tach typed, "How do christians in the US deal with the fact of possible extra-terrestrial life?"

Abraham 3:12-13 (Abraham is shown many stars.) Thus I, Abraham, talked with the Lord, face to face, as one man talketh with another; and he told me of the works which his hands had made; And he said unto me: My son, my son (and his hand was stretched out), behold I will show you all these [stars]. And he put his hand upon mine eyes, and I saw those things which his hands had made, which were many; and they [stars] multiplied before mine eyes, and I could not see the end thereof.

Moses 1:29,33,35,37-39 (LDS in the US believe in extra-terestrial life) 29 And he [Moses] beheld many lands; and each land was called earth, and there were inhabitants on the face thereof. 33 And worlds without number have I created; and I also created them for mine own purpose; and by the Son I created them, which is mine Only Begotten. 35 But only an account of this earth, and the inhabitants thereof, give I unto you. For behold, there are many worlds that have passed away by the word of my power. And there are many that now stand, and innumerable are they unto man; but all things are numbered unto me, for they are mine and I know them. 37 And the Lord God spake unto Moses, saying: The heavens, they are many, and they cannot be numbered unto man; but they are numbered unto me, for they are mine. 38 And as one earth shall pass away, and the heavens thereof even so shall another come; and there is no end to my works, neither to my words. 39 For behold, this is my work and my glory—to bring to pass the immortality and eternal life of man.

tach typed, "If they don't deny it, how do they deal with Jesus the saviour?"

Moses 1:33 33 And worlds without number have I created; and I also created them for mine own purpose; and by the Son I created them, which is mine Only Begotten.

tach typed, "Did [He] come to earth just to wash terran sins away, or alpha-centaurians sins are included too? If not, does an alpha-centaurian jesus exist?"

I cannot easily locate references in a timely fashion, however, LDS are of the mind that the Lord also atones for all of His work, meaning his work outside of this earth, but this is not the official stance I believe and I'm only going by memory.

tach typed, "Also - Adam's original sin also doomed alpha-centaurians? If so, why?"

Adams transgression did not doom alpha-centaurians (aliens), because (of three things) alpha-centaurians are not literal descendants of Adam, they were not put under Adam's dominion (Genesis 1:26,28; 2:19), nor did they exist in the same time frame (before the time of repentence Alma 42:4-10 wherein Adam was not subject to death) as Adam at the time of the transgression. The penality for sin (death) extends by lineage (Exodus 20:5:34:7; Numbers 14:18; Deut. 5:9; D&C 103:26).

Genesis 2:19 19 And out of the ground the Lord God formed every beast of the field, and every fowl of the air; and brought them unto Adam to see what he would call them: and whatsoever Adam called every living creature, that was the name thereof.

Gen. 1:26 26 ...and let them (mankind) have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth.

Alma 42:4 4 And thus we see, that there was a time granted unto man to repent, yea, a probationary time, a time to repent and serve God.

tach typed, "Also, alpha-centaurians are human? God did made them as its image?"

By revelation we believe that the Lord created many worlds.

If need be I will explain further and try to cite more references if asked.