Rabbi Chasdai Crescas (14th century) writes that nothing in the Torah outlook precludes the existence of life on other worlds. The verse "Your Kingdom is one which encompasses all worlds... (Psalms 145:13)" implies the existence of more than one world. According to the Talmud there are 18,000 (at least!). The existence of these other worlds and the fact that they rely on Divine Providence make it reasonable to assume that life does exist there!
The Sefer Habrit states that extraterrestrial creatures exist but that they have no free-will. He adds that we shouldn't expect creatures from another world to resemble earthly life, any more than sea creatures resemble land animals.
Now you might ask, what possible purpose could there be for the existence of 'Martians' who possess no free will? This problem prompted Rabbi Yosef Albo (author of Sefer Ikkarim) to view their existence as illogical.
However, a possible purpose for their existence can be found in the work Tikunei Zohar, which states that in the future each tzaddik (righteous person) will rule over a star and have an entire world to himself. This world with its entire population would serve to enhance the tzaddik's spiritual growth.
