Question:
Why did God create all the other planets if we are the only people inhabiting a planet?

Answer:

All things were made for His own glory and pleasure.  “For by Him were all things created, that are in heaven, and that are in earth, visible and invisible, whether they be thrones, or dominions, or principalities, or powers: all things were created by Him, and for Him” (Colossians 1:16).