PSA: Why Pluto isn't a planet · 7:32pm Jul 28th, 2015
First off, Pluto's really, really cool. Pun not intended.
Like, look at that thing. A couple years ago, it was a smudgy, low-resolution blob at the edge of the solar system. Now it's this gorgeous thing, with mountains, an atmosphere, a geologically active surface and massive glaciers made of nitrogen ice.
For the most part, the awe-inspiring pictures New Horizons has been sending back seem to have rekindled a bit of the interest in space exploration. A couple friends of mine who can only name Jupiter's Galilean moons after a quick Google search were talking about the New Horizons imagery. It's nice to see.
Unfortunately, those pictures and findings also seem to have rekindled that irritating "Pluto is totally a planet" debate. I'm not naive enough to think I can stop that debate single-handedly, but I can at least give my followers (those of which care, at the very least) a bit of closure, or explanation.
Here's the big, take-home message: Pluto's demotion was of a political nature, not a scientific one. See, before Pluto was demoted, it was the ninth known body in the solar system big enough to be called a planet. At that time, the International Astronomical Union (IAU) had two criteria for a planet: it had to be large and massive enough to maintain a roughly spherical shape, and it had to be orbiting the Sun, not another planetary body.
Then Ceres was discovered, and then Eris, and then Makemake and Haumea. The first is the largest body in the asteroid belt (the one with the infuriating bright spots), and the others are Kuiper Belt objects beyond the orbit of Pluto.
At that point, the IAU had to make a decision. They didn't want the solar system to have more than ten planets, as that would make them too difficult to memorize, or something asinine like that. So they added a third, more nebulous criteria to the "planet" definition. That criteria was that the object had to have "cleared the neighbourhood" in its orbit, meaning that any other body sharing roughly the same orbital path had to have been either ejected via gravitational slingshot, caught in a stable orbit as a moon, or repurposed as an impact basin on the object in question. This criterion is what killed Pluto's planet status.
I should also note that this definition only applies to our own solar system. The definition does not apply to exoplanets for a number of reasons, mainly because exoplanets cannot be directly imaged with current technology.
Basically, the decision was to either lose one planet or to add four or more. And, quite honestly, having to memorize thirteen planets in middle school would be a lot more annoying than memorizing eight. So unless you can come up with a better planetary classification that includes Pluto but excludes the other dwarf planets (and all future candidates), Pluto's staying a dwarf planet.
Not being able to remember 13 vs remembering 8 isn't really a good reason for planetary status.
The fact Pluto's orbit is
1) really elliptical, (part of which is actually INSIDE Neptune's orbit)
2) inclined quite a lot
Is also not a good reason to not include Pluto. What if we found a 2x Earth sized, Earth like planet that had a similar orbit. Would that be classed as a planet or dwarf planet?
3277253 At risk of sounding like a know-it-all, the orbit doesn't matter. The criteria asks whether or not there's any crap (asteroids, comets, et cetera) occupying roughly the same orbital path. For example, Mercury has a similar level of eccentricity in its orbit (~0.21 to Pluto's ~0.25). But it still counts as a planet because it's not part of a belt of material. If said super-Earth was in our solar system and had not "cleared the neighbourhood," it would be classified as a dwarf planet regardless of its size, orbit, inclination, et cetera. That said, it'd probably have a deep enough gravity well to clear its orbit anyway (unless it was still in the middle of accreting, it which case it'd probably be called a protoplanet).
Finally, someone with sense. Also, I hate it when people think demoting Pluto to a dwarf-planet somehow makes it any less interesting. Spoiler alert; it doesn't. Pluto is the same amount as interesting as it was as when it was a 'true' planet.