Most people have trouble believing in the Power of GOD. So they look for excuses to support their weak faith in an Omnipotent GOD. They can look straight at a verse with all its power exposed and then come to a condescending view of GOD. Therefore they will conclude that mansion can't be planets they must be bodies. But examine the context of the scripture. Is a place (Mansion) or a thing (body) being discussed?

Joh 14:2 In my Father's house are many mansions: if it were not so, I would have told you. I go to prepare a place for you.
Joh 14:3 And if I go and prepare a place for you, I will come again, and receive you unto myself; that where I am, there ye may be also.

The first words overlooked are "if it were not so, I would have told you". Jesus is saying "if there were NOT many planets (manions) I would have told you that yours was the only one". But He did not tell them that Earth was the only Mansion.

Now notice he can't be talking about a body because He is NOT talking about a THING. He is talking about a PLACE and even in context uses the word PLACE. I go to prepare a thing or a place?

At the pre-trib Jesus is going to take us to one of those PLACES and not one of those THINGS.

In the heavens there are many of these PLACES and he went to get one ready for us. He did not go to prepare THINGS; He went to prepare one PLACE.