Alas, poor Pluto…

I’ve decided to dedicate this post to a question that I get asked pretty frequently: “Should Pluto be a planet?”

In a nutshell my answer is a resounding, “NO”; however, I will dispute the International Astronomical Union (the governing body of Astronomy who made the decision) and their definition of what a planet is. The decision that was handed down by the IAU back in summer 2006 was basically an official “final” decision on an argument that’s been raging for almost three hundred years: “What is a planet?”.

This might seem like a relatively simple question, but if you really think about it, it’s not. There are millions of things orbiting the Sun and other stars, how do you classify them broadly? It’s not a simple task. Originally, the five planets visible from Earth with the naked eye (Mercury, Venus, Mars, Jupiter, Saturn) were noticed to differ from the stars by the Greeks who called them “planetae” or “wanderers” and named them after Gods whose traits correlated with the apparent nature of each (we use the Roman names today because they incorporated the Greek astronomical knowledge later on). Fast forward now several centuries to 1609, when Galileo used one of the first telescopes to view Jupiter and its four largest moons. This was the first time in recorded history that man ever actually saw a planet’s disk fully resolved, that is, saw it as something other than a speck of light in the sky. Once the telescope was invented (not by Galileo, but Hans Lippershey) and perfected, new objects in space began to be discovered. In 1781, William Herschel made the first modern planetary discovery when he found Uranus. At the start of the 19th century, scientists began finding a slew of “new planets” at the distance between Mars and Jupiter. Not knowing any better, the scientists added them to the growing list of planets. Textbooks from the early part of the 19th century have as many as 11 planets listed (Mercury, Venus, Earth, Mars, Ceres, Pallas, Vesta, Jupiter, Saturn, Uranus), but as more and more of these new planets were discovered, astronomers realized that these smaller bodies found between Mars and Jupiter were actually a new category of celestial bodies and reclassified them as asteroids. The solar system was beefed up to 9 planets after the discoveries of Neptune (1846) and Pluto (1930) and that’s how it remained for over 85 years.

Since its discovery in 1930 by American astronomer Clyde Tombaugh, Pluto has always been viewed as an oddball in the solar system. Although it still has never been well-imaged, we know that it’s less than half the size of Mercury and has a surface area roughly comparable to the land area of New England. It’s also odd that after the relative easy separation of terrestrial inner planets and large gassy outer planets we had tiny icy Pluto. Pluto was basically a surprise, ahead of it’s time; a celestial body we found at a time before we could explain it. Starting in the early ’90s, astronomers began finding other icy objects beyond Neptune; they were named Kuiper Belt objects (KBOs), after the astronomer who theorized almost four decades earlier that there might be a ring of small icy objects orbiting out past Neptune. Most of these KBOs were small and asteroid-like and posed no question about their planethood, but as the number grew, astronomers began wondering whether Pluto was actually a tiny planet or just a large KBO. It was rationalized that Pluto would be the limit, nothing smaller would be considered a planet. But in 2005, a body larger than Pluto (Eris) was discovered in the Kuiper Belt; you might have heard of this object referred to as “Planet X” or “Xena” by a lot of popular news outlets. Once an object larger than Pluto was discovered, astronomers had a dilemma, add Eris to the solar system to agree with the already questionable classification of Pluto or reconsider Pluto’s planetary status.

So, the astronomers rightly decided to reassess the definition of a planet with direct regard to Pluto and they correctly deemed that the tiny icy body was not a planet but a large KBO. However, the part of the ruling that I will disagree with is the IAU’s formal definition of what is a planet. The IAU ruled that by definition a planet must satisfy 4 criteria: a) it must orbit the Sun, b) it must be large enough that its gravity has pulled it into a spherical shape, c) it must have cleared its orbit of debris, and d) it isn’t a satellite. They also decreed that Pluto or any other object that satisfies a),b), and d) but not c) is a new type of celestial body known as a “dwarf planet”. As of right now there are 8 planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune) and 5 dwarf planets (Pluto, Eris, Ceres, MakeMake, Haumea) in the solar system.

That was the ruling. Now for the arguments against it.

First off, the idea that a world orbiting another star that’s not the Sun isn’t a planet is ridiculous. Of course it’s a planet, it’s just a different type of planet. In the current usage, “dwarf planet” is even more misleading because dwarf galaxies and dwarf stars are subclasses for smaller galaxies and stars; a dwarf galaxy is a small galaxy, a dwarf star is a small star, but a dwarf planet is not a planet. That’s just going to confuse people. I think that the term “planet” should be broadened to encompass specific subclasses such as exoplanets and dwarf planets.  What are dwarf planets? Tiny planets. What are exoplanets? Planets that orbit other stars. Boom, done. Simple language, simple ideas, but under the current definition you can’t use the word “planet” to explain either exoplanets or dwarf planets unless you’re saying “is not a planet”.

Another technical wrench that could be thrown into the IAU’s decision regards criterion c), that an object must clear its orbit of debris. This makes sense; it rules out asteroids and KBOs because they aren’t really the main object orbiting at that distance from the Sun, they’re smaller parts of the the larger feature or “belt”. However, this wording here could technically be read to strip Jupiter of its planethood. You see, Jupiter has groups of asteroids known as Trojan asteroids that follow in front of and behind it in its orbit. So, unless we have a very specific definition of “debris”, Jupiter has other non-satellites in its orbit, so TECHNICALLY, if you want to be a stickler, it shouldn’t be a planet either. Of course, I’m not saying Jupiter shouldn’t be a planet, I’m just saying that the IAU has to do a better job in its definitions.

So there you have it, Pluto is no longer a planet, but a non-planet object known as a “dwarf planet”. In fact it’s actually in a subclass of dwarf planets known as “plutoids”; this in itself is ridiculous because the only current dwarf planet which isn’t a plutoid is Ceres, which is the only asteroid dwarf planet, so they should really just eliminate the term dwarf planet and leave all objects in the asteroid belt as “asteroids” and all large objects in the Kuiper Belt as “plutoids” and then that eliminates the whole “planet”-“dwarf planet” issue. But that’s just my opinion and apparently that isn’t that highly regarded in the world as of yet. One day though.

And don’t even get me started on planetary nebulae…


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