Here's why:

* Prepositional Phrase: It begins with a preposition ("around") and ends with a noun phrase ("the earth").

* Adverb: It modifies a verb, adjective, or another adverb. It tells us where something is located.

For example, in the sentence "The satellite orbits around the earth," the phrase "around the earth" tells us the location of the satellite's orbit.

It's not an adjective phrase because it doesn't describe a noun. Adjective phrases modify nouns.