Would the discovery of ETs (extra-terrestrials) threaten organized religion?
The discovery of ETs would pose no more of a threat to Judaism than would the discovery of a new species of rabbit.
It would be limiting G‑d's power to say that He could not have placed life on other planets. In fact, there is a reference in the biblical Book of Judges (5:23) to an inhabited place called Maroz, which the Talmud identifies as a star.
But Jewish thought has always believed that the most weird and wonderful creatures are to be found right here on earth. We can explore the remotest extremities of space but still remain alien to our own humanity. The real secrets of the universe lie hidden in the depths of the human soul.