbit the sun. In a similar way, new discoveries led scientists to question the historical view of what a planet is. The new definition isn't perfect, say its supporters, but it's more accurate than the old way of thinking. In spite of "historical reasons," Pluto must go.

13 By now, people are used to the idea that Pluto isn't a planet, even if not everyone agrees with that idea. But is the definition of planet settled? Probably not. First of all, the IAU's definition applies only to objects in our own solar system. But astronomers have discovered huge, round objects circling distant stars. Most people would agree these objects are planets, but they aren't covered by the definition. Shouldn't a good definition of planet work beyond our own solar system? Furthermore, astronomers have even found some solid round objects in space that don't orbit any stars at all. Are they planets?

14 No matter what happens to Pluto, two things are certain: Science will keep making new discoveries, and scientists and nonscientists alike will argue about what each new discovery means.