Is Earth the only planet God created life on... is it possible there are other civilisations on other planets?
Would aliens be Christians too?
What would it mean to Christians if there was life in other planets?
Are there any hints about this in the Bible.. either way?
I believe there are other creations of God. Even if we just think of angels, they are another type of creation. In heaven there are beasts (living creatures).
Rev 19:4 And the twenty-four elders and the four living creatures fell down and worshiped God who sat on the throne, saying, "Amen! Alleluia!"
Around God's throne, are some fairly funky creatures.
Eze 10:14 Each one had four faces: the first face was the face of a cherub, the second face the face of a man, the third the face of a lion, and the fourth the face of an eagle.
As for aliens in the traditional sense, I believe in the last days that when Satan is cast to earth that he will masquerade as an alien.
Rev 12:9 So the great dragon was cast out, that serpent of old, called the Devil and Satan, who deceives the whole world; he was cast to the earth, and his angels were cast out with him.
Rev 13:4 So they worshiped the dragon who gave authority to the beast; and they worshiped the beast, saying, "Who is like the beast? Who is able to make war with him?"
As for "are they Christians", creatures of God, will serve God according to the truth revealed to them. They are not saved in an identical way to man, the following would suggest angels desire to see into what we have in Christ, but some knowledge is denied them.
1Pe 1:12 To them it was revealed that, not to themselves, but to us they were ministering the things which now have been reported to you through those who have preached the gospel to you by the Holy Spirit sent from heaven—things which angels desire to look into.
Men are saved by Christ, by no other method.