There's solid proof against alien visitors (the ol' speed of light limitation bugaboo).
For those Christians who interpret Genesis literally, life on other planets may or may not present a problem, depending on how strictly the early Genesis passages are read.
For those Christians who view Genesis as a representation that God is the Creator and that humans are His greatest creation, life on other planets is a scientific curiosity but does not conflict with their beliefs whatsoever.
"There's solid proof against alien visitors (the ol' speed of light limitation bugaboo)."
And pre Einstein, that wasn't even known. So are your really so sure that - say 500 hundred years from now - that knowledge won't be overturned too? Even if it isn't, what about suspended animation, etc.
Consider this: Given our rate of advancement (and assuming we don't blow ourselves up by then), at some point might we not be able to seed another planet with life? If so - and given the countless numbers of galaxies, solar systems and planets out there - might someone else not have done the same thing to us already?
Is it the Christian belief that God will step in and specifically prevent us from doing that in our future?