Why we lost Pluto

Eight planets. How annoying is that? It’s as though the entire scientific community is conspiring to make us stupid by reversing everything we learned in grade school.

Like most red-blooded Americans, I was upset when they demoted Pluto to “dwarf-planet” status. What did that even mean? And who cared about those other non-Disney-related dwarf planets it’s suddenly known with?

Come closer and I’ll let you in on the joke. It all starts with planet Ceres.

Ah, Ceres. No, I’d never heard of it, either. I believe that if they’d told us the history of that little guy, we’d all have come to embrace the dwarves.

Once upon a time, (1801 to be exact,) an Italian astronomer discovered Ceres, thereafter commonly known as the 8th planet (Neptune and Pluto hadn’t been discovered yet). For years, every school textbook listed it among the planets and people got very excited about it.

Then, just like Pluto, it got put down.

Why? Well, I haven’t yet told you where it lives. And this is where it gets a bit embarrassing for the people who published textbooks in 1801: little Ceres is between Mars and Jupiter, smack-dab in the middle of the asteroid belt. Of course, they didn’t know about the asteroid belt then. Their telescopes were only just becoming powerful enough to see that far. In the 50 years that followed, so many asteroids were found in that area that they stopped getting excited and naming them … and little Ceres was quietly erased from the list of planets.

So what does this have to do with Pluto? Everything. It turns out, Pluto resides in an asteroid belt of it’s own, known as the Kuiper Belt, and the reason most of us don’t know that name is that our current telescopes are only now getting powerful enough to spot all the little balls of ice floating at that distance.

So here’s the joke: just like Ceres, Pluto has much more in common with those asteroids than it does with any planet. In retrospect, we should have realized that we were jumping the gun once again, but … it’s funner to discover planets than asteroids, now, isn’t it?

So will they change their mind about the other 8 planets, just when we’re settling into this? No. I assure you, those are safe. (The one thing that our main planets have in common that nothing else we’ve found so far does is this: they circle our sun in perfectly round orbits. Every asteroid and dwarf planet has an elliptical orbit, not necessarily centered around the sun, suggesting that they were products of collisions. Dwarf planets, meanwhile, differ from the common asteroid only by being much, much bigger and having enough self-gravity to become round.)

But if we find something else hiding out between here and Neptune, I’m going to tell my kids not to bother learning its name.


Incidentally, the discovery of the other 3 dwarves is fascinating story, one that I can’t tell nearly as well as the man who discovered it. I highly recommend that you read “How I Killed Pluto .. and Why It Had It Coming” by Mike Brown.
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