Rennet is enzymes used in producing cheese, which come from the stomach of an animal. During the manufacture of rennet, the raw material becomes unfit for canine consumption, the measure of what constitutes food. Simply put, as a result of this process, the end product is considered a whole new thing, called a davar Hadash. As a new thing there is strong halakhic rationale for declaring the new food source kosher and parve.
The assumption is that any food stuffs produced by a Star Trek replicator would also pass through such a transformation, breaking down matter into its building blocks before reassembling as whatever you wish.
Additionally, replicated food could be considered created out of nothingness. In this case, it would be like food descended from heaven.
While replicated food would be kosher and parve, some rabbinic concerns have been brought through the ages about a permitted food appearing to be a forbidden food. When parve margarine was debuted, rabbinic authorities required that, even in the fanciest of establishments, it be served in its wrapper so there would be no confusion.
As for food found on other planets, all plants are kosher. We have clear rules for animals that live on land or in the water, including reptiles and insects. Birds present a different story. Instead of qualifications for kosher birds, we have a list of those we cannot eat. While some would allow any alien birds for food, others would require a mesorah, a tradition, of eating a bird to be in place.
Rabbi Shlomo Kluger (1785-1869; Ha'elef Lecha Shlomo 1:YD:111) was asked about birds that come from America. He responded that we only eat birds with a mesorah, there were no Jews in America before 100 or so years ago, ergo there cannot be a mesorah and all American birds are prohibited. He concludes with the warning that whoever fails to heed him will in the future have to answer for his actions. Based on specifics in the responsa, it is most likely that Rabbi Shlomo Kluger was addressing the issue of the kibbitzer hen that some people thought came to Europe from America via India, and that he did not have turkey in mind. Nonetheless, it is difficult to see why his logic should be any different for turkey than for the generic American bird that he discusses.
On the other hand, "the wild turkey has a crop, its gizzard is peelable, it has an "extra" toe, and its eggs have the indicators of kosher eggs, all signs indicating the turkey may be kosher."
J. David Bleich, "Mitzvoth in the polar regions and in earth orbit"
-"The issue of Sabbath observance aboard a space ship is a novel extension of the much older question of Sabbath observance in the polar regions and adjacent areas in which daylight and darkness extend for months at a time rather than alternating in periods of approximately 24 hours."
-A response from 1886, Rabbi Simcha Ha'Levi Bamberger, "...Why should a person, even during weekdays, place himself in a state of doubt with regard to reading of the Shema and prayer? At the minimum, do not remain in that country on Shabbat [where] there is doubt with regard to what to do..." (Letter to his son who was considering a business trip to Norway)
Position of the Tiferet Israel, R. Israel Lipschutz (19th c.)
--'The day is determined objectively rather than individually by each traveler....Shabbat occurs at the North Pole the same day as it does on the rest of the globe and is objectively determined by the 'revolutions' of the sun in the sky.'
--The traveller should adopt the 'clock' of the 'place from which he departed'.
On conversion - Looking at life on earth, there are varying opinions. The key seems to be who would be considered intelligent life with free will and a moral responsibility.
David A. Weintraub Religions and Extraterrestrial Life: How will we deal with it?
Q: What does Jewish tradition think about the possibility of life beyond earth?
Rabbi Hasdai Crescas (1340-1410) Light of the Lord - 'Space is infinite and infinite space contains a potentially infinite number of worlds. In such a universe, nothing in physics and nothing in scripture or Talmudic writings can deny the existence of ET life.
Rabbi Pinchas Eliyahu Horowitz of Vilna (1765-1821) - [Based upon the Talmud teaching, 'God flyer through 18,000 worlds on His chariot at night'] He suggests 'intelligent life exists on these 18,000 worlds, though these life forms may not be similar to terrestrial life forms.'
Student of Rabbi Crescas R. Yosel Also (1380-1444) "...because no other creatures could have free will, there would be no reason for them to exist; therefore, they do not exist."
R. Horowitz of Vilna, Sefer Ha'Brit, 'ETs would have no free will and no moral responsibility, but...they might still exist.'
Aryeh Kaplan (1934-1983), "The basic premise that of all possible species only many has free will, is well supported by the great Kabbalist, Rabbi Moshe Kordovero in his Pardes Rimonim. Using tight logical arguments, he demonstrates that there can be only one set of spiritual worlds. Although God would want to maximize the number of recipients of His good, His very unity precludes the existence of more than one such set. Since this set of worlds deals specifically with God's providence toward man because of his free will, this also precludes the existence of another species sharing this quality."
R. Norman Lamm The Religious Implications of Extraterrestrial Life (1986)
"The discovery of fellow intelligent creatures elsewhere in the universe, if indeed they do exist, will deepen and broaden our appreciation of the mysteries of the Creator and His creations. Man will be humble, but not humiliated."
'Judaism is only for those who identify as Jews here on Earth: "Torah was given to man on earth and its concern is limited to terrestrial affairs..."