During the preparations for the first manned flights into space, modem-day Torah Sages were asked the question: What does the Torah tell us about the possibility of sentient life existing elsewhere in the universe? Their answer was usually along these lines: We do not know of any specifically negative answer in the Torah, so that essentially, there is no contradiction between the opinions of those who claim that such civilizations exist and any basic tenet of Jewish belief. Still, the overall spirit of the Torah implies that there is no life elsewhere in the universe, other than that which exists here on Earth.
"And You [G-d] created heaven and earth ... and on earth there are trees and grasses ... and birds, fish, domestic animals, wild beasts and human beings." "In view of this statement," continued the Rabbi, "It is highly unlikely that there is life the moon, for it states explicitly that: "on the earth, there are trees and grasses, birds, fish, domestic animals, wild beasts and human beings." "This implies that only here on Earth were these creatures created."
"In the last few decades, a growing number of astronomers have promulgated the view that alien civilizations are likely to be scattered among the stars like grains of sand, isolated from one another by the emptiness of interstellar space. Just for Earth's own galaxy, the Milky Way, experts have estimated that there might be up to one million advanced societies... Now two prominent scientists say the conventional wisdom is wrong. The alien search, they add, is likely to fail."