The Lem system contains nine planets, one of which is habitable. The habitable planet, Lem VII is an ocean planet, and serves as Sidonia's first colony. The outer gas giant, Lem IX, contains both a significant ring and several moons, operating as both a source of raw materials for Sidonia and a safe place for the assaults against the Large Mass Union Ship.
The Lem systems other planets are unknown at this time; presumably, none of them are habitable aside from Lem VII. Given that Lem VII is planet in the 'Goldilocks' zone of habitability, it can be assumed that Lem I - VI are close to the sun, somewhere between Earth and Mercury's orbits in Earth's solar system.
Sidonia interacts with several planets in the Lem system:
Once discovered, the Lem system is a powerful catalyst within the pacifist movement on Sidonia. At least 100,000 members of Sidonia wish to attempt the colonization, and an initial colony is started on Lem VII, with a support colony on Lem VII's moon. This colony, however, is short-lived - the Large Mass Union Ship destroys the moon colony, and Lem VII proves to be inhabited with Gauna.