Sometime ago I heard a factoid that children had to learn more and more planets in school until we finally discovered that those things were not planets. It was probably from a CGPGrey video.

FactCheck🔗

• Sources¹
• Wiki on Former planets

Seem so.

We were pretty good at classification🔗

What surprised me immensely was the amount of knowledge we had so far back. I know that historical-folk surprise me often, but here they almost always knew what they were looking at, considering the magnitude of knowledge and mapping these are almost rounding errors that are so visible only because so few bodies satisfy our classification of planet.

Eris caused recategorization?🔗

One of the startling facts about Eris is that appaerently it is the 9th most massive thing orbitting the sun… did you notice? It’s more prominent than pluto.

I love one of the announcement sites

The actual reason we now know how many planets we have, and more importantly what that even actually means is August 16th 2006 (2006-08-16), when the IAU published their definition which is in wide use since. Interestingly enough the definition strictly covers Solar System planets, not exoplanets, those have their own definition in the works and it’s way closer to “defining SI units”.

The reason for this IAU definition started with the discovery of Ceres and many other bodies, initially called planets, but later renamed to asteroids.

When you find a body you think is special, but suddenly start finding hundreds of similar bodies, some of which are larger than the first, it’s probably not that special.

We still have Pluto, but it’s among its friends the asteroids of the Kuiper belt.

1. While I did read through some of the cited sources, this isn’t something too much rides on so I didn’t verify this info beyond Wiki too much. ↩