The author explains the evidence for they would help from astronomy. He says that if planets are behaving in a way that cannot be explained by what is already known, then another planet is searched for which would explain their behavior. This, he says, is actually how the more distant planets were discovered. We look, then, for something that would explain what is not inexplicable from what we already see.
N.T. Wright, Simply Christian: Why Christianity Makes Sense
faith, wonder, worship