#zestytalks

Alright, so as for why Pluto isn’t considered a planet… Fair warning, this is lengthy

For starters, it’s not the only former planet! Back before Copernicus realized that the planets orbit the sun we thought everything in the solar system orbited Earth, and the Sun and Moon were both considered planets!

So we came up with the first rule for a planet; it must orbit the sun! If it orbits another object then it’s a moon or a satellite (we did need this distinction; we called the moons of Jupiter and Saturn planets too).

Once we got that debacle sorted out we started calling all the large asteroids in the asteroid belt planets too. Ceres, Vesta, Pallas, Juno, Astreae, Hebe and Iris were all considered planets. I mean, they orbit the sun so they’re not a moon, right? But then we’d have to include every single hunk of space crap we found in the belt, and it’s estimated that there’s 1.1 million to 1.9 million asteroids over the size of 1 kilometer, and millions of smaller particles too!

So we came up with a new rule; a planet must be large enough to reach hydrostatic equilibrium, or in other words to be rounded into a spherical shape under its own gravity. All the objects in the belt came to be known as asteroids or minor planets since they no longer fit the description!

You might point out that Ceres IS rounded under its own gravity… and you’d be correct! Which is why it’s considered a dwarf planet even today! I’m pretty sure it was still lumped into the category of minor planet at the time though, since the term dwarf planet didnt exist at the time and it kept things simple. Please correct me if I’m wrong though, I’d love to learn more about Ceres!

We were comfortable with these two rules for a long time, and when we discovered Pluto it fit right into the category of planet! However we started having some trouble when we discovered Eris, and it 2005 it was named the 10th planet. In fact, Eris is actually larger than Pluto! Or… it’s debated? They’re both very similar in size but we don’t have much data on Eris because it orbits the sun 3 times further away than Pluto, or 68 times further away than the Earth! (In other worlds 68 astronomical units). It’s so hard to observe…

But it brought up the question of where do we draw the line? There’s no telling how many dozens of small Kuiper Belt Objects we’d end up calling a planet, and if you include one you have to include them all…

So in 2006 a third rule was put into place; an object has to be massive enough to clear its orbital path, or in other words it has to be able to clear out any debris in its path with its own gravity. That excludes all objects in the Kuiper belt, as well as all objects in the asteroid belt (so now Ceres can have a place to fit in!). These objects were called dwarf planets! Since then we’ve discovered many more dwarf planets (two of which were called Easter Bunny and Santa because of the times of the year they were discovered, and were later renamed Makemake and Haumea. Irrelevant but fun!).

So that’s why Pluto’s no longer considered a planet! I don’t think of this as a bad thing, if anything it just made it go from being in last place as far as planets go to being in first place as far as dwarf planets go, and it’s been affectionately called the king of the Kuiper belt.

Nomenclature is hard though, and space doesn’t care about exact definitions. The asteroid Vesta is way larger than the Saturn’s moon Mimas, but Mimas is round while Vesta is not. If it hadn’t been impacted by two asteroids Vesta would likely still be round and it would be considered a dwarf planet, but it isn’t round so it doesn’t get that title! And what do we consider round enough to be hydrostatic equilibrium? And all planets have objects in their Lagrange points, they technically aren’t considered clear from the planet’s orbit but they do follow the planet as it orbits the sun! What’s the exact number of space specks we’ll allow a planet to have to still be considered a planet, because I’m sure there’s a planet in some far off solar system that has that exact number!

My point is this is all really complicated and tedious and there are no exacts and I’m in way over my head so! Please don’t argue with me about Pluto needing to be included in the season or needing to be considered a planet. Maybe I’ll give them a spot in the seasonal design, along with a few other dwarf planets, if I have enough time on my hands and motivation to design such a thing. As for if it needs to be considered a planet, take that one up with NASA and not me! (/lighthearted)