Thanks for taking my theory seriously. Not many people have 8-) Many great Christian thinkers, like C.S. Lewis and Peter Kreeft, have taken seriously the possibility of extraterrestrial life, but haven't tried to reconcile it with the fact of the Incarnation.
You're right in what you say. Christ's death would certainly have atoned for the sins of other rational beings in the universe.
But imagine the reverse. Imagine if Christ took on a Martian nature to redeem Martians and humans. It just wouldn't seem right.
If there were other beings, all which would have been made “in God’s image”, and created for God’s benefit - why would God work so hard for us to get it right here on Earth?
Might be easier to just send the flood, forget about Noah, and concentrate on Cygnus X-1 where they have a better handle on God.
However, no matter WHAT planet beings were on, sin would be present.
And, God cares about the tiniest sparrow - so even if there were a billion other created planets with beings on them, He would STILL care about us.
I wonder what purpose God might have for creating other planets with beings on them? Maybe He tried a whole bunch of times until He got the results He wanted.
Hoever, being God, and knowing what beauty is (and we recognize it as well because we are made in the image of God), I’m guessing He didn’t need a bunch of trys, and if He did, they would all be pretty similar to what He created on earth.