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 others go away. However, a planet replaces playing a land, and when a new one appears the previous one goes on the bottom of your library. The number in the 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. Cards' abilities are effected by gravity, but I'm not sure what the general effect is.