The Question I Always Get: Pluto

I get asked a lot of questions at the Planetarium. Many are about black holes (no, none are close to us). Many are about the Sun exploding (it won’t). And many—like…a lot—are demands to know what happened to Pluto and how astronomers like me could be so cruel to such a beloved object.

For the record, I had absolutely nothing to do with Pluto’s 2006 reclassification from planet to dwarf planet/Kuiper Belt Object (KBO). But I do happen to agree with it. So in honor of the anniversary of its discovery let’s talk about what exactly did happen to Pluto, why some folks got so upset, and why I personally think it’s justified. 

Odd Planet Out

Pluto was discovered on February 18, 1930 by a young man named Clyde Tombaugh searching through images taken the previous month by the Lowell Observatory in Arizona. What he was actually looking for was “Planet X”, a theoretical fifth gas giant that some bad math had suggested must be out there perturbing Uranus’s orbit (turns out we just had guessed Neptune’s mass incorrectly—but that’s a story for a different post). 

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Pluto was quickly found to definitely not be a gas giant. When we first discovered it, we guessed that it might have about the same mass as Earth. That number was revised continually downward throughout the 20^th century. Today we measure Pluto’s mass at about 0.002 Earths. That fits with our measurements of its radius, about 0.19 Earth radii (or about 2/3 the radius of the Moon). It means Pluto is notably smaller than anything in our solar system classified as a planet. Heck, it’s less than half the size of Mercury, and Mercury is a wee little planet!

Then there’s Pluto’s orbit. It’s quite different from the orbits of the planets. The planet orbits are all pretty circular, although Mercury likes to be a little on the eccentric side (ha, that was a pun. A bad one, but I’m not good at puns, ask anyone on the Planetarium team). Pluto’s orbit has an eccentricity of about 0.25—decidedly uncircular.

What’s more, because it’s so eccentric, the Sun is definitely not in the middle of Pluto’s orbit. Technically all the orbits are ellipses with the Sun at one of two “foci”. It’s just with most of the planet orbits they’re close enough to circular that the Sun is very close to being in the center of the orbit. Pluto’s is eccentric enough for the Sun to be noticeably to one side, which means Pluto is much closer to the Sun at some points in its orbit than at others (and, in fact, it gets closer to the Sun than Neptune does at certain points). 

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And then there’s the tilt, or inclination of the orbit. The orbits of the planets all align pretty well with the Sun’s equator (here Earth is actually the weird one, with its orbit tilted by over seven degrees). Pluto’s orbit has an inclination of over 17 degrees, and with the wide radius of its orbit that means it’s doing a big up-and-down that sticks out in any diagram of the solar system.

Point is, Pluto does not orbit like a planet. And with its small size and composition of ice and rock, it doesn’t really look like any of the planets either. So why did we spend 76 years calling it one?

What’s a Planet Anyway?

When Pluto was discovered we didn’t have a definition of what made something a planet. As a result over the years our solar system has officially had between 5-16 planets. For instance, when we first started discovering asteroids in the 1800s, we called them planets because they orbited the Sun and at the time we called everything else orbiting the Sun a planet.

Then we realized they’re small compared to the planets. And we realized their orbits are more inclined and eccentric than the planet orbits tend to be. And we realized there’s a lot of them in a relatively small amount of space, unlike the planets which are spread out. Sometime around the mid-19^th century astronomers clued into the fact that these were a new class of object and made up the name “asteroid” for them (a word, by the way, that still has no official definition) and we went back to having, at that point, eight planets.

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When we found Pluto, we started calling it a planet pretty immediately, before we knew how weird it was. When we did know it was weird, we kept calling it a planet because…well, what else were we going to call it? It wasn’t a star, or a moon, or an asteroid. And we were already calling it a planet anyway, it was easy to just keep doing so. 

Then in 1992, we discovered a thing called Albion. About as far out as the farther end of Pluto’s orbit, it suggested there might be more out there. And indeed six months later we found another object out there. And another. And another. And another. Turns out our solar system has a second belt besides the asteroid belt between Mars and Jupiter. Since its existence was predicted by Gerard Kuiper in 1951, it was dubbed the Kuiper Belt.

And all of these Kuiper Belt Objects are small, icy things with inclined, eccentric orbits hanging out mostly past Neptune’s orbit. Which sounds an awful lot like Pluto.

The Controversy

Still, Pluto was waaay larger than any other thing we were finding in the Kuiper Belt. It was not necessarily illogical to still see it as special and to keep calling it a planet, and it was definitely the easiest thing to do. That, of course, is when things got tricky.

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In 2005 an object was discovered very far from the Sun. Technically it’s not generally referred to as a KBO, but as a trans-Neptunian object or a scattered disk object, but it hangs out in that general region of the solar system. Its discoverers referred to it as Xena, after the warrior princess. And Xena was a problem. 

It’s basically the same size as Pluto. If Pluto is a planet, then it’s hard to argue that Xena is not. If you’re not going to call Xena a planet, it’s hard to justify calling Pluto one. Suddenly the astronomical world needed an official definition of what made a planet a planet.

In 2006 the International Astronomical Union (IAU), which is in charge of all official designations in space, declared that to be a planet you had to 1. orbit the Sun (which Pluto does), 2. be round (which Pluto is), and 3. gravitationally clear your orbit (which Pluto, as an undeniable member of the Kuiper Belt, does not).

Now to be clear: not all astronomers agree with this definition, and it’s not particularly well worded. But the goal was to stop there from being a potential raft of new planets being added to the solar system, since nobody knows how many other Pluto-sized objects are hiding in the far reaches.

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And the world went ballistic. This scientific reclassification of an inanimate ball of ice and rock became a horrific act inflicted on what seemed to suddenly be everyone’s favorite planet. To this day I get children young enough that Pluto has not been a planet in their lifetimes asking me about this awful injustice because their parents are still upset about it. One parent once asked me how astronomers dared to do something like this to the children, and I admit I was too busy processing that statement to give a good response. 

The IAU tried to assuage everyone by creating the “dwarf planet” designation, but that really just confuses the matter. It doesn’t meant something is still a planet in any way, it just means it’s something that hits points 1 and 2 in that official planet definition, so essentially round asteroids and KBOs. And some of them aren’t that round! Full confession, I hate the term “dwarf planet”.

For the record, it was in the same year of 2006 that the IAU also gave Xena an official name: Eris, for the Greek goddess of discord and strife. It was a name well-earned.

Perspective

Look, there was no way we were keeping nine planets. The precious My Very Elegant Mother mnemonic was going to change no matter what thanks to the discovery of Eris. It was either going to be eight planets or 10+ and keeping the eight big things that are not part of either of the belts just makes more intuitive sense. Though I agree the planet definition’s wording could use some work, classifying Pluto as something else works, given its orbit and size and composition and placement. 

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Changing old labels in light of new information is part of what keeps science accurate. If we stuck with old labels for the sake of nostalgia, we’d all still be calling dolphins fish. And, as an icy outer solar system lump, I can assure you that Pluto was in no way harmed by this reclassification.

And you know what? It’s time we all stopped putting planets on such a big pedestal! Being a planet doesn’t make you special. It mostly just makes you big. Being interesting makes you special in the solar system. The planets are interesting, don’t get me wrong, but they’re not the only ones.

Look at the outer solar system moons like Europa and Titan that we think could support life! Look at asteroids like Bennu and Ryugu! Look at comets like Churyumov-Gerasimenko! Look at the Moon! These are all non-planet objects that we’ve sent (or are sending) spacecraft to because they are fascinating worlds, no matter how they’re classified.

And Pluto is too! When we sent the New Horizons spacecraft flying past it in 2015 it revealed a dynamic, magnificent, intriguing, unique world that we are still studying today. Pluto was never awesome because it was a planet. It was awesome just because it was Pluto. 

And that will never change.

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