On the one hand, I think it would be great to teach the Trinitarian God.
On the other, if they are rational beings, they aren’t really part of our Church, which is based on the sin of Adam, the first human nature, and the assumption of a human nature by God which all men can participate in. If these aliens aren’t descended from Adam, they have no part in that.
I’m not saying God doesn’t have a place for them. But we can’t just baptize them. The covenant of law and the covenant of grace God made with man is for humans. We don’t really have the authority, insofar as I understand it, to baptize aliens into the Church, anymore than we have the authority to ordain women as priests or dissolve a sacramental, consumated marriage. That’s not to say they can’t worship God.
In some ways, it seems like being Jewish versus being a gentile who recognizes the one true God in olden times. The covenant of the law was with Jews, but gentiles could still worship. And that’s okay. Except in this case there’s no conversion from one to the other. If God wishes to incorporate aliens-not-descended-from-Adam into his covenant of grace, or form a separate relationship with them, that is for him to do. Not something we have the authority to do. As I understand it.
In a sense, from a Thomist standpoint, whatever biological species we are, perhaps the essence of both the human and the alien would be the same: a rational animal. The other biological differences are properties or accidents which are not part of the essence itself (maybe). But that doesn’t change the fact that our Church is based on descent from Adam, his original sin, and the new Adam (Christ). Apart from that descent, that original sin from Adam, human nature, I don’t think they could belong to the sacramental life of the Church.
All that said, I’ll obviously support whatever the magisterium formally teaches, if they ever do.
(I’ve been on some long-winded ramblings this week.)