Rules for custom planets:
Pretty much just make your planet make sense, and if it doesn't make sense, give a reason why(Leviathan crash, things that otherwise warp reality, etc).
Use the template here(Template taken from a Star Wars RPG rulebook, it can be modified if you like this is just my personal preference)
Planet Name:
Planet's Moons:
Atmosphere:
Hours per day:
Days per year:
Climate:
Dominant Enviroment:
Gravity:
Sentients:
Technological Development:
Government:
Economy: Exploration
Significant Feature:
Cultural Quirks:
Other: