Adding to what Claire said, the fact that "Earth" is capitalized is what tells you that it is a proper name. That is why "the" is not needed. Had "earth" not been capitalized, it would not be a proper name. In that case it would be the same as "land" or "sea" and would need "the":
"Most of the earth is covered with water."
"The sea covers most of the earth."
"Much of the land is used for growing crops."