First, is there anyway to defend you debries field, I tried to send ships but the mission is impossbile, and secound when you get a % chance of moon creation, is that a % chance the next time you are attacked, or everyday, or something eles.
Thanks for the help

The percentage is the chance it had to form THEN. Moons are created immediately after a battle, if they are going to form. Leaving your DF or anything like that makes no difference.

And no, you can't defend a DF.