Pluto is NOT (not?) a Planet


Sometimes, sacrifices must be done for the greater good. In recent years, a beloved astronomical object has undergone such a tearful melodrama… Sadly, Pluto is no longer a planet.

What the hell? That’s stupid! Who on Earth is responsible for betrayal???

You do know I’m not talking about Mickey’s dog, don’t you?

Still… You’re saying that my whole early education was based on a lie!

I know! That saddens me too… But, however heartbreaking and weird that is, it does sound like Pluto was miscategorized back then…

Pluto is (officially) NOT a Planet

Neil deGrasse Tyson, one of today’s greatest science popularizer, has quickly been designed as the main responsible for Pluto’s demotion. After all, he was Director of the Hayden Planetarium, the first scientific authority to have dared attacking Pluto’s beloved status in kids’ books. This is what he explains on the Verge:

So, if I understand correctly… Pluto is too small to be a planet, right?

Pluto is in fact smaller than our own Moon! Its diameter is about 2300 kilometers. This is nearly New York – Chicago! In comparison, the smallest (other) planet, Mercury, is twice as large, and thus 8 times more massive! Even worse, this ratio is the same as between Pluto and its Moon, Charon! This means that the object of the Solar system which is the most like Pluto is Pluto’s own moon… Isn’t that fishy?


But that’s not all. In 2005, another object called Eris (formely Xena) has been discovered by Michael Brown. But the International Astronomical Union refused to call Eris a planet, even though Eris is (slightly) bigger than Pluto. In revenge to this rejection, Michael Brown designed a plan to kill Pluto’s status of planet, which he recounted in his latest book.

Why can’t we say that Eris is just the 10th planet?

The trouble is that there are many more objects slightly smaller than Pluto and Eris. Historically, in the 19th Century, the first ones to be found were Ceres, Pallas, Juno and Vesta. Granted, the biggest one, Ceres, is only 950 kilometers in diameter, which is much less than Pluto, but these were also discovered over a century before Pluto! At that point, people naturally learned about the 11 planets of the Solar system (Neptune hadn’t been discovered either).

Why have these 4 planets been demoted?

In the decades to come, many more Ceres-like planets kept being discovered around the same area as Ceres, namely, between Mars and Jupiter. Today, we have listed over hundreds of thousands of these. And surely, no one wants to learn about the 100,000 planets of the Solar system! This led scientists to recategorize Ceres and his lookalikes as a specific kind of astronomical objects: Asteroids, of the Asteroid Belt.

And I guess that the same is going on for Pluto and Eris?


But isn’t there a clear definition of what a planet is?

Well, no. Hummm… Actually, in 2006, the International Astronomical Union (IAU) gave a new official definition for planets. For an astronomical to be a planet, it now needs to satisfy three conditions.

  1. First, it must orbit the Sun, which Pluto does.
  2. Second, roughly, it must be massive enough to be round, which Pluto is.
  3. Third and finally, it must have absorbed nearly all other astronomical objects of its near surrounding, which Pluto has not.

This last condition is what led Pluto to be categorized as a dwarf planet, rather than a regular one.

So, that’s why Pluto isn’t a planet!

Officially, yes (although it does trouble me to claim that a dwarf planet is not a planet, hence the title of this article…).

Pluto is (clearly) NOT a Planet

But there are many other (more relevant?) facts that make Pluto so different from other planets that it probably doesn’t deserve to belong to the category of planets.

Like what?

Like its orbit. Pluto’s orbit doesn’t have much in common with the orbits of other planets. First, it’s much less circular. In fact, at its closest, Pluto is closer to the Sun than Neptune is! This is illustrated by the following image from

Second, unlike all other planets, Pluto doesn’t orbit around the Sun on the same plane as all other planets do, as illustrated by the following picture from

Pluto's Tilted Orbit

Waw! All other planets have circular obits on a same plane? How unlikely is that?

It’s actually not unlikely at all! Rather, it’s a mere consequence of the laws of mechanics and the fact that planets have absorbed most of their surrounding, as explained by MinutePhysics:

Essentially, the motions of the planets away from the solar plane (or rather, in the direction of the angular momentum) have been cancelled by repeated collisions with particles whose average velocities lie in the solar plane.

So, isn’t the fact that Pluto doesn’t orbit in the solar plane related to its not-having-cleaned its surrounding?

That’s the gist! In fact, Pluto has so clearly not cleaned its surrounding that its Moon Charon is actually so large compared to Pluto that it makes Pluto orbit around the center of gravity of the system Pluto+Charon. And weirdly, enough, this center of gravity is located outside of Pluto! In other words, Pluto is orbiting a point out of Pluto, as displayed below (from Wikimedia):

But there’s probably an even more convincing reason why Pluto isn’t fit to be a planet…

What is it?

Exactly! What is Pluto?



I mean that what makes Pluto so different from planets is precisely what Pluto is. While the 4 terrestrial planets of the Solar system (Mercury, Venus, Earth and Mars) are mainly made of silicate rocks and metals, and while the 4 gas giants (Jupiter, Saturn, Uranus and Neptune) are mainly made of… well, gas, Pluto is rather a huge iceberg! So, crucially, Pluto is essentially not the thing that planets are!

Is that so bad?

Yes! Because there are many other objects that are made of what makes up Pluto. As these objects approach the Sun, the Solar wind melts them, leaving a long bright tail!

Are you talking about comets?

I am! What resembles Pluto’s material the most are comets. And if Pluto were suddenly to be brought closer to the Sun, our eyes wouldn’t see a planet. They’d see the largest and most breathtaking comet ever! And this would actually make Pluto more awesome than any of the planets of the Solar system, with the only exception of our beloved Earth!

Waw! That blows my mind!

I know! Let me recap. Actually, no. Let me let the awesome CGPGrey recapitulate:

Why Pluto (ever) was a Planet

Let me now get to the truly awesome part of Pluto’s story! What makes Pluto really special is rather the History of its discovery, which also explains why it ever was regarded as a planet.


What’s so interesting about that?

Well, in fact, let me get all the way way back to the 18th Century. At that time, the great Isaac Newton had just revolutionized science by introducing the fundamental laws of mechanics. These powerful laws succeeded in explaining all motions, from the fall of an apple on Earth, to the rotations of the planets far in the sky. Crucially, Newton’s discovery was largely based on his careful study of the orbits of the planets, described by Kepler’s laws. So, in some sense, planets are the cause of our first real understanding of physics!

What does this have to do with Pluto?

I’m getting there! In 1781, William Herschel discovered Uranus, which he initially named The Georgian star in honor of British King George. Amusingly, because people had been learning about the 6 planets of the Solar system for millenia before Herschel’s discovery, no one could even imagine that there were planets left to discover. This led Herschel to first miscategorize Uranus as a distant star. But, eventually, it got its deserved status of planet. There were now 7 planets. Which leads us to the year 1846.

What happened that year?

Early in this year (maybe even in 1845), British mathematician John Couch Adams predicted the location of an 8th planet. He sent its computed coordinates to the Royal Greenwich Observatory. As the story goes, the Observatory, outraged that a man doodling on papers would tell it what to do, refused to make the observation! Eventually, the Royal Greenwich Observatory famously became known as the observatory which did not discover the 8th planet, and Adams got left out of the History of planet discovery.

Wait… How did Adams predict the 8th planet?

Great question! Astronomers noticed that Jupiter was perturbing Uranus’ trajectory when the two planets were on the same side of the Sun. This is merely a consequence of Newton’s laws, and the fact that Jupiter effectively pulled on Uranus when it was close enough. However, weirdly enough, Uranus was also undergoing perturbation, at a point where neither Jupiter nor Saturn could have been the cause! This led Adams to make his bold prediction!

But Adams didn’t get this prediction confirmed…

Indeed. Yet, meanwhile, French mathematician Urbain le Verrier independently made the same prediction. The only difference is, that le Verrier asked the Berlin Observatory to have the prediction confirmed. Which they did. To their great astonishment, as they followed le Verrier’s indications to point their telescopes in the right direction, there it was! Neptune, the 8th planet, was discovered! Check Paul Hewitt’s awesome story-telling of this great achievement of science:

Let me guess… Pluto was predicted just like Neptune was!

Exactly. At that time, people weren’t sure that Pluto existed though. But, because Neptune did feature wobbles in its trajectory, even when it was far from other giant gas planets, a long-lasting search for Planet X, the 9th planet of the Solar System cause of Neptune’s wobbles, was launched. Starting in 1906, with the wealthy American Percival Lowell. Unsuccessfully though, as Lowell died in 1916.

After legal issues of inheritance, Lowell’s search of Planet X finally resumed long after his death and using his wealth. The task was given to young Clyde Tombaugh to systematically scan the limits of the Solar system. Tombaugh used a clever technic to search for Planet X: He searched for moving dots! More precisely, he took pictures of the same locations in space every two weeks. By quickly shifting pictures back and forth, Clyde Tombaugh found out a possibly moving object of the Cosmos. This is what’s explained by this BBC documentary:

Tombaugh’s detection was then confirmed a few months later. Then, the Harvard College Observatory officially announced the discovery of the 9th Planet in 1930, and named it Pluto. So, in essence, Pluto was a planet, because theorists predicted a 9th planet and experimentalists found a moving object in the sky.

Discovery of Pluto

Pluto Size

Didn’t experimentalist see how small Pluto was?

Pictures made by telescopes back then were extremely blurry. The only clue they had was that a blurry spot was moving on a static background! That’s why the observations weren’t accurate enough to refute the prediction that Planet X was nearly as large as Neptune. As measures got more and more precise, Pluto’s estimated size kept decreasing, decades after decades. This endless decrease even led some to predict a vanishing size for the mysterious Pluto, as in the article on the right, subtitled “The Pending Disappearance of Pluto“.

So, Pluto isn’t the Planet X theorists predicted?

Exactly. As it turns out, what disturbs Neptune’s trajectory isn’t a single large object, but a huge collection of small objects which form the Kuiper belt, which Pluto is a member of. Amusingly though, in the 1950s, American astronomer Gerard Kuiper was rather imagining that the big Pluto had formed out of a collection of small objects which would thus no longer exist. But, as it turns out, Pluto isn’t big, and, hence, this collection of small objects still exist today.

Has the Kuiper belt been observed experimentally?

Yes, quite recently. In 1992, David Jewitt and Jane Luu observed the first other objects of the Kuiper belt. Since then, thousands of other objects have been found in this area, confirming Kuiper’s alternative to the massiveness of Pluto.

Let’s Wrap Up

What’s fascinating about the demotion of Pluto is the underlying story of scientific advancement. Science is not a dusty old book of definite knowledge, but, rather, an ongoing construction and structuring of the History of our observations of Nature. This advancement requires frequent questionings of established world views. Not because we like to be rebels. But because History teaches us that we’ve always had a flawed image of the Universe we live in. And this inquiring spirit has led to increasing progresses of our picture of the world. These progresses are the beautiful and exciting aspect of science, and what hugely excite me!

Let me end with today’s picture of the Solar system, which CGPGrey presented at the end of his video. Namely, our Solar system has a Sun, 4 terrestrial planets, many asteroids, 4 giant gases and a Kuiper belts with many dwarf planets:

The Solar System

But as CGPGrey highlights it himself, this image is just an image. In particular, it is infinitely NOT rightly scaled.
So, will this picture remain true?

I don’t know. But I doubt it will remain like that. In particular, there may still be giant gases lying behind the Kuiper belt. Or some other astronomical objects we have never even managed to think of…

One comment to “Pluto is NOT (not?) a Planet

  1. It’s a bit off-topic, but (IMHO), the far more upsetting revelation about our planets was the Sussman/Wisdom paper “Numerical Evidence that the Motion of Pluto is Chaotic” (1988; ). The two MIT professors had created The Digital Orrery; they used this computation device to note that the planets (and the now-classified dwarf planets) have chaotic orbits. Their computation device ran at about 10Mflops on 150W of power; I keep hoping that someone will port their code to an iPhone/iPad. 🙂 The horizon of predictability for Pluto is around 20 million years — far too long for us to have observed astronomically.

    The irony: while Sir Isaac created calculus to map the forces and dynamics of the “clockwork universe”, that same engine was used by Sussman/Wisdom to prove that no clockwork mechanism could ever deterministically model the motions of the solar system (and the galaxy and the universe) over the long run. Did Newton suspect this? Would he be pleased with the finding? Would he be surprised to know that the general public is essentially oblivious to this bit of science over 25 years after its publication? I was hoping that Neil deGrasse Tyson and the writers of the new Cosmos would cover this, but haven’t seen anything yet.

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