How do you return occupied planets?

[Caliell]

So I managed to successfully invade a planet, but my forces for some reason captured the planet that originally belonged to my ally. I do not wish to keep the planet, but I do not know how to return it. In previous version of the game, that was possible, but now I do not see how it is possible.

Is there a way to transfer sectors/systems that had been captured by your armies?

[Aegmar]

yes, you can transfer systems with a trade deal. You have to be at peace (or both even) and the one you want to give the system has to own a system that borders on the one you want to transfer.

[tempest.of.emptiness]

Also, the problem might resolve itself at the end of the war. Typically the star base of a system like that will become controlled by your ally when it is defeated, even if you are the one who defeated it, when the system formerly belonged to your ally (because they will have claims on the system as a result of it being formerly theirs).

When you invade the planet, it will show that you are occupying it, and continue to do so until the war ends. At the end of the war, however, control of the system (if it is fully occupied) will revert to the player who controls the star base. When that happens, control of any planets in the system will be surrendered to that player. So if your ally has credit for controlling the star base then they will automatically get the planet too when the war ends.