Planets are a strange idea I had. They're like world enchantments in that they have a global effect and when one enters the battlefield, the previous one goes away as a SBA. However, playing a planet uses your land-drop for the turn. The number in the lower-right corner is the gravity of the planet. Gravity normally ranges from 1 to 6, though some effects can cause it to go higher or lower. Gravity has a general effect on all spells. In addition, some cards explicitly reference it.

Planets: Earth, Center of Exploration, Freytal, Bastion of Prosperity, Grodun, the Stern Empire, Pultoid, Dwarf Planet, Solthanar, Shadow's Home, Sora, the Endless Sky

Related cards: Planet's Blessing, Glider Drake, Duroc, Destroyer of Worlds, Collapse, Mine Guardian, Gravity Elemental, Fractured Surface