The Sphere classify planets into three main branches, based mainly on their climate and ability to support colonization.
Red or Dry worlds: These worlds are very dry, generally having large inland seas instead of oceans, dry climates, lots of deserts, mountains, badlands and tundra. As with most planets in the Sphere, they are quite warm with small cold areas.
Green Worlds: Worlds with roughly an equal amount of landmass and surface water. These worlds tend to be somewhat cooler then Dry worlds, yet still quite warm. They are classified green as plant life is usually the dominant lifeform before colonization, covering nearly all the landmass. These worlds tend to lack deserts.
Blue World: Worlds with over three quarters of surface water, with landmass covering the last quarter of surface area. These worlds tend to have the most volatile weather and be the coolest as well. There are only two known Blue worlds in Sphere space, Kintar the Kintaru homeworld and a second. The second is the only known world with polar icecaps.