When Pluto became officially redefined as a dwarf planet, many people were devastated. How could they give poor little Pluto the boot? It was a planet for ages! Why change that now?

Turns out there are some really good reasons - starting with the fact that Pluto never really should have been called a planet in the first place.

Calm down, let me explain…

Let’s start at the beginning (of the 1900s, that is). There was growing hype about the search for a mystery planet that was expected to orbit beyond Neptune. Astronomers had noticed a little wobble in Neptune’s orbit every now and then, which they believed could be explained by the gravitational tug of a more distant planet. This is how Neptune itself was discovered, so it made perfect sense.

So the search for Planet X was underway!

Dr. Percival Lowell, founder of Arizona’s Lowell Observatory, was one of the champions in the search for the unseen Planet X. He estimated that it was about 7 times as massive as Earth and was similar in many respects to the outer planets.

Ultimately Pluto was found where they were looking for Planet X, so of course it was dubbed a planet. But we didn’t really know much about it at that point. We made a lot of assumptions about things like its size, which was thought to be comparable to Neptune (which is 17 times as massive as Earth).

But the more we learned about Pluto, the stranger it became.

Several oddities forced us to see that Pluto’s not really so planet-y after all. For one thing, its size estimate kept going down, down, down as we studied it more. It became kind of a joke that it was in danger of vanishing altogether based on the trend of size estimates. We eventually discovered that the little guy is even smaller than our Moon.

But while Pluto’s small size is what everyone thinks it was demoted for, there are other factors that set Pluto apart from all of the planets. One is its orbit. Have a quick glance at the image above and you can see that it’s clearly different from the others. The planets orbit on a pretty level plane, but Pluto’s orbit is skewed.

And you can kind of tell from the above image that Pluto’s orbit is also highly eccentric - it’s more oblong, more of an oval than the other planets’ orbits are. And while the other planets orbit in neat, concentric ellipses, Pluto’s orbit actually crosses Neptune’s. (Don’t worry - they’re never going to collide. They’re locked in an orbital resonance.)

Even weirder, Pluto’s moon Charon was discovered and it turns out that its massive enough that it doesn’t even really orbit Pluto - they actually orbit each other. Now, that’s technically true of any planet/moon pair, but in every other case in our solar system the center of motion isn’t outside of the planet itself. Pluto and Charon orbit around a point in free space.

Pluto’s weird...so what? Can’t it be weird and be a planet too?

It sure can! The trouble is that there wasn’t a specific definition of what a planet was in the first place. It was always just pretty obvious, so there wasn’t a reason to come up with an exact definition.

But Pluto really threw us for a loop! The kicker was that we eventually found tons of other stuff orbiting in Pluto’s region of the solar system (which came to be called the Kuiper belt). We were forced to officially define what a planet was because we had to figure out if those other bodies, which we call Kuiper belt objects or KBOs, we were discovering out there were planets too.

We even found some KBOs that are bigger than Pluto...so that made us ask what exactly a planet is, because if Pluto made the cut there was no reason to exclude these new guys we were finding.

We began to realize that Pluto and the KBOs were a unique group of objects.

As we continued studying Pluto & the other KBOs, we learned that Pluto is a lot more like KBOs than it is like a planet. So what were we to do?

This actually isn’t the first time we’ve had to demote a planet - the process just happened a lot quicker the first time. Some astronomers suspected there should be a missing planet orbiting between Mars and Jupiter. So, they started looking for it and lo and behold - there was Ceres!

But we quickly found tons of other bodies in that region that shared common characteristics, so we figured it was probably not a classical planet after all. They even came up with a new name for it - asteroid. They called the region the asteroid belt.

Why was it different for Pluto?

Ceres and Pluto have extremely similar stories, but there’s a key difference. In the case of Ceres, we found the other objects pretty quickly. For Pluto, however, it took decades and decades and decades for more of these objects to be discovered and for us to learn more about their odd characteristics.

So we were faced with a situation where we had to reassess the number of planets in our solar system, because we knew it was NOT 9. But was it 8 or 10+?

After tons of debate and deliberations, the International Astronomical Union (IAU) was forced to finally come to a decision. They defined a planet as meeting the following 3 criteria:

As far as size goes, Pluto’s just fine! It’s the third criterion there that gave it the axe. All of the planets have SOME stuff floating around in their orbits, but the debris in Pluto’s orbit amounts to about as much mass as Pluto itself (plus recall that it crosses Neptune - so its orbit is far from clear!).

Still not on board with Pluto’s demotion? I’ve got great news for you! Pluto doesn’t care whether we call it a planet or not. It’s gonna go on Pluting just the same either way. But when we study the characteristics of different objects in the solar system, it makes far more sense to include Pluto in the category of objects that it shares the most traits with - and that’s the Kuiper belt objects.


Look at it this way…Pluto went from being the runt of the planets to the King of the Kuiper belt! 👑

By Ashley Balzer, Lake Afton Public Observatory Volunteer