Rant: Get Over It; Pluto Is NOT a Planet

Pluto is NOT a planet. Get over it.

For some reason, people seem to be offended that the International Astronomical Union stripped it of its ranks and recategorized it as a Dwarf planet.

And as a good scientist, I'm horrified how many people make their scientific decisions based off of raw emotions and no logics or fact!

That isn't to say that, in practice, science isn't driven by politics and emotions; but just because your immature little brother is being bad, doesn't mean you should as well.

So in order to set a good example to the emotionally driven ignorant masses, let's get to work on why Pluto cannot be a planet.

The Crux

The key to all of this is that there is no way to make a simple and impartial definition of a planet that includes Pluto with the other eight planets.

This is important enough to repeat:

There is NO simple, impartial definition we can apply to the word "planet" that includes the standard 9 planets, and only those 9 planets.

Lets Try Our Hand At This

Let's try our best to make a definition for planet that includes Pluto and see what happens:

So first thing first, let's try the most obvious definition. Let's try to limit by size. Since Pluto is about 2,400km in diameter, we'll try the following definition:

Definition #1: "A planet is any body bigger than 2,000km in diameter, whose primary gravitational influence is the Sun."

So now let's vet this one to see how it works out for Pluto.

If we consider the second part of the statement, "...whose primary gravitational influence is the Sun". As it turns out, depending on where Pluto is in its orbit, the gravitational pull between Pluto and Charon is somewhere between 40 and 110 times as strong as that of Pluto and the Sun! Thus the first thing we need to consider is a better definition of how the gravity of the Sun affects the orbit of the planet!

So let's try another definition.

Definition #2: "A planet is any body bigger than 2,000km in diameter, whose local system orbits the Sun, and who is not the satellite of another planet."

But whoops! It turns out that then we'd have to have 10 planets, because Eris--which was discovered in 2005--is a few kilometers bigger in diameter, and is almost 30% more massive--than Pluto!

Indeed, to put the size of things into perspective, it turns out there are seven moons of established planets that are bigger than Pluto, including our own moon!

Furthermore, there was no scientific reason why we chose a diameter of 2,000km as the minimum limit in the first place. It was just an arbitrary number that was pulled out of thin air in an effort to make Pluto a planet, but other similar objects not!

Indeed, if we tried to scientifically pick a minimum diameter, we'd probably want to pick the critical radius where gravity pulls the object into a spheroidal shape. This This diameter depends on the material of the body, but is often around 400km to 500km in diameter, which is about 5 times smaller than Pluto, and includes at least several Kupier Belt Objects, as well as the largest asteroid in the Asteroid Belt, Ceres!

Indeed, as we discover more and more Kupier Belt Objects, this definition could lead to dozens or even hundreds of planets!

So size (and mass) turned out to be a really bad way of defining what is and is not a planet.

So lets this another way entirely. What can we say about the orbits of the planets?

Well, let's look at how circular the orbit is; which is called its eccentricity. If we look at most of the planets, the eccentricity is such that the oribtal path is reasonably close to circular.

But this fails for Pluto. Pluto's orbit goes from around 30 AUs at its closest to the sun, to nearly 50 AUs at its farthest. Thus its orbit is nearly 1.7 times longer than it is wide making it pretty oval in shape.

But it gets worse. This test also fails for Mercury, whose eccentricity is almost as bad as Pluto's, while the asteroid Ceres has about the same eccentricity as Mars.

So if we try to use the shape of the orbit as a criteria, we endanger the planetary status of Mercury, while possibly allowing the asteroid Ceres as a planet!

So what if we look at the inclination of the orbit? Most of the planetary orbit's lie very nearly in the same plane as the Earth. In fact, most of the planets lie within 3.5 degrees of this plane.

However, this is very different for Pluto, whose orbit is inclined 17 degrees out of the Ecliptic Plane!

Even worse Mercury, at 7 degrees, has problems again!

It is inclined by over twice the amount of any other planet, and is getting pretty close to the orbital inclination of the asteroid Ceres, which is at about 10 degrees!

So once again, if we try to use an orbital parameter to define what is a planet, we endanger Mercury or include Ceres!

So it appears that we need to appeal to something else than orbit. Which leads me to wonder about Moons? Pluto has three moons, and all the other planets seem to have moons, while Ceres does not.

But wait; Mercury has no moons, while many other bodies in the solar system (such as some asteroids and Kupier Belt Objects) do. So this makes a horrible way of defining a planet as well.

An Analog From History

I keep bringing up Ceres for a reason. Ceres discovered in 1801, was the first asteroid found. However it was not always classified as an asteroid. Indeed, for about a half century, Ceres (along with the other three first found asteroids) were listed as planets in books and tables. Thus, for a good part of the 1800s, we had more than nine planets, and none of them were Pluto!

But as the number of astronomical bodies that all had nearly this same orbit grew, it became obvious that these were not best categorized as planets, but that they were best categorized on their own.

Indeed, this was a wise decision, as over the past 200 years the number of asteroids found has climbed over 100,000! Aren't we glad we don't have to learn about the 100,000 planets!

Kupier Has A Belt

So as it turns out, Pluto is smack in the middle of the Kupier Belt, and its only special attribute seems to be that it is one of the largest Kupier Belt Objects found so far.

Now since you are here, you probably don't know anything about the Kupier Belt. This is for good reason, because even though it had been conjectured to exist in the 1940's, it wasn't until the early 1990's that we were able to discover any object in it other than Pluto. Anyway, as a quickie overview, you can think of it much like an asteroid belt that is beyond the orbital distance of Neptune and upto 50 AU away. It contains thousands or even tens of thousands of objects, all in a similar orbit, and all with a similar chemical composition.

Indeed the composition of Pluto is much more like that of a comet, and the scientific evidence is mounting that many of the comets originated as Kupier Belt objects.

The Nail In The Coffin

Thus to continue considering Pluto a planet would be the same fallacy as having continued to cald the first four asteroids planets.

It would also mean coming up with an absurdly complicated definition of planet because we are too emotionally attached to what we knew

But since immediately after Pluto was found nearly 80 years ago, it has been conjectured that Pluto should not be called a planet because it doesn't belong with the others.

Good science is about progress, change, and admitting when we are wrong. And as we discover more about the structure of our solar system, we will have to continue to admit mistakes, change our definitions, and push scientific progress for the good of humanity.