I think everyone knows about how Triton likely had a binary partner that was tecnically its moon, but there is more: Triton was very likely bigger before and had what's called a reliquidization during the capture.
The tides from Neptune and collisions with its moon system would have melted Triton and make it lose a lot of material, as well as absorb some from the past moons.
Why is this important?
Because if Earth is not Proto, then Triton shouldn't be who he was pre-capture.
Also, if Earth and Luna are siblings because of the shared origin (and composition), then Triton and the current inner Neptunian moons could also be considered "biological" siblings, since those little moons formed from the disc of debris from the capture (mix of past Triton and the past Neptunian moons' material). I think it's sweet that all of Neptune's moons see each other as siblings even without this being addressed.
So, since the show started out as educative I thought it was worth mentioning this:
The Alpha Centauri System wasn't near the Sun when it formed.
Stars move at different speeds through the galaxy, and so, the Alpha Centauri System is our neighbour NOW, but it wasn't back then and won't be in the future. Same can be said of Arcturus.
The one that could really have been there was the old star, that I think it's meant to be Coatlicue. Its supernova was the one that supposedly triggered the formation of the Sun and some sibling stars (aka stars from its birth cluster) by compressing the cloud of gas that would form them.
Planet X, the Fifth Giant and Planet 9 are different hypothetical planets.
Here is a summary of each theory and how they relate to each other:
Planet X: A massive transneptunian planet that caused disturbances in Neptune's orbit. It doesn't actually exist, the disturbances were a data error, but they found Pluto while searching for this one.
Planet 9: A massive transneptunian planet that would be shepherding some extreme transneptunian objects (ETNOs), creating a cluster. This is the one they are still trying to find and it's sometimes called Planet X too.
Fifth Giant: An ice giant that was ejected out of the Solar System by Jupiter (maybe assisted by Saturn) due to the early instability of the system. They used to think it could have stayed as Planet 9, but the consensus right now seems to be that it's highly unlikely.
The show mixed the 3 into the Planet X character.
Another thing is that X looks bigger than the other ice giants when they should be similar, but they've never been good at representing the sizes (Mars also looks bigger than he should be), so I don't think this is particular to Planet X. Besides, it could be salvaged by saying that he has similar mass to the others but is less compact (like how Uranus is bigger but lighter than Neptune).
Some extra trivia below:
Fifth Giant's mass:
From the latest estimations, it would have a mass of 15 to 17 Earth masses, likely closer to 15. For reference, Uranus is 14.5 Earth masses and Neptune 17.1.
However, while these masses come from the simulations with higher success rate, it is also possible that the Fifth Giant was 10 to 15 Earth masses, making him the lightest ice giant. This is funny to me because it's been expeculated that, if that's the case, the Fifth Giant's lower mass could be due to forming near the most massive planets, aka the gas giants (they left less material for him to form from).
I've heard this being called "having its lunch stolen" (referring to Jupiter taking material from Mars's orbit during the Great Tack), so does that mean Saturn stole X's lunch? Maybe Jupiter too?
Planet 9's mass:
Planet 9 would be between 5 to 10 Earth masses and 2 to 4 Earth radii, making it a super-Earth (lower masses) or mini-Neptune (higher masses). Lower mass seems more likely.
New Planet 9:
They found a Planet 9 candidate via infrared (AKARI-IRAS). However, if it really is Planet 9, it's captured because the object they've observed seems to have a very tilted and retrograde orbit. This one seem to be an ice giant by mass too.
