with the new version, you can invite friends with a link.
It's written that his/her planet will be placed next to mine.
but which one of my planets? I tell you that it's not the mother planet, because I invited a friend of mine and his planet has been placed 22 systems far from my planet in G1, but my mother planet is in G4.

So the question is: how can I know where friends' planets will be placed?

So the best way to keep friends close to you is to invite them at the beginning when you have one planet

IMHO it's a good thing that your friends are in other solar systems because otherwise you could block your solar systems with your friends planets.
Answering your question - in my opinion you can't know.

I've not had time to test it properly but as far as I remember it's something along the lines of:
follow the normal positioning technique for new users of Gal 1/2/3/...9 then compare this to your list of planets.
so close to one of your Gal 1 planets is the most favourable and likely position

In fact I invited another friend and he's near the same planet of mine in G1.