The word "planet" derives from an ancient Greek phrase meaning "wanderer," so called because the ancient astronomers observed that a handful of "lights" were in constant motion, each charting its path across the sky against the relative stillness of the fixed stars. Most notable among these astronomers was Ptolemy, whose geocentric conception of the visible universe reigned until the advent of the telescope sixteen centuries later.
 
The Ptolemaic System contained seven of these "moving lights," which were thought to orbit the Earth, including our nearby Moon and concluding with the distant, malefic planet Saturn, whose orbit inscribed the outermost boundary of our solar system.
 
In the 17th Century, acceptance of the heliocentric system, and the rise of Rationalism in general, resulted in the proliferation of astronomical observatories across Europe. Increasingly powerful telescopes exponentially increased the range of vision, allowing astronomers to expand the historical boundaries of our solar family to include three new planets; Uranus in 1781, followed by Neptune in 1848, then Pluto in 1930.
 
The ninth planet was controversial from the very beginning, with its eccentric, elliptical 248-year orbit, its incredible remoteness and its size, less than that of our moon. (In fact, Pluto is smaller than at least two of Saturn's moons.) Over the last century, the floodgates opened wide; astronomers began to identify thousands of objects, from asteroids to comets to moons in orbit around other planets, all within our solar system.
 
"Pluto," according to a recent article at space.com, "has been considered a planet since its discovery, but this position has come under threat with the discovery of 2003 UB313 (aka Xena), an object larger than Pluto orbiting out further in the Solar System. The International Astronomical Union will be meeting in August to decide on the fate of Pluto."
 
Even more than that, they will be trying, again, to hammer out a suitably scientific definition for that ancient wanderer of words, "planet." Until as recently as the 90s, there has never been any need for a definition that did more than separate those stars that wandered from those that did not.
 
Mike Brown, Professor of Planetary Astronomy, Division of Geological and Planetary Sciences at California Institute of Technology explains, "Astoundingly, no precise scientific definition of the word 'planet' currently exists." Up until now, there have been

1. Planet : any of the celestial bodies (other than comets or satellites) that revolve around the sun in the solar system

2. Minor Planet/Planetoid/Asteroid : any of numerous small celestial bodies composed of rock and metal that move around the sun (mainly between the orbits of Mars and Jupiter)

(*The astronomical community may differ slightly in their definitions of these three words, which are used often interchangeably but, accordingly to Dictionary.com, they are, in fact, interchangeable.)

"Continent," Brown suggests in another article, is a good example of a word that "should have some scientific definition," and yet, does not. There is no single definition, scientific or otherwise, that can be made to apply to all seven of the universally agreed-upon continents. "Why is Europe called a separate continent?" Brown continues, "Only because of culture. You will never hear geologists engaged in a debate about the meaning of the word "continent."
 
Where do we draw the line?
 
Based on research from Space.com:

Purely historical. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto are planets. Nothing else in the solar system is a planet. This definition is definitely historically valid, but... completely lacking in scientific motivation, [it] makes the word "planet" meaningless as a scientific description.

Historical plus. Mercury through Pluto are planets, as is any newly discovered object larger than Pluto. This definition is, we believe, the one in most common colloquial use throughout the world, even if people don't realize that this is the definition they are using. Indeed, if Sedna had been larger than Pluto, most would have hailed it as a 10th planet. This definition -- like the previous -- is historically consistent, but -- like the previous -- still fails the scientific test. Why is Pluto the cutoff size? Is there really a big enough difference in size between Pluto and Sedna and Quaoar that one should be called a planet while the others are not? The scientific answer remains a resounding no.

Scientific I. Gravitational rounding. Any object which is round due to its own gravitational pull and which directly orbits the sun is called a planet. It is strictly scientific, yet historically valid, as all objects that we call planets... are indeed round. More importantly (and by a complete coincidence) the dividing line between... round and... not round is just a few times smaller than Pluto. So why not take advantage of this coincidence and simply define planets to be objects which are round? To do so means that we must admit several other bodies to the class of "planet;" Sedna, Quaoar, the asteroid Ceres, and perhaps a dozen Kuiper belt objects. As Kuiper Belt researcher Dr. Marc Buie, says, "I believe the definition of planet should be as simple as possible: One [criteria] is that it can't be big enough to burn its own matter - that's what a star does. On the small end, I think the boundary between a planet and not a planet should be, is the gravity of the object stronger than the strength of the material of the object? That's a fancy way of saying is it round?"

Scientific II. Roundness plus size. A synopsis: A planet is a body that directly orbits a star, is large enough to be round because of self gravity, and is not so large that it triggers nuclear fusion in its interior. "I think there's a consensus moving in this direction," Stern said. The actual definition will, at least, be more complex than that. Stern favors calling the smaller objects dwarf planets, for example. Other astronomers prefer the term minor planet. Another term bandied about is Kuiper Belt planets.

Kirsti Melto elaborates on this possibility. "Astronomers will likely continue using the term planet, or they might switch to planetary mass object. And, either way, they will put other terms in front of it to define each type, such as gas giant, terrestrial, asteroidal and perhaps even traditional or historic. Even the sorts of adjectives that might be used is not totally agreed upon. The terms Trans-Neptunian and cisjovian are based on location, not composition."

Population classification. This definition requires a little more explanation and a little more understanding of the solar system, but, in the end, leads to the most satisfactory definition of "planet." Just like the solar system very naturally divides itself between round objects and non-round objects, it also very naturally divides itself between solitary individuals and members of large populations. The best known example of a large population is the asteroid belt. The solitary individuals are much different. In their region of space there is only them (Earth, say) and then a collection of much, much smaller objects (the near-earth asteroids), with no continuous population in between.

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune all count as solitary individuals by this definition. Pluto and Quaoar do not. Pluto is clearly a member of the Kuiper belt population. Since there is a clear scientific distinction between solitary individuals and members of large populations it is instructive to come up with words to describe these objects. The large populations can each be described by the particular population (asteroid belt, Kuiper belt...). What about the solitary individuals? Isn't the best word to describe them "planet"?

Is there any historical basis for saying that a planet is a solitary individual that is not a member of a large population? Yes! As mentioned earlier, historically Ceres and the first few asteroids were initially classified as planets. Only when it became known that there were many, many asteroids in similar orbits was it decided that they should no longer be classified as planets. Historically, there is a clear distinction between planets and populations. Historically, then, Pluto, too, should no longer be considered a planet.

We are thus left with a final concept of the word planet. Every object in the solar system quite naturally can be classified as either a solitary individual or a member of a large population. The individuals are planets. The populations are not.

Mike Brown speaks for himself and the rest of his team, those responsible for discovering the Minor Planets Quoar and Sedna as well as Pluto's troublemaking baby sister (2003 UB313 aka Xena and, quite possibly #10) when he says; "We declare that the new object, with a size larger than Pluto, is indeed a planet. A cultural planet, a historical planet; I will not argue that it is a scientific planet, because there is no good scientific definition which fits our solar system and our culture, and I have decided to let culture win this one."