The Rosharan system is comprised of a series of enormous gas giants which crowd the outer reaches of the system. However, nobody has been able to observe them directly, as their manifestations in Shadesmar is minor.
There are three worlds in the habitable zone, all of which are inhabited to one extent or another. There is Ashyn, the burning world, which suffered a cataclysm long ago. People there live in very small pockets of survivability, including the famous floating cities.
The middle world, Roshar, is the showpiece of the system. Roshar is home to a diverse and unique ecology containing dramatic megafauna and fascinating symbiotic relationships between creatures (both humanoid and not) and Splinters of Investiture.