How to help the alien start the Galaxy Travel:
1. Go to the Hangar > Aliens > scroll down and tap the Alien Visitor Hall
2. Tap an alien
3. Tap Galaxy Travel to send them off on their Galaxy Travel
4. Once the alien reaches the last planet on their journey they will have returned to their home planet!
5. After the alien has reached their home planet, they'll be able to return to the other planets they'd visited on their journey any time they like!