The Twelve Colonies were founded by tribes from Kobol, the birthplace of humanity. There were originally thirteen tribes, but one tribe went to a planet called Earth. The constellations of Earth II's (our Earth) Zodiac will be named after these colonies.
The Twelve Colonies spin around a single star on elliptical orbits. Closest to the sun is sweltering, volcanic Canceron, the greatest source of Tylium in the system. Next is a Trojan orbit of planets -- three worlds following the same orbital path, separated by roughly the same distance (one-sixth the orbit's diameter). In order, these planets are Aquaria, Caprica, and Virgon. They are centers of art, culture, and system-wide trade.
Two gas giants follow, each in its own elliptical path and each itself orbited by a number of moons. The first of these behemoths is Zeus, named after the King of the Gods, and its moons are Leonis, Picon and Scorpia. Another large planet named Aerilon shares Zeus' orbit, positioned exactly opposite the giant. The second gas giant is Hera and her charges are Gemenon and Libran, with Sagittaron likewise spinning opposite. These giants and their moons are the heart of the system.
At the outermost edge of the system dwells Tauron, followed by a third gas giant named Ragnar. Tauron was the first world settled after the fall of Kobol. It was the springboard for all expansion in the system.