Why isn’t Pluto a Planet?

Surely you’ve heard something about the New Horizons mission. If not, come out from under your rock.

I’m not going to say much about the mission in this post. I want to talk about Pluto’s planetary status.

For me, and many of you reading this, Pluto was always a planet in science class. We always had nine planets. We remembered their order by saying “My Very Excellent Mother Just Served Us Nine Pizzas” Or something similar. Maybe you even had a homework assignment to come up with a new sentence to remember the planets by. I certainly did.

Maybe you remember a time before Pluto was even discovered. Congratulations on your long life! And props to you for being on the internet! But for most of us, we grew up with nine lovely planets, and probably adored the littlest one, Pluto.

But, in the early 90’s, Pluto’s status was called into question when other similar sized objects were discovered in the Kuiper belt. It wasn’t until the mid 00’s that this became an issue of public concern. Pluto was demoted and everyone was sad. I remember also being sad. Why can’t Pluto be a planet anymore? Don’t pick on him ’cause he’s little. Everyone was upset, and offered Pluto their condolences.

Now there’s talk about giving Pluto planetary status again.

But I’ve found myself on the other side. Pluto’s not a planet. As much as we all miss the days when he was, the truth is that he’s not actually a planet, and he never was.

There are three criteria for an object to be considered a full-sized planet. The object must meet all three.

  • It must orbit the sun
  • It must be massive enough to assume hydrostatic equilibrium (It must be round).
  • It must have cleared it’s “neighborhood” of all debris.

Pluto only meets the first two criteria. Thus, it is a dwarf planet, which id defined as being round and orbiting the sun. There is still debris in Pluto’s orbit, therefore it cannot be considered a full-sized planet.

Let’s talk about labels again (see previous post about labels here). A label is used to help us explain something in a simple way. The words “planet” and “dwarf planet” are both labels that explain an object in space. We know what a planet is, and we know what a dwarf planet is. We also know the difference between the two. We know that Pluto does not meet the criteria to be called a planet, so the label “planet” does not describe Pluto. Pluto does meet the criteria to be a dwarf planet, so the label “dwarf planet” does describe Pluto. Do you see what I’m saying?

If the whole point of labeling these bodies to to help us understand what they are, then mislabeling them creates confusion and misunderstanding. Calling Pluto a planet is incorrect because it’s not actually a planet, it’s a dwarf planet. There’s a difference.

If you are knowledgeable on the topic of gender identity, the following will make sense to you, if not, you can skip it. Calling Pluto a planet is like calling a trans man a woman. Just because he started out being called a woman, and we’ve always known him as a woman, doesn’t mean that he is. Just because Pluto started out as a planet, and we’ve always known it as a planet, doesn’t mean it is. Deep down inside, our trans man has known that he wasn’t a woman. He looked like a woman, and was called a woman, but he’s really been a man all along. Pluto looked like a planet, and we called it a planet, but it’s really been a dwarf planet all along.

So if we were to grant Pluto planetary status again, we’d also have to give the rest of the dwarf planets planetary status. Ah! Did you know there were others? There are five widely recognized dwarf planets: Pluto, Ceres, Eris, Makemake, and Haumea. There are also tons of other objects in the solar system that are dwarf planet candidates, which we don’t have enough information on.

I hope you have a better understanding of Pluto and it’s planetary status. Keep your eyes and ears open for the flood of new information coming in from the New Horizons mission. Soon we’ll know more about Pluto than we thought we would!

Pluto. Taken from New Horizons. July 13th 2015.

Pluto. Taken from New Horizons. July 13th 2015.


Part of the Daily Blogging Challenge July 2015

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