In 2008, the International Astronomical Union (the IAU) re-defined a planet in such a way that Pluto is excluded. That was nearly 13 years ago, but the desire to call Pluto a planet remains prevalent, even today. Thus, I want to address why the definition of a planet changed and why Pluto now falls outside of that definition.
First, we need a little history. The term “planet” derives from a Greek word which means “wanderer.” Why? Well, since ancient times, people have looked up at the stars and tracked their movements. Most of the stars followed a very regular, predictable pattern as they moved across the sky in a yearly cycle. However, there was a handful of peculiar stars that did not follow this same regular pattern. Since they deviated from the normal and appeared to follow their own courses, they were called wandering stars.
It seems strange to us to think of the planets as stars, but consider that in a time before telescopes, planets did not look different from stars: they were both lights in the sky. Thus, the only notable difference between a star and a planet is the latter’s peculiar, wandering motion. Thus, to an ancient astronomer, planets were not necessarily a special type of stellar body with a unique composition and place in the universe, they were just stars that moved differently.
A better understanding of stars came about in the 1500’s. Men like Nicholaus Copernicus so thoroughly studied the movement of the stars and planets, that it became apparent that the planets orbited around the Sun. This revelation was one of the triggers for the scientific revolution, since men like Sir Isaac Newton were then able to come along and explain why the planets orbited the Sun (Newton’s universal law of gravitation cleared up that problem).
Note that now the definition of a planet could change. To the ancients, a planet was just a wandering star. To the men in the scientific revolution, a planet was a body that orbited the Sun. Now, the nature of planets was seen to be distinct from the nature of a star. Notice how the definition changed with a growing understanding of the Solar System and the Universe as a whole.
Now, some may be fine with that definition. After all, planets orbit the Sun, what else could possibly orbit the Sun? Well, as it turns out, there are more than just planets that orbit the Sun. One such group of objects are the asteroids. The first asteroid was not observed until 1801. It is the largest asteroid and it is named Ceres. Now, we have an additional wrinkle in our definition of a planet: if a planet is something that orbits the Sun, should every asteroid be considered a planet? First of all, that would be impractical, because of the huge number of asteroids and because every once in a while, one strays off course and collides with a planet (whereupon it becomes a meteor [while falling to a planet] and a meteorite [when it lands on the planet]). Thus, maybe we can modify our definition of a planet once more to include only the large, spherical objects that orbit the Sun. The asteroids are simply part of the asteroid belt, so they are a separate, different sort of thing from a planet.
While the first asteroid was discovered in 1801, Pluto was discovered in 1930, 129 years later. It was discovered as astronomers were looking for “Planet X”. To explain Planet X, while most of the planets are visible to the naked eye, Neptune is not. The first indication that Neptune existed was when astronomers noticed a strange wobble in the orbit of Uranus. The best explanation for the wobble was another planet with a larger orbit, and low and behold, Neptune was discovered. Then, a wobble was discovered in the orbit of Neptune, so a ninth planet was proposed. Until it was found, this hypothetical planet was called Planet X. Astronomer Percival Lowell began a search for this Planet X (he actually coined the name Planet X) in 1906. His project continued after his death, and it was in the search for Planet X that Pluto was discovered.
As a side note, there is some debate as to whether or not Pluto is Planet X. While Pluto is in a similar orbit that was expected for Planet X, it differs from the predicted size of Planet X, so some astronomers regard Lowell’s Planet X as a non-existent planet. Regardless of whether or not Pluto is Planet X or whether it was simply found during the search for a non-existent planet, Pluto became recognized as the ninth planet in 1930.
Thus, Pluto held its position as the ninth planet for several years. Then, things began to change. Astronomers began to propose that Pluto was not alone. Perhaps there were other objects outside of, or near, Pluto’s orbit. Finally, in 1992, visible evidence of the Kuiper Belt was found. By the way, “Kuiper” is pronounced as “ky-per.” The Kuiper Belt is like the asteroid belt, in the sense that it too is a large collection of small, rocky objects orbiting the sun in a specific region of the solar system. The problem for Pluto began when astronomers began to notice that there were other objects in the Kuiper belt that were or a similar size to Pluto.
Three of these objects in the Kuiper Belt that are similar in size to Pluto are Eris, Makemake, and Haumea. Eris is nearly the size of Pluto while Makemake and Haumea are a little smaller. Remember Ceres, the first asteroid discovered? It also is a large body. All four of these objects are spherical, like a planet, and all of them, except Ceres, have moons, like most of the planets.
Astronomers faced a dilemma. If a planet is simply a large, spherical object that orbits the Sun, then should not Ceres, Eres, Makemake, and Haumea also be called planets? However, that opens up the possibility that new planets could be discovered every few years as more and more objects are found in the Kuiper Belt. Besides, all of these objects, plus Pluto, differ from the other planets because they are found in a field of rocks (either the Asteroid Belt or the Kuiper Belt), so maybe it would be easier to give them a new classification.
The IAU opted for the latter option. They changed the definition of a planet such that a planet must fulfill three requirements:
- It must orbit the Sun,
- It must maintain a spherical shape,
- It must have cleared its orbit.
“Cleared its orbit” needs a little explanation. A large object, like a planet, has enough of a gravitational pull that any loose objects in its path get caught in its gravitational pull and fall to the planet. That is why stray asteroids that cross the path of a planet end up falling into the planet. This is also why stray asteroid remain in the orbit of a planet: the planet pulls them to itself. Thus, planets “clear” their orbits of stray debris.
Pluto, Ceres, Eris, Makemake, and Haumea all orbit the Sun, maintain a spherical shape, but they do not clear their orbits. That is why they are found in the Asteroid Belt and the Kuiper Belt. Thus, they fall outside of the new definition of a planet. Instead, all five of these objects are called dwarf planets. While some people may be upset at this new definition of a planet that “excludes” Pluto, keep in mind that the definition of a planet has changed since ancient times. Changing a definition in order to accommodate new discoveries is not a new thing. Instead, it is often a necessary thing. Also, Pluto is not so much “excluded” from the planets as it is put in a new group, the dwarf planets. Thus, Pluto is not “lonely” all by itself: it has “friends,” it is just that its friends are less familiar to most people. Unfortunately for Pluto, it so happened to have been found before most of the rest of the dwarf planets, so at the time, it was easier to simply call it a planet. It was only when an abundance of other dwarf planets were found that it became apparent that Pluto was not a unique planet, it was simply one of a whole new class of objects orbiting the Sun.
These changing definitions of a planet remind us that science is a human endeavor, and as the body of science is a human construct, it is prone to errors and re-writes in order to accommodate new information. Do you know what will not change, no matter how much humans learn or know? The Word of God, the Bible, which was given to us so that we may know the truth. If we start with it as our foundation, then our knowledge will not be subject to revision.
Thoughts from Steven
Heart of America Science Resource Center 