Was going through BGG forum as usual.

This question came up, if it's legal to connect two planets with Z-axis that already have a "normal" connection.

I was like, it can't be, but then I read the FAQ.

Q: Can Z-Axis navigation routes be placed on two planets
that are already adjacent?
A: Yes. The only restriction when placing/moving these is
that both ends of a Z-Axis navigation route can never be attached to the same planet

We have never played like this, can anyone explain why this isn't just stupid.

This means people can minimize the amount of Z-axis to their two home worlds.

Doesn't make any sense at all to me.

There are two Stage I Event cards which allow to reallocate single Z-axis route, replacing it by existing one.

So that having any Z-axis connected to a starting planet is usually risky at the beginning of the game.

You are allowed to z-connect two planets that already have a connection on any of the three dimensions (x,y, or z).

You are NOT allowed to z-connect a planet to itself.

You can do it, although (as already said) it's subject to the "Navigation Error" event card. Also, it could make you more of a target in a game with 3+ players.