Frequently, when we are discussing astronomy with customers (it comes with the territory when you sell telescopes) the subject of Pluto comes up and the customer usually asks us: “So, what happened to Pluto? All of a sudden they decide it isn’t a planet? What do you think of that?!”. A lot of these folks are somewhat upset about it. Not very upset mind you, because it isn’t like the Astronomers who downgraded Pluto slashed their tires or anything, but there is some concern in their voice.
Maybe it is because Pluto was the only ‘planet’ to be discovered by an American, maybe it is a love of the Disney dog with the same name, maybe it is because the mnemonic devices for remembering the planets they learned as a child are now defunct (Mother Very Thought Made a Jelly Sandwich Under No Protest, i.e.), most likely it is just a disruption of what many considered to be an orderly solar system without real reason. After all, if some scientists can just waltz in and say Pluto isn’t a planet they can do it to any of our favorite planets! It seemed unprecedented and without cause.
But the fact of the matter is: there were very good reasons for the re-classification, and things like this have actually happened before.
So where do we start in discussing Pluto? How about where it lives: The Kuiper Belt.
The Kuiper Belt was theorized back in the 1930 with the discovery of Pluto. It seemed to many an Astronomer that it didn’t make sense that Pluto was the only planet beyond Neptune. Add to that its smaller size and composition seemed to indicate that Pluto might be just one of many. For a long time, the Kuiper belt was mostly a theory, but with many elements that made sense (especially with regards to other Solar System objects such as comets), the theory changed a lot over the decades, and nothing could really be proven as technology was not up to finding more objects past Pluto. Even the best astrophotograph of Pluto by the Hubble Space Telescope was mostly a pixelated mess.
There’s a new kid in town
The problems started with the discovery of a new object past Pluto: Eris (at first nicknamed ‘Xena’ after the TV show character). Eris was larger than Pluto, and it became apparent that there might easily be more planets, so it didn’t seem correct to call Eris the 10th Planet just yet. There was much hemming and hawing, but mostly the issue was put on the backburner. The question was deliberately avoided and you would be hard pressed to find any posters printed that show Eris in place as the 10th planet. But soon things came to a head.
Enter the World’s Sexiest AstroPhysicist
The issue came to a head when Neil Degrasse Tyson, Director of the Hayden Planetarium in New York had his Planetarium make a exhibit on the planets of our solar system.
The Planetarium’s display idea was simple: put planets in the categories they fit in rather than their orbit order. It was easy enough: rocky planets like Mercury, Venus, Mars, Earth, and Gassy planets like Neptune, Saturn, Jupiter, Uranus.
Trouble was, Pluto didn’t fit into either of those categories. It certainly wasn’t a gas giant, and it wasn’t rocky like the inner planets. In fact, its composition was more in common with comets than planets! In fact, if Pluto decided to wander closer into out solar system it would actually grow a tail like comets do from the solar winds. It was obvious this wasn’t going to fit anywhere. So Pluto had no display at the Hayden. Soon the media got wind of Pluto’s absence and things started to cascade. In the meantime, a few more objects had been discovered in the Kuiper belt, so astronomers couldn’t ignore the issue anymore.
Just What is it?
The problem, at the core, is that there had never been a real definition of what a planet is. Planet simply means ‘wanderer’ in greek and they were called that because the visible planets meandered independently of the stars. But that was hardly an ironclad definition. In fact this lack of definition had caused trouble before: Ceres.
Ceres was discovered in 1801, and was almost immediately declared to be a new planet. Textbooks and other materials of that era actually would list Ceres as being a planet between Mars and Jupiter. For over 50 years it stayed that way. But soon more and more objects were being found in what is now known as the Asteroid Belt. These objects were soon reclassified as asteroids and Ceres was no longer considered to be a planet, but and asteroid. Like Pluto, it received a demotion.
Define Your Terms
So the international astronomy community started to work on new definitions for what a planet actually is. At first they stumbled, badly. In an effort to not lose Pluto as a planet it was first decided that a planet was pretty much anything in the solar system that was large enough to have a spherical shape and wasn’t a satellite of another planet. This, of course, was a failure of a definition as not only did it mean that all the new Kuiper belt objects were now planets, but Ceres was also a planet again! This obviously was not going to work.
So a new definition was worked on, this time it was decided to not only define a planet, but also develop a new definition for what Pluto et. al. were as well. It was decided that a planet was:
a) Large enough to be spherical from its own mass
b) Has sufficiently clear its orbital path of other large objects.
And that was that. Pluto had the first part, but didn’t even come close on the second. And so Pluto was demoted for not cleaning its room.
For objects that were round but didn’t meet the other qualification, a new term was invented “Dwarf Planet“. The definition of a dwarf planet was that it simple be massive enough to be spherical (larger objects become spherical because of the mass of the gravitational pull on themselves overcomes other factors). So Pluto, Eris, Ixion, Orcus, and even Ceres. Yes, little Ceres got promoted by this new definition. The only dwarf planet not in the Kuiper Belt.
And so is the story of how Pluto became a planet. This actually was the short version, you can imagine how long the full tale was.
Oh, and for a bit of trivia, Mike Brown, the guy who discovered Eris, which might have been the tenth planet but instead triggered the de-planeting of Pluto? Well, he now twitters under the name plutokiller.
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Filed under: Astronomy, NASA, Science History
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