Sometimes, depending on what the Block/Planet's purpose is, it could make more sense to have its thread started by touching ANY side. This would be useful for "dangerous objects" that must be avoided, but as it is right now, that's nearly impossible to accomplish correctly.
By the way, if this gets considered, it may be important to allow this as an option. I've seen some levels where there were objects that obviously function better by only having their tops activate triggers, plus other mechanisms could break levels if they could be started by touching any side.