Why Isn't Pluto a Planet Anymore?

When I was a kid we were all taught there were nine planets. I clearly remember the illustrations in grade-school textbooks. We’re here on Earth, then there’s Mars, then the big one, then the one with the rings, then those other two that I could never remember which order they belonged, and then, way out there all by itself, there was Pluto. Pluto was a teeny little thing compared to the rest, and I recall teachers talking about how cold it is on Pluto, since it’s so far from the Sun. Growing up, Pluto was a planet any kid could identify with: Small, lonely, out there all by itself away from the other planets. But Pluto was a planet, darn it. At least it had that going for it.

This changed in 2006 when Pluto was demoted from its rank of ninth planet. In the blink of an eye, then there were eight. The loss of Pluto’s planetary status didn’t exactly cause rioting in the streets, but it did make many people stop and wonder. How did this happen, and who decided the fate of Pluto? Furthermore, and more philosophically, when is a planet not a planet?

When Johann Gottfried Galle discovered Neptune in 1846 he and others in the astronomy community realized that there had to be something beyond. Galle was working off the theory that Uranus was being acted upon by another planet further out in the solar system, but the gravitational pull of Neptune alone didn’t seem to explain it. This undiscovered mystery planet was later dubbed Planet X.

Many would search for Plant X in the years that followed and come up empty, until Clyde William Tombaugh found what looked like an unknown planet on February 18^th, 1930. The body was named Pluto, and presumed to be the elusive Planet X. But that assumption didn’t last long. Pluto was much smaller than expected, and most astronomers didn’t believe it had the mass necessary to affect much larger planets like Uranus and Neptune. But the world knew of Pluto’s existence, and it became the ninth planet of our solar system.

Pluto is much smaller than the Earth, only about .6% of the Earth’s volume. In fact, Pluto is less massive than many moons in the solar system, including ours. Pluto's small size means means it has only a fraction of the gravity we feel here on Earth. It is theoretically composed of frozen nitrogen and ice, with a rock core. Pluto even has its own moons: Charon, Hydra, Nix and S/2011 P 1. It takes Pluto 248 years to orbit the Sun, and a day on Pluto spans a little over 6 Earth days. Temperatures on Pluto can dip down to 400 degrees below zero.

For seventy-five years Pluto was our ninth planet, until astronomers made some interesting discoveries that demoted it down a notch. How did this happen?

In January of 2005 Mike Brown’s team at the Palomar Observatory discovered what was first referred to as the tenth planet. Later named Eris, this new celestial object would launch a firestorm in the astronomy community. Slightly larger than Pluto, Eris too had its own satellite. But many believed Eris was not a planet at all, and if Eris wasn’t a planet, what did that mean for Pluto?

In 2006 the International Astronomical Union put an end to the debate when they published an official definition for a planet. To be a planet, an object must:

1. Orbit the Sun.

2. Have enough mass to be a sphere by its own gravitational force.

3. Have cleared the neighborhood around its orbit

Pluto met the first two points, but failed on the third. Therefore, Pluto, Eris, and several other recently discovered objects, were reclassified as dwarf planets.

Many people objected to this new classification. Some disagreed on a scientific basis, while others protested from a purely sentimental point of view. But Pluto was gone from the official discussion of planets, and we were down to eight.

Dwarf planets like Pluto are Eris are now believed to be the largest known bodies of the Kuiper Belt, an array of icy objects orbiting the sun beyond Neptune. Other notable dwarf planets discovered throughout history include Ceres, Haumea and Makemake. Still out there is the notorious Planet X. Even though modern astronomy has suggested it probably doesn’t exist, some still believe in the possibility.

The confusion over Pluto serves to show just how little we know about what’s really going on out there in space, and what wonderful discoveries still await us. It seems like every week astronomers are making new progress in mapping the celestial world. Not only are they finding objects in our own solar system, but they’ve even discovered planets orbiting other stars.

There is always great speculation about what could really be out there waiting for us. From those still searching for Planet X, to those who believe the sun has a twin called Nemesis that will one day orbit back into the solar system and cause havoc, there is never a shortage of theories.

We can say a tearful goodbye to Pluto knowing that the science behind its demotion has brought new discoveries. The textbooks now may only show eight planets, but they’ll contain more information than ever about our universe. Perhaps astronomers will discover a true ninth planet someday. Until then, we can fondly remember the days when the little planet Pluto took up that last spot in the solar system.