JOEL - In a vision given to Moses, the Lord said,

"Worlds without number have I created; and I also created them for mine own purpose,there are many (worlds) that now stand, and innumerable are they unto man" (Moses 1:33, 35).

Joseph Smith taught that old physical worlds pass away while new ones are being formed (Moses 1:35, 38), that each system of worlds has its own laws (D&C 88:36-38) and Jesus Christ is the creator of all these worlds (D&C 76:24; 93:9-10).

"That by him, and through him, and of him, the worlds are and were created, and the inhabitants thereof are begotten sons and daughters unto God.(D&C 76:24)

King Benjamin tells us that the atonement applies to those who are sons and daughters of God making them free from sin:

"And now, because of the covenant which ye have made ye shall be called the children of Christ, his sons, and his daughters; for behold, this day he hath spiritually begotten you; for ye say that your hearts are changed through faith on his name; therefore, ye are born of him and have become his sons and his daughters.
And under this head ye are made free, and there is no other head whereby ye can be made free. There is no other name given whereby salvation cometh; therefore, I would that ye should take upon you the name of Christ, all you that have entered into the covenant with God that ye should be obedient unto the end of your lives." (Mosiah 5:7-8)

The atonement must apply to any who are called "begotten sons and daughters unto God", otherwise they would not be called such.

In a poetic paraphrase of Doctrine and Covenants 76, Joseph also wrote that Jesus Christ was not only the Creator but also the Savior of these worlds:

And I heard a great voice bearing record from heav'n,
He's the Saviour, and only begotten of God -
By him, of him, and through him, the worlds were all made,
Even all that career in the heavens so broad,
Whose inhabitants, too, from the first to the last,
Are sav'd by the very same Saviour of ours;
And, of course, are begotten God's daughters and sons,
By the very same truths, and the very same pow'rs.
(Times and Seasons, 1 Feb. 1843, 82-83)

Concerning the universality of the Savior's work, Elder Marion G. Romney declared,

"Except for his mortal ministry accomplished on this earth, his service and relationship to other worlds and their inhabitants are the same as his service and relationship to this earth and its inhabitants" (Improvement Era, Nov. 1968, 46).

So as far as we know Christ's atonement covers everyone on every world that has existed or will exist in this universe. People of other universes, past and future, may have their own separate Godhead and Savior who will atone for their sins.