[SPACE 4e] System Design help? (star systems that is...)

[Ze'Manel Cunha]

For the double planet, I prefer doing one bigger, usually about 0.2-0.4G bigger, this makes the sattelite "planet" almost a moon, except the two planets actually both orbit around a center of mass which then orbits around the star, so only one orbit around the star is taken up.

The two planets are likely tidally locked to each other and at least 1-2 million miles apart, but there's no reason they wouldn't rotate around their center of mass in a stable orbit, while the center of mass orbits the sun.

This then allows you to have the third planet in the life zone some 100 million miles or so away from the center of mass of the double planet.

For other effects on the system, just consider the total mass of the two planets as if it were one larger planet in the center of mass orbit.