Oh my poor friend, buckle into your spaceship and prepare for 10 Gs of space knowledge that you did not ask for, because boy do I have a rant for you.
So our story begins in 2006, when the IAU revoked Pluto's planetary status. Contrary to the common narrative, this really has very little to do with Pluto and everything to do with the fact that astronomers just. Kept finding celestial bodies that probably should be called planets too. Like poor Ceres, a planet in our asteroid belt that everyone forgot about like that one kid who sits in the corner of the cafeteria that no one talks to. Anyways, in '05 Eris is found and the IAU says, âwe need to come up with a new definition for planets, because we canât just keep calling everything a planet. Thatâs too messyâ as if science was just going to fit into the neat little boxes we make for it and not chew them to pieces like the nasty little creature that it is. Well, in 2005 top NASA engineer and planetary scientist Alan Stern writes a paper talking about how we should define subcategories for planets to distinguish them, with dwarf planets being one of the two. The IAU says âHey thatâs a neat concept! What if we steal that terminology and literally nothing else.â
The IAU then defines three criteria for what constitutes a planet:
It must orbit the sun. (Not âa star.â Very explicitly our sun.)
It can maintain a spherical shape on its own, AKA hydrostatic equilibrium.
It has to have âcleared the neighborhood of its orbit,â an arbitrary bullshit term that they had to come up with more arbitrary bullshit terms to justify (we will be coming back to this)
There are plenty of things wrong with definition, some of which have been discovered since the change, but plenty that we already knew about. For a start, âIt must orbit our sunâ is the most blatantly human-centric definition Iâve ever heard in my miserable little life. We have discovered thousands of celestial bodies orbiting other stars in our galaxy. Are those not fucking planets? We literally call them "Exoplanets," which translates roughly to "planets out of," like outside of our solar system. There's no reason that they couldn't be planets if we wanted them to be. (In fact in 2015 a proposal to the IAU tried to add exoplanets to the definition. No word yet on whether that's a thing now or not.) Another big issue is that we've actually discovered that Mercury doesn't meet hyrdostatic equilibrium. It keeps its spherical shape thanks to outside forces, so by the current definition it is also a dwarf planet.
"But Inferno," I hear you saying, "these are both technicalities and you're just being pedantic. Go back to living under your creative writing rock and let real scientists do the hard work." Well let me introduce to you the real nail in our astronomical coffin: Our own goddamn moon.
First lets talk about clearing the neighborhood of the orbit, because believe it or not it is relevant to the discussion. So in 2006 when the IAU came up with this rule, there wasn't actually a standardized definition for what the hell it actually meant? This term was also stolen from a paper by (well lookie here) Alan Stern and Harold F. Levison, and it refers to a planet having swept its orbit clean of smaller bodies over time by either pulling them in, flinging them out, or capturing them in orbit. How much mass exactly? Well the IAU decided to go with a completely different metric given to us by Steven Soter (colloquially called Soter's Constant) which basically divides the mass of the planet by the mass of the surrounding matter, and if the result is greater than 100 times, it maintains planetary status. This is a very dumb metric, and it has already been pointed out by many greater than me that the list of planets that haven't cleared their near orbits can include Neptune, Jupiter, Earth and Mars because the term manages to be both super specific and obnoxiously vague at the same time.
Back to the moon! Fun fact for the day: The moon doesn't orbit the earth. Before everyone starts screaming in the reblogs, allow me to explain. See, the moon and the earth actually share a point of orbit called the barycenter, and that point is outside the center of the earth. It's about 4,671 km away from the core. Here's a really shitty youtube video to illustrate:
Second free fun fact for the day: That barycenter is moving outward! It's very slow, to be sure, about 3 cm a year, but it is moving. Which means, to absolutely no one's surprise, that the moon has not been captured in Earth's orbit. In fact, it would be more accurate to describe our relationship with the moon as a proto-double planetary system than anything else.
In fact, in 1975, infamous scientist Issac Assimov (a name I demand be forever stamped into your brain) did some math and discovered something very interesting. He did a little gravity math, and he discovered that the sun's gravitational force actually has more of a pull on the moon than the earth's gravitational force does. It's the only "moon" in our solar system that behaves like this. By this logic, the moon doesn't orbit the earth... the moon orbits the sun. (insert dramatic space-based piano sounds here). In 2017, a paper was published that took all this and pointed out that by the criteria above stated, the moon actually meets the critera to be a planet by IAU definition.
Yeah, that's how fucked up the current definition is.
To summarize:
1) In 2005, Alan Stern suggested we make an inclusive categorization system that used the term "dwarf planet" as a subcategory.
2) In 2006, the IAU completely ripped the the term dwarf planet from any meaningful context and used it to make an exclusive categorization system because "the science was too messy"
3) The definition was leaking water when it was first made and is actively taking on water thanks to the real science that's been done in the two decades since.
4) The moon is canonically a planet under IAU definition, fuck you.
Other points of note:
> Another point of research by Alan Stern notes the fact that the term "planet" has been used completely outside of the IAU's standard in papers to refer to things including our own moon and Jupiter's many moons. Clearly how scientists are using the term is just as questionable.
> Alan Stern is also one of the head engineers that was on the New Horizons Pluto satellite missions, so I don't know why the hell he isn't considered a leader in the subject. Let the man cook, goddamnit.
> Real, actual scientists are still actively arguing about how the definition should work but there is relatively general agreement on both sides that the current system is Bad(TM).
> In response to a comment I've gotten often along the lines of "Pluto didn't get deleted from the sky:" I mean, in a sense, it kind of did? Yeah it still exists, but it has been pretty explicitly categorized as "not a planet," which is just a touch stupid.
> Like it or not, science isn't a democracy. It's based on empirical evidence and the scientific method, not a minority of the scientific community deciding that it's not clean enough for them so they get to change the definition. The reason you even know about the change is that they used Pluto as a scapegoat and engaged in a massive round of propaganda to win over the common folk.
> Legally, Pluto is a planet in Illinois and New Mexico. Ha get got fuckers
> Finally, there's just no fucking reason that the system has to be exclusionary, and that's the part that really irritates me. We could have had dwarf planets be a subcategory of planets, and used literally the same criteria we have now, and I would have no argument against it. I'd actually be quite happy with that outcome. The fact that the current definition excludes dwarf planets is really the stupidest part of all of this, and it's even more frustrating that we actually had such a system designed, and the IAU decided to just ignore it because they could.
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