Compound nouns are not formed according to precise laws of grammar, but rather commonly accepted usage. "Planet Earth" is a short way of saying "the planet called Earth". Earth here is the name of the planet, not the material from which it is made. A planet of "earth" (if the term existed) would suggest a planet made of earth or full of earth. "Rose flower" is not a compound noun. As for "butterfly of love", I have never heard of this term but it could exist and look good in context. One day, someone might coin the expression "love butterfly" and the "of" would disappear.

It may just be considered an idomatic expression. It also could have originated as a clarifying statement that eventually had the comma dropped. In otherwords, people might have often said, "the planet, Earth" just like they would have said "the planet, Venus," and the comma eventually was dropped because the expression became so common.