JOEL - There are some past church members who said they heard church leaders talk about these possibilities, but in truth this hollow earth or planet theory has not been seriously considered among any of the past or present leadership of the church. Some people have even suggested that the lost tribes are on their way to earth in a big spaceship (remember Battlestar Galactica?).
The official position of the LDS church is that "we have no knowledge of the location or condition of that part of the ten tribes who went into the north country." (Little, James A. & Richards, Franklin D., Compendium. p. 88 (See also 1 Nephi 22:4)) James E. Talmage wrote "it is plain that, while many of those belonging to the Ten Tribes were diffused among the nations, a sufficient number to justify the retention of the original name were led away as a body and are now in existence in some place where the Lord has hidden them." (Talmage, James E. The Articles of Faith. p. 340)
Many church leaders have offered their opinions on where the lost tribes are and what their condition is based on interpretations from some scriptures and comments from past church leaders, but these opinions have not been found to be in total agreement with each other.
Elder Bruce R. McConkie writes "There is something mysterious and fascinating about believing the Ten Tribes are behind an iceberg somewhere in the land of the north, or that they are on some distant planet that will one day join itself with the earth, or that the tribe of Dan is in Denmark, the tribe of Reuben in Russia, and so forth. A common cliché asserts: 'If we knew where the Lost Tribes were, they would not be lost.' True it is that they are lost from the knowledge of the world; they are not seen and recognized as the kingdom they once were; but in general terms, their whereabouts is known. They are scattered in all the nations of this earth, primarily in the nations north of the lands of their first inheritance." (Bruce R. McConkie, "A New Witness for the Articles of Faith)
In one of his concluding testimonies in the Book of Mormon, Mormon said: "I write unto all the ends of the earth; yea, unto you, twelve tribes of Israel" (Mormon 3:18). Obviously Mormon believed they were somewhere on the earth or he would not have said this. And in 3 Nephi we read, "And as surely as the Lord liveth, will he gather in from the four quarters of the earth all the remnant of the seed of Jacob, who are scattered abroad upon all the face of the earth." (3 Ne. 5:24), suggesting that they all exist somewhere on the surface of this earth.
So as far as we know the lost ten tribes are scattered somewhere probably on this earth among us, some presumably in the northern countries(Jer. 3:18; 16:14-15, D&C 110:11), north of their Palestinian home, north of Assyria from where they escaped. We do know that they are not lost to God(3 Ne. 17:4), since Jesus Himself visited them after He appeared to the Nephites(3 Ne. 15:12-16; 17:4). Many of them may have lost their identity and have been assimilated into other local populations. But whereever they are they will eventually show themselves in a remarkable way(D&C 133: 26-32) and will accept the gospel in these latter-days as a result of the existing missionay work of the church.