Pluto: Not Really A Planet??

August 28, 2006

Since childhood, I always thought of Pluto as the ninth planet, the planet farthest from the sun. That's what I read from astronomy books, and that's what every quizmaster said in quiz bees then. I memorized the names of the planets and their relative distances from the sun. And they were always in my heart, those nine planets, from nearest to farthest: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto.

But when I was in high school, I was shocked when I heard that Pluto was not really the farthest planet from the sun. Neptune was the farthest! I didn't believe it at first, but after I read the latest science books and encyclopedias, I found out that Pluto's orbit is so elliptical ("very elongated oblong") that the Sun's position inside its orbit is way off-center. One side of its orbit reaches the very far end of the solar system, while the opposite side is inside Neptune's orbit. Pluto crossed Neptune's orbit on February 7, 1979. Since then, Neptune became the farthest planet.

So at that time, my idea of Pluto being the farthest planet from the sun got pretty shattered. Almost.

During that time, I also learned that Pluto will cross Neptune's orbit again some time in 1999, escaping Neptune's orbit. When it happens, Pluto will once again become the farthest planet from the Sun. And so it did on February 11, 1999.

But recently, I learned from the science news that astronomers came into an agreement last week that Pluto is not really a planet! That there are officially only eight planets in the solar system! What a shocker again!

I learned that Pluto's planetary identity has been in question since years ago. For instance, there are some asteroids found recently that are a bit larger than Pluto. Therefore, some asteroids are larger than some planets?? The astronomers found out that there's really no scientific definition of the word "planet". What exactly is a planet?

To answer that question in the definitive, the astronomers convened in the International Astronomical Union (IAU) in Prague, Czech Republic. In that meeting, the majority of the astronomers agreed on the following definition of a planet (a.k.a. Resolution 5A). For a celestial body in the solar system to be called a planet, it has to satisfy all of the following:

1. It must be in orbit around the Sun.
2. It should have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
3. It should have cleared the neighborhood around its orbit.

Criterion #1 means that the celestial body should revolve around the Sun. Criterion #2 means that the celestial body should have a nearly round shape. And Criterion #3 means that no other celestial body of comparable "size" (mass) should orbit near the body in question.

Pluto fails to meet Criterion #3 because there are thousands of huge, icy and rocky bodies orbiting near Pluto's orbit. These bodies are called Kuiper Belt Objects (KBOs), and they lie inside an asteroid belt-like region called the Kuiper Belt. The belt is located just beyond Neptune's orbit. Some of the KBOs are as large as Pluto, even larger! (In addition, Pluto itself is a Kuiper Belt Object.) Pluto therefore fails to clear the Kuiper belt from its orbit. Because of this, Pluto fails to be a planet under this definition. (Update: Pluto's failure to meet Criterion #3 is not really due to the fact that Pluto's orbit overlaps the orbit of Neptune, which was what I posted previously.)

This definition is radical enough. It booted Pluto out of the elite nine bodies of the solar system. It will surely change textbooks, encyclopedias and science websites around the world. But it doesn't yet sink totally into mine. It boggles the mind. That's Pluto.

See also these links:
National Geographic
IAU Website
Fox News
Bad Astronomy
Wikipedia