I believe it would be illegitimate, because it comes off as someone who is unwilling to allow anything that proves them wrong as anything more than something that they can change into being proven they are right.
I'm not saying that life outside off of this planet (or out of our solar system) would disprove god, but it's difficult to take someone serious when they constantly re-arrange the parameters of the argument.
Anytime you just go "see? that proves god's existence" as your answer, it's stupid. Because it's much easier to say it doesn't prove GE (shortened for ease of typing..yes, I realize this sentence explaining the abbreviation is actually longer to type out), because you actually have proof of life outside of our planet.
There is no mention of G providing life outside of here. Sure, G made the "Heavens and the Earth", but there's nothing said about 8.8 light years away from here, that G made a mistake and only made microbes on a comet heading our way in 80K years.
Think of the reverse. It would be like if science changed the test it used to validate (or invalidate) a hypothesis, or changed the rules of what is considered a passing test. It invalidates the scientific argument.
I'm certain that someone else could've worded this a lot clearer.
