Why couldn't Pluto have stayed a planet?

For an unassuming little ball of ice several billion miles away from the sun, a lot of people were strongly invested in Pluto’s status as a planet. It was the end of the mnemonic to remember the order of the planets - My Very Excellent Mother Just Served Us Nine Pizzas - and we had a lovable Disney animal named after it. It’s no real wonder that people were upset.

But the truth was, Pluto had never fit in very well with the other 8 planets. The orbit of Pluto, unlike the rest, which all lay in a very tightly defined plane, was wildly askew. It’s tilted by about 20 degrees relative to the rest of the planets. Also unlike the rest of the planets, which travel around the sun in almost perfect circles, Pluto’s orbit was extremely elongated. And it was a tiny planet. Pluto is big enough to have compressed itself into being a sphere (instead of the lighter and irregularly shaped asteroids), but it had a moon that was almost as big as itself. None of the inner 8 planets behave this way.

But we were more or less okay with ignoring these problems with Pluto and leaving it as a planet, since up until that point we hadn’t found any other objects of a similar size in the solar system; everything else was much smaller. It made sense to leave it as a planet if it was a unique object. But this is where Pluto started running into problems - we began to realize that Pluto wasn’t unique. Our ability to detect similarly sized objects got a lot better, and all of a sudden we had other objects that were about the same size as Pluto. Most notably, there was Eris, which was even further out from the sun than Pluto, and bigger than Pluto.

Now a decision had to be made.  Either we allowed Eris in to the solar system as the 10th planet (if Pluto was a planet, Eris surely was), or we would have to come up with a better definition of what a planet was.  The problem with including Eris and Pluto as planets was primarily that Eris’ discovery was proof that Pluto was not unique, it was merely the first in a class of objects we’d been unable to detect so far.  That lack of uniqueness meant that we’d be constantly adding new planets to the list as more of them were discovered, and we were guessing that there would be a lot of these objects out there.

So the International Astronomical Union decided to impose a set of 3 criteria that any object in the solar system must pass in order to qualify as a planet.  To gross public dismay, Pluto no longer qualified.  The criteria were the following: a planet must orbit the sun, and not another, smaller object. (In other words, it can’t be a moon.)  It must also be sufficiently massive to have compressed itself into a sphere, and finally, it should have cleared the area around its orbit of other objects.  Pluto failed the last criterion, which meant it went into the newly minted “dwarf planet” category. This dwarf planet classification distinguished them from the irregularly shaped objects that orbit the sun (like the asteroids), but swept them out of the main ‘planet’ category.

Ultimately, the decision to reclassify Pluto was a choice to make our definitions more consistent, which in the long term means there are fewer revisions of our textbooks. Our understanding of the solar system will gradually become more and more complex as time goes on, so it makes sense to let our classifications also reflect the complexity of nature. In the mean time, our very excellent mothers will just have to serve us nachos or naan instead of pizzas.

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