Why Isn't Pluto a Planet Anymore?

If you're like me, in elementary school you learned about the nine planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Pluto was discovered and classified as a planet in 1930 by astronomer Clyde Tombaugh of the Lowell Observatory in Arizona. (The existence of this planet was actually proposed years before by American Percival Lowell, who had theorized that slight disturbances in the orbits of Uranus and Neptune were caused by the gravitational pull of another planet. Lowell even went so far as to make calculations to estimate where to find this planet!) Right away, the solar system's newest planet was considered an oddball, however. Its orbit is so eccentric (far from circular) that it actually gets closer to the sun than Neptune for 20 of its 248-years-long trip around the sun. Its axis of rotation is tilted 57 degrees with respect to the plane of its orbit around the Sun, so it spins almost on its side.

In 2006, the International Astronomical Union (IAU) General Assembly met in Prague to discuss Pluto's "planethood." At the conference, researchers endured eight days of contentious arguments, with four different proposals being offered. One controversial suggestion would have brought the total number of planets in the solar system to 12 - including Ceres, the largest asteroid, and Pluto's moon Charon. Near the end of the Prague conference, 424 astronomers voted to define a planet as having these three characteristics:

1. It must orbit the sun.

2. It must be spherical in shape due to its own gravity.

3. It must have "cleared the neighborhood" around its orbit.
   
   

We know that Pluto orbits the sun. While it is large enough to have become spherical, Pluto is not big enough to exert its orbital dominance and "clear the neighborhood" surrounding its orbit. (In addition to having 5 moons, Pluto shares its orbit with other objects in the Kuiper Belt, a region of the Solar System beyond Neptune that is populated with many small icy bodies.) From then on, only the worlds Mercury through Neptune would be considered planets. Pluto and its like - round objects that shared the neighborhood of their orbit with other entities - are now classified called dwarf planets. All other objects orbiting the sun would be known as small solar system bodies.

It is important to note that Pluto did not change; rather, scientists changed how a planet is defined. What was once a loose word used to describe a large object within the solar system was now specific. Each year on August 24, the international scientific community recognizes Pluto's historic downgrade with a holiday called Pluto Demoted Day. But just because Pluto lost its planet status doesn't mean it isn't fascinating. Pluto has a very prominent heart-shaped glacier the size of Texas and Oklahoma on the surface! Pluto has blue skies, spinning moons, and mountains as high as the Rockies. There are snow-capped peaks on Pluto, and it snows - except the "snow" is red! The largest of Pluto's five Moons, Charon, is so big that Pluto and Charon orbit each other like a double planet. And Charon is so big that its gravity actually causes Pluto to wobble in its orbit.

The debate about Pluto's planetary status goes on, even among astronomers. Yet it seems unlikely the IAU will revisit the planet definition controversy any time soon. Astrophysicist Ethan Siegel said, "The simple fact is that Pluto was misclassified when it was first discovered; it was never on the same footing as the other eight worlds." Mike Brown also chimed in: "So, hey, Pluto is still not a planet. Actually, never was. We just misunderstood it for 50 years. Now, we know better." The solar system's transformation from nine planets to eight (at least by the standard IAU definition) was a long time in the making and helps encapsulate one of the greatest strengths of science: the ability to alter seemingly steadfast definitions in light of new evidence.