Pluto was the only planet in our solar system discovered by U.S. astronomers.
The five brightest planets Mercury, Venus, Mars, Jupiter and Saturn are visible with the naked eye and were known to people since ancient times. Uranus is, under good conditions, just barely visible to the naked eye, and was officially discovered in 1781 by Sir William Herschel, an Englishman, who initially believed that he had found a new comet.
About 60 years later, English mathematician John Couch Adams and French astronomer Urbain Le Verrier studied small irregularities in Uranus' orbit and concluded that there must exist another planet beyond Uranus. Following their theoretical predictions, Johann Galle and Heinrich D'Arrest discovered Neptune.
Finally, Pluto was discovered in 1930 by an American amateur astronomer Clyde Tombaugh at Lowell Observatory in Arizona, successfully ending a long search for the so-called "Planet X". As most of us are probably aware, a well-known Disney character obtained his name after this planet.
At the time of discovery, and for the next 76 years, Pluto was considered the ninth planet of the solar system. The fact is that there is no brighter object than Pluto at a similar distance away from us, so there was no reason to dispute its status.
However, it turned out that Pluto is made mostly of rock and ice, and its reflective, icy surface made it appear very bright, and tricked us to believe it was bigger than it really was. In reality, Pluto is about two-thirds the size of our own moon.
Then, in the 1990s, technological advances made possible much more precise observations, which led to discovery of Eris and other objects of what is today known as the Kuiper belt.
The first estimates indicated that Eris was larger than Pluto, so briefly it was classified as the 10th planet of our solar system. Soon, many other objects of similar size were discovered and the calls for revisions of Pluto's status as a planet started circulating.
The situation culminated in 2006, when the International Astronomical Union (IAU) officially demoted Pluto to a dwarf planet, reducing the number of planets in our system to eight.
At about the same time, the NASA mission New Horizons was launched toward Pluto. It traveled for nine and a half years at the speed of 30,000 miles per hour, and finally reached the closest approach point (about 8,000 miles) to Pluto this month.
The spacecraft New Horizons gave us unprecedented pictures of this celestial object. It also gave us the updated measurements, which sparked an old debate.
Pluto is actually slightly larger than Eris (though Eris is still more massive since it is denser).
So what do we do now? Do we give Pluto back its status as a planet?
One of the reasons people are so passionate about giving Pluto its planet status back is national pride -- Pluto is the only planet of our solar system discovered by U.S. scientists. Actually, New Mexico and Illinois passed a law that Pluto will always be considered a planet.
There are also more subtle reasons.
One of the requirements for something to be considered a planet, that the IAU setup in 2006, was that it must gravitationally clean-up its own orbital environment.
Pluto belongs to the group of objects known as the Kuiper belt, so it clearly does not meet this requirement. This is also the only requirement that Pluto does not meet. There is a strong impression that this requirement was formulated on purpose, just so that Pluto fails the test.
In my opinion, Pluto will not get its planetary status back, at least not soon. It was demoted only 10 years ago. If IAU keeps going back and forth every 10 years, it will not look like a serious organization and people will be confused with many different definitions being written in school textbooks.
Ultimately, it does not matter whether Pluto is called a planet or not. People get together, exchange arguments, give new names to the things around them. But Pluto is whatever it is, and has not changed a bit since we demoted it into a dwarf planet. We can label things what we want, but it is irrelevant because nature uses its own logic which is sometimes superior to ours.