Answer:
One of Eight Planets : Earth
Spiral galaxy, One of the estimated billions of galaxies: Milky Way
Galaxy: Milky Way
Explanation:
According to the definition, appositive or apposition is a grammatical construction which represents similar noun phrases and nouns where one serves or defines the other one.
Here, the ultimate examples of apposition are mentioned in the given statement as Earth: One of Eight Planets, Milky Way: Spiral Galaxy and one of the estimated billions of galaxies: Milky Way.
With such kinds of words, it's easier to understand the noun or renames with a short definition written as a phrase.
