The heavens proclaim the glory of God.

A good question would be why God made other planets and did not put life on them. It's true what the Bible says in Psalm 19:1:

The heavens proclaim the glory of God. The skies display his craftsmanship.

So what do the planets declare about God? Sure their gravity keeps us in place but what else? They show us just how special the earth is! If the planets weren't there how long would it have taken for us to realize that the earth has to be a perfect distance from the sun? Or, that it has to be the right size, and have the correct gases in our atmosphere to support life?
If we were close like mercury we'd be too hot. If we were larger we'd have too much gravity. If we were smaller we might be like Mars.
Surely the heavens do proclaim the glory of God!