Earth, one of eight planets, resides in the spiral galaxy the Milky Way, which is one of the estimated billions of galaxies in space. Identify the essential appositive and the noun it renames or identifies.

The essential appositive in this sentence is "resides in the spiral galaxy the Milky Way" and the noun in identifies is Earth. As one can observe, other appositives present can be eliminated without losing the meaning of the whole statement.

I see two appositives in this sentence.

First is "Earth, one of eight plants". The appositive is 'one of eight planets' and it refers to Earth.

Next is " ... Milky Way, which is one of the estimated billions of galaxies in space". The appositive is "which is one of the estimated billions of galaxies in space" and it refers to Milky Way.