Stewardship

Within Christendom the worlds differ in many great respects, despite their unity around The Church. However, one manner is which the planets of the region are aligned, aside from the aforementioned Church, is their general sharing of the view of Stewardship.
Stewardship is the essence of Christian doctrine with regards to the Cosmos. It is a view which ultimately establishes Humanity as having been given stewardship over the universe by its God and creator, and are thus responsible for its upkeep. It should be noted that the notion of Humanity is not strictly limited to Humans. In fact, many non-Human species have been inducted into Christendom.
Within Christendom the view on which races can be considered their "brothers and sisters" among the stars is based in large part on their "alieness," although that would make it seem more arbitrary than it is. A distinct mark of Humanity is Reason. As being made in the image of God entails having a rational soul, then those aliens which have a "knowable" mind can be included as his children and as fellow stewards of creation. Conversely, those aliens whose minds are so "alien" as to be essentially incompatible are considered as beasts or some advanced organic system, a notable example being The Spread.
Although in the earlier years of Christendom Humans were much more xenophobic, the acceptance of aliens into the Church has become recognized as doctrine, particularly in light of their mental similarities.