Changing Empire in mid game, is it possible?

[Ruatha]

I just started my first SE4 game as a pbm game with two of my friends, now we have got a part into the game and are reluctant to restart (The others are... not me).
The problem is that I chose Gasgiant with methane as homeplanet, thinking that the planet types were randomly and evenly dispersed, that is not the case.
There are few Gas giants with methane and when I try to uild a colony on one of them (with a green star at the corner) they become domed colonies.
The only colony that isn't domed is my original home planet. It will take 50 turns to discover Rock colony and then i can build colonies on rock planets with methane atmosphere.

My question is:
Is it possible to swap empire with a computer player in midgame, and if so, how?