on earth

1.  Also, in creation; in the world. Ever, anywhere, of all possible things. These phrases are all used for emphasis in questions or, less often, in a negative context. For example, What on earth is he doing with a spade? or Where in creation did that child go? or How in the world do you expect me to carry all those bags? [Late 1700s]
2.  like nothing on earth. Incomparable. For example, That perfume smells like nothing on earth, or Her new hair color is like nothing on earth. [c. 1900]

The adverb has one meaning:

Meaning #1: used with question words to convey surprise