Dual planet system

[Mindseye]

Using the world creation system in t20 I rolled up a size 7 moon orbiting a size 8 planet. How do I go about figuring out the orbit for that moon? I have a week grasp of what a barycenter is and Roche's Limit, but the math is beyond me. Is there a "simple" formula to describe this, or at least a ballpark idea to work with? I'd assume the distance would need to be farther than the distance between Terra and Luna, but beyond that, I'm clueless.

da Wookiee

 

[Piper]

Does T20 give you the ability to generate mass and density?
Once you have those, the Roche formulae are here
The barycenter calculation seems fairly straightforward, and is given here

If the math is still boggling, you can post the planet/moon data here and I'll take a shot at it, if you like.

Regards

 

[Mindseye]

Nothing for mass or density, just diameter and density of atmosphere. Unless that is enough information to estimate mass and density, I think I'm stuck.

 

[Piper]

I ran the numbers using the densities of Earth for both bodies. The worst case Roche distance comes in at around 15 thousand kilometers.

I made some assumptions (probably bad ones) to figure the barycenter. I assumed the moon orbited at the 15000 km Roche limit and both bodies have a density equal to Earth's. This places the barycenter within the body of the size 8 planet.

This seems really wrong to me, but I checked my numbers and they look right. If anyone is up to tackling this, PM me and I'll e'mail my spreadsheet to you.