I really don't see such a discovery shaking the faith of JWs. There are already far more damning scientific facts that disprove JW teachings, such as the age of modern humans which is approximately 200,000 years old and not the 6040 years that Watchtower claims. Yet these scientific facts don't do much to shake most indoctrinated JWs because they either:
1. Claim that the scientific findings are in error or are false.
2. Come up with crackpot, pseudoscientific explanations to reconcile scientific findings with the bible.
I think the age of humankind and the mountains of scientific evidence in favor of evolution are actually far more faith-damaging than any discovery of extra terrestrial life, ever will be. Why? Because unlike the origin of species and the age of mankind, the bible is very silent on the subject of extra terrestrial life. There is really no bible verse that one can point to to show that the bible denies the existence of ET life. The bible is as silent on ET life as it is on life in the Americas. So finding ET life, while it would be surprising, really wouldn't do much to shake JWs' faith in the bible or Watchtower.
My guess is that Watchtower would initially deny the finding, saying that scientists have made a mistake or that its just false propaganda by those trying to discredit the special creation of life on earth in favor of propagating the idea that life evolved elsewhere just as it evolved here.
Then after a while - years, or decades - Watchtower would finally get "new light" and say that the bible is really silent on the issue of ET life and that it is a book meant specifically for humans and so it should not surprise us to discover that Jehovah created life on other planets, while the bible is silent on the matter, because the bible is not a science text book yadda yadda yadda . . .
Did you know that the cousins of the JWs - the Seventh Day Adventists - already believe there is extra terrestrial life out there? Yes, Ellen G. White believed there are other unfallen worlds out there! If you ever talk to an SDA ask them their thoughts about the possibility of life out there and see what they say. See if the term "unfallen worlds" (or similar) rolls off their tongue. If the SDAs can believe that, then it shouldn't be hard for JWs to adjust to such a discovery.