The surface condition of the crapapult allows it to be built on Cerys, but when I tried yeeting Pu-239 there, it doesn't seem to be counted towards unlocking the earring... Could you have a look at this issue?

This is by design. It needs to be yeeted on Rubia. The intent is to make the planets feel better integrated with each other.

The problem is that I have no idea how to best convey this to the player besides you just noticing it doesn’t work. I am open to suggestions, but keep in mind that this detail probably won’t be remembered if conveyed while starting Rubia.

If possible, how about sending a chat message when the player place a crapapult outside of Rubia for the first time?