Pluto Pique

Pluto Not A Planet

Pluto And Your Mom


Okay, so the second meme is kind of funny in that it appeals to my inner teenager. It’s still Stupid (as are most things that appeal to my inner teenager).

I’m pretty sure that none of these pictures actually depicts Pluto. The first one is definitely Ganymede, the largest moon of Jupiter, although the color appears to have been tweaked a little. The last image depicts…well, I’m not sure. But it isn’t Pluto.

Here are the best images we have of Pluto until the New Horizons probe flies past it in 2015.

Pluto Images Hubble

Look, I understand the Pluto grief, especially in the United States. Pluto was the only Solar System planet discovered by an American, so its demotion in 2006 by the International Astronomical Union (IAU) must have seemed like a blow to American pride. Besides, kids around the world grew up learning about the nine planets. Sure, Pluto was a bit of an oddball with its eccentric, highly-tilted orbit, but that’s what made it cool. Pluto was kind of like that quiet kid who never really got to know anybody, but he could make those awesome paper airplanes that would fly for miles. Yeah…Pluto was like that kid.

So yes, I get why people were upset…angry…maybe a little betrayed…by the sudden switch. Except it wasn’t so sudden. The motivation that eventually led to Pluto’s demotion had been brewing for years. Astronomers started to find other ice-rocks similar to Pluto orbiting the Sun far, far out in the frigid realm of the Kuiper Belt. Some of these objects were as large as Pluto, if not larger. It was becoming increasingly difficult to defend calling Pluto a planet if you weren’t going to afford planetary status to all those other rockcicles.

I imagine that astronomers were getting uncomfortable attending parties with all the irate parents demanding to know if their kids were going to have to memorize ten – excuse me, twelve – scratch that, thirteen planets. I envision a lot of awkward throat clearing and desperate attempts to change the subject.

It didn’t help that there wasn’t even a formal definition of the word planet. Much like Supreme Court Justice Potter Stewart, who knew obscenity when he saw it, some astronomers felt that they knew a planet when they saw it. Unfortunately this highly subjective means of identifying planets wasn’t exactly rigorous. It was time to make a decision.

That’s exactly what the IAU did during their conference in Prague in 2006. At long last, after years of debate, an official definition existed for the word planet.

The definition, which did almost nothing to quell the controversy, has three criteria. In order to be considered a planet, an object must:

  1. Orbit the Sun. Pluto passes this test, but so do all the other objects that were making things difficult to start with, so no help there.
  2. Be large enough to pull itself into a sphere by its own gravity. Remarkably, Pluto also passes this test. That means that Pluto’s demotion has absolutely nothing to do with its size! Do you hear that, funny-but-stupid meme in the middle?
  3. Clear its orbital neighborhood of debris. And here is where Pluto loses out – along with all the other bodies whose planetary status hinged on this very decision. Pluto orbits the Sun in a region called the Kuiper Belt, which is occupied by zillions…no, gigajillions of other bodies. The combined mass of these bodies is greater than the mass of Pluto itself, so you can’t really say that Pluto has cleared it’s neighborhood. In fact, Pluto starts to look a bit like a hoarder.

Now I don’t really have a problem if you’re disappointed in Pluto’s demotion. (Although come on, man…it’s been seven years. Time to move on.) I do have an issue with people who don’t understand the scientific reasoning that went into the decision. Just so we’re all clear:

  • It’s not the size that counts, as long as a planet is large enough to be round. Pluto’s got the mass to pull it off.
  • There are lots of objects besides planets that orbit the Sun. Yes, planets consume the vast majority of the non-Sun mass in the Solar System, but when you consider numbers alone, they are a tiny minority. Pluto is in good company.
  • The IAU’s definition of planet says nothing about moons. If it did, then Mercury and Venus would be excluded from planetary status.

For my part, I like Pluto. I can’t wait to see the detailed photos that return from the New Horizons probe when it zips past the Pluto system. But I’m not going to worry too much about what we call the object in the pictures. You shouldn’t either.


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