Answer:
Essential appositive: Milky Way
Noun it identifies: Galaxy
Explanation:
An appositive is a noun or in some cases, noun phrase that come before a noun to give it more meaning.
Therefore, from the sentence, "Earth, one of eight planets, resides in the spiral galaxy the Milky Way, which is one of the estimated billions of galaxies in space", the essential appositive is "Milky Way" and the noun it identifies is "Galaxy".
Step-by-step explanation: