Pluto isn't a planet...and the debate matters!

I got a heart tattoo (sniffles) guess that doesn't matter now...

It’s all fun and games until someone loses a planet
    In 2006, the International Astronomical Union decided that Pluto was no longer a planet and was instead to be referred to as a “dwarf planet”.  Outcry ensued and eleven years later it has not abated.
    The physicist Sean Carroll writes in one of his recent books “Pluto is the ninth planet and it’s my book so I’ll call it what I like”, while Neil deGrasse Tyson writes in one of his own “Pluto isn’t a planet, get over it.”  There’s even an episode of Rick and Morty where Jerry delivers a speech to the Plutonians, declaring that Earth’s scientists were mistaken in reclassifying it.
    The man largely responsible for the monumentous decision, Mike Brown, uses the twitter handle @plutokiller and has the Death star destroying Alderaan for his banner picture.  So perhaps it’s all a matter of whimsy and tongue-in-cheek sport.   Pluto is, after all, the furthest planet/dwarf from the Sun.  Does it really matter what we call it? 
    I am going to argue that it does, not because  astronomical terminology is crucial to our lives but because this debate reflects something important about how Science operates.  So hold onto your preconceptions folks!  Well, actually don’t.  Let go of our preconceptions.  But hang onto something.

I gotta suggestion...

I’m a Believer
    I remember hearing the Pluto news on the radio and thinking it was pedantic nonsense.  You can’t just change what Pluto is because someone decides to tweak a definition!  I had images of pencil-pushing smart-alecs smarming away to themselves at how clever they were, with no concern for public opinion.
    Don’t misunderstand me here, public opinion does not dictate truth and reality is not flexible.  But the definitions of words are, and the accepted meaning of a word should reflect its common usage.  If everyone agrees on a particular definition, an organisation would be foolish to redefine it. 
    I also remember thinking the whole thing was bad for Science PR because organisations like the IAU should serve the public not dictate to them.  If we use the word “planet” to refer to something which Pluto clearly is, that’s enough reason to preserve its status.  But here’s the thing: Pluto doesn’t match the public definition of a planet.  That’s why the IAU changed it.     
    What I was getting wrong eleven years ago was that the IAU genuinely was taking public opinion into account.  The reclassification of Pluto was done out of respect for the lay public, not in spite of them.

A typical IAU member...I assume

The First Planets
    Every ancient culture monitored the skies, charting the mysterious lights which roam above our heads, and every single one of them made the same discovery.  The majority of the twinkling dots follow a clear pattern, changing position on a predictable 365 day-cycle...but five of them do not. 
    Five of the bright sky-things move on bizarre trajectories, weaving and wailing without rhythm or logic.  The Greeks called these five objects “wanderers” (planetes in Greek) because they appeared to wander as if conscious beings.  They were assumed to be Gods and were identified as Hermes, Aphrodite, Ares, Zeus and Chronos, later re-named for their Roman counterparts Mercury, Venus, Mars, Jupiter and Saturn. 
    The first definition of “planet” was therefore extremely simple.  A planet was one of the bright lights which moved in non-predictable ways.
    But thanks to the work of people like Eratosthenes, Ptolemy, Newton, Buridan, Copernicus, Kepler, Brahe and Galileo, we figured out that the planets were following a pattern, albeit a complex one. 
    The Sun was sitting at the centre of a circular plane with the planets orbiting at different speeds, one of which was the Earth we stood on.  Sometimes Earth would be behind another planet and sometimes it would overtake it, giving the impression of the other planet zig-zagging across the sky - what astronomers call retrograde motion.
    To further complicate things, it turned out this view was only about 90% accurate.   Firstly, planets move in ellipses rather than circles and secondly, they aren’t going around the Sun at all.  Planets and the Sun are actually orbiting each other, it’s just that the Sun is so much bigger so its movements are small.  If you assume the Sun is stationary with planets moving around it (what you were probably taught in primary school) you will get the wrong answers when trying to account for planetary motion. 
    Nature does complicated things so we have to accept equally complicated explanations, even if they contravene what we learned when we were young.

THIS! IS! ASTRONOMY!!!

Six and Beyond
    By the 18th Century, the definition of a planet had evolved to “something which shares a common centre of mass with the Sun and has a fixed elliptical orbit”.  In fairness, that definition is a mouthful so “things which orbit the Sun” will do in a pinch.  And there were six planets rather than five, because Earth was one of them.
    Then in 1781, the astronomer William Herschel discovered that one of the dimmest stars visible to the naked eye does the retrograde-motion thing.  By carefully measuring its position with a telescope, Herschel realised this object wasn’t a star at all, it was orbiting our Sun.  This made Herschel the first person in modern history to discover a planet, yielding Mercury, Venus, Earth, Mars, Jupiter, Saturn and George.
    The name George didn’t catch on in France however, where King George was despised, so it was eventually renamed after the God of the sky: Uranus.  One of the most majestic and powerful figures in classical mythology.  Today, it has come to mean something else...well...strictly speaking it should be pronounced “yor-ann-us” but the other way is definitely more fun.  As a physics teacher I’m pretty sure I’ve heard every permutation of this joke but I have to be honest, I still find Uranus hilarious.
    Then in 1801, Giuseppe Piazzi discovered the eighth planet, Ceres, lurking between Jupiter and Mars.  Ceres was the smallest planet discovered to date, at least ten times smaller than the moon, but it orbited the Sun just like the others, so Jupiter was bumped down the list to become the sixth planet, Saturn the seventh and so on.  Inconvenient, but as a scientist you change your view when the data forces you.
    A few months later Heinrich Olbers discovered another planet at the same distance to the Sun, which he named Pallas.  Then in 1804 Karl Harding discovered Juno.  In 1807 Olbers discovered Vesta and in 1845 Karl Hencke discovered Astraea.
    The thirteenth planet was a little different though.  This one was discovered by equation rather than telescope.  In 1821, Alexis Bouvard was taking precise measurements of Uranus (hur hur hur) and found that it didn’t move in a standard ellipse.  Instead, it seemed to be pulled to the side as if there were another object attracting it and in 1846 Johann Galle finally observed it with a telescope, giving us Neptune. 
    Then Karl Hencke discovered the planet Hebe in 1847 along the same Mars/Jupiter orbit as most of the others.  The fifteenth, Iris, was discovered the same year by John Russell Hind, the sixteenth, Metis, in 1848 by Andrew Graham and the seventeenth, Hygiea, in 1949 by Annibale Gasparis.  Hold on a moment...

Back up, back up
    Any textbook on astronomy in the 1850s would have listed our solar system as boasting seventeen planets.  But as our telescopes got better we discovered more and more objects floating between Mars and Jupiter and by the 1860s there were over a hundred of them, which led to a problem. 
    When people heard the word “planet” they imagined great big round things with their own orbits, not scraggly space-debris circling the Sun like a moat around a castle.  Either we kept the definition of planet to mean “thing which goes round the Sun” or we start using it the way the general public used it, even though it would disqualify the rocks between Mars and Jupiter.  After much deliberation we went with the second option. 
    Although never formally defined, astronomers started using the word planet to refer to what the general public thought the word meant.  This meant we needed a new word for the thousands of rocky clumps swimming between Mars and Jupiter and the term “asteroid” was coined.     
    Really, the problem arose because language evolves slower than Scientific knowledge.  We get a word like planet in our vocabulary and it hangs around for hundreds of years, colouring our perceptions.  If we discover that reality has nuances to it, we either keep using the old terminology or we invent a new word to describe the stuff we didn’t originally know was there.   

Seriously, what the hell is going on here? Goofy's a dog...walking a dog...

The goofy story about Pluto
    In 1906, the astronomer (and millionaire) Percival Lowell decided it was time we discovered a ninth planet.  He had good reason to suspect there might be something there - minor disturbances in Neptune’s orbit - but mostly he was motivated by the passionate desire to look beyond the edge of what was known.  He poured a lot of money and resources into searching for “Planet X” and hired some of the world’s best astronomers to work at his observatory.
    Sadly, Lowell died in 1916 before Planet X was discovered, but the mission continued in his absence.  Under the direction of Vesto Slipher (who also discovered the redshift effect) Clyde Tombaugh was set the task of searching the sky beyond Neptune and on February 18th 1930, he captured images of what Lowell had hoped for - a ninth planet, roughly the size of the Earth.
    Planet X-fever gripped the world and international headlines proclaimed the discovery of the first proper planet since Neptune.  A competition was held to decide what we were going to call it and over a thousand names were suggested.  The name Pluto was proposed by eleven-year-old Venetia Burney, and ultimately won by popular vote.  
    By 1948 however, precise measurements were taken on Pluto’s size and it turned out we had been a little premature in declaring it the same mass as the Earth.  It was actually about a tenth as heavy.  Never mind though, it was still bigger than Mercury. 
    Except it wasn’t.  By 1978 we learned that Pluto was actually about a six hundredth the mass of the Earth, smaller than Mercury and even our planetary moon, making it the smallest planet in the solar system.  But it still satisfied the main criterias for it to be a “planet”.  It was orbiting the Sun, it was big enough to be round and it occupied a unique orbit.  Except it didn’t.

This is why I wear braces folks

The Second Belt
    In 1992, the astronomer Jane Luu discovered a second object floating on Pluto’s orbit which she nicknamed Smiley but was given the official designation 1992-QB1.  Then in 2003, the astronomer Mike Brown discovered an asteroid at the same distance, which he called Sedna.  He went on to discover Haumea and Orcus in 2004, and then Makemake in 2005.  But then, most disconcertingly, Brown discovered Eris, which turned out to be 25% heavier than Pluto. 
    We can argue that Pluto is a planet on the grounds of it being round, and we can dismiss all the small rocks nearby as asteroids.  But when we discover objects heavier or bigger than Pluto on the same orbit, it’s time to rethink things.
    Turns out there are over 2,000 objects orbiting past Neptune and Pluto is only one of them.  Our solar system doesn’t have one asteroid belt, it has two!  This second one has been called the Kuiper belt (pronounced Kie-pur) and its asteroids are very different from the ones we’re familiar with.  A lot of them are huge chunks of ice and rock, often many times bigger than planetary moons.  Pluto, it turned out, was Ceres all over again - the first object discovered in an asteroid belt and accidentally labeled as a planet. 
    So what do we do?  If we keep calling Pluto a planet then we're misleading people.  It’s not very big and it’s not a lone body, it’s just a fat asteroid which happened to get noticed first.  But if we want to keep calling Pluto a planet, we need to redefine what that word actually means.
    Eventually the IAU decided to repeat what was done in the 1860s.  The definition of planet was fixed in people’s minds, so we left it and came up with a new word to fit the new thing: “dwarf planet”.
    The definition of a planet is the same as it always has been.  Something which a) goes round the Sun, b) is roughly spherical due to gravity and c) has cleared its orbit path so it’s the only dog in town.  A dwarf planet is something which hasn’t done the third one...it’s big enough to be interesting, but it’s part of an asteroid belt.  This means our solar systerm really has six dwarf planets: Ceres (reclassified from asteroid), Pluto, 2007-OR10, Eris, Haumea and Makemake.  And there’s a good chance more will be discovered in the Kuiper belt with time.
    I think the IAU made the right call.  They were faced with either inventing a new word or changing the meaning of an old one.  And the former option is usually the better idea.  You can’t force people to change the words they’ve always used, but you can introduce new ones.

This should clear up any confusion

When I was a young warthog...
    People get annoyed about the whole thing because Pluto, it would appear, has been unfairly demoted.  But the thing is, it hasn’t at all.  Pluto hasn’t been changed into a different thing - we just discovered what it was all along, like taking the mask off a Scooby-Doo villain.
    Imagine you had nine spoons of sugar in front of you.  You’re told by everyone that it’s definitely sugar in each one and you believe that for a long time.  If you eventually discover the end one is really salt, what you’d  say is “oh, I guess we made a mistake”.  It would be bizarre to say “I’ve always been taught there are nine tablespoons of sugar and I still believe that’s true.  I’m going to redefine what I mean by sugar as ‘any white powder’.” 
    You’re welcome to do that of course, but in doing so you’re bending the definition away from what everyone means.  You’ve also redefined the word to include things like sherbert and powdered glass.  Unless you’re extremely stubborn (in which case can I watch you eat your powdered glass cake?) you know what the sensible thing to do is, even if you don’t like it.  The intellectually honest approach is to accept that you were taught a mistake.  It wasn’t anyone’s fault and nobody lied to you, but you got told something incorrect. 
    So why do people object to learning the truth?  Why do people get upset when a faulty fact is corrected?  Shouldn’t that be a good thing?
    In the process of writing this blog I consulted with my father, a passionate astronomer (the guy has a five-foot Russian-built telescope with a motor to compensate for Earth’s rotation in his garden shed) and he made a very important point: for a lot of people, this kind of thing can be more about emotion than intellect.  If you grow up learning something, it can feel like the rug being pulled out from under you if it turns out to be wrong.
    This is a fair point.  When I tell the Pluto story to my younger students they are fine with it.  I explain that there was a large asteroid which got mistaken for a planet and as soon as we realised the mistake we corrected it.  There is no objection to this because “it was mistakenly identifed as a planet” is part of the fact they learn.
    It’s only when we are victims of the mistake that it can be a human instinct to fight back.  Intellectually we might accept Pluto’s status, but emotionally we are irritated because we are creatures of habit and familiarity. 
    The same way people objected to Ceres and Pallas being reclassified in the 1860s, people in the 2000s objected to Pluto going the same way.  And, just like Ceres and Pallas, people growing up after that decision are fine with Pluto being a dwarf planet.  Finding out as an adult that one of your childhood facts was wrong can feel like a piece of your childhood has been knocked away.  Nobody likes having their childhood messed with.

There are many things wrong with this poster. Most of them are wearing a brown jumpsuit

Why it Matters
    Science offers us insight and knowledge, but it comes at a price - we have to be prepared to let go of familiar beliefs if they turn out to be wrong.  This is one of the hardest parts of Science but it’s also one of the most important.  It’s the reason we no longer believe the Earth is the centre of the Universe.  It’s the reason we no longer believe the planets are Olympian Gods.  It’s the reason we make progress in the first place.
    And it doesn’t have to be a bad thing.  Alright, we lost a planet.  That sucks.  But technically we gained six dwarf planets as well, so if you want a solar system full of planets, the 2006 ruling gave you exactly that.  And, most importantly, we gained a deeper understanding of how complicated the solar system really is.
    There are eight planets, hundreds of moons, thousands of asteroids in two different belts (as well as two clumps of asteroids called the Greeks and Trojans orbiting near Jupiter) and probably dozens of dwarf-planets.  Not to mention comets from the Oort cloud.
    We had to abandon our simple view of reality to get to this astonishing point, and it’s very probable some of what we currently “know” will turn out to be wrong ten years from now.  When people are young, they learn a simple view of reality, just as out entire species did.  Science is the thing which allows us to move beyond that and gain a more sophisticated and beautiful view of the Universe.  It can be painful letting go, but it can be eye-opening and wonderful as well.

Don't tell me this isn't cooler than what you learned in primary school!!!

   
Right, now let's deal with this whole "conventional current" malarky...

Image credits
Pluto: img-purch
Mr Arnold from Jurassic Park: blogspot
Arrogant IAU Member: ehowcdn
King Leonidas: huffingtonpost
Fred Durst: impericon
Pluto and Goofy: urdogs
Double belt: blogspot
Diagram: trivialdiscourse
The Last Jedi: Wallpapersite
Orbit animations: exploremars