How to change the game options?

Hey does anyone know how you can see the game options specifics.   For example, I want to add a new choice in the game options for extreme planets.   I wanted to see what each option means...i.e.   frequent vs common vs occassional, etc.   Does occassional mean 20%  or 15%, or something else entirely..    Anyway, I was hoping this is a text file that I could mod.  Does any one know?