I wonder when we will have the first dollar trillionaire. (Rockefeller doesn't count) It's likely that some people will own entire planets in the future.
I remember a documentary by a wealthy persons son who interviewed one of the founders of Kinkos who makes a comment something along the lines of wanting to look down on Earth from the Moon someday and being able to say, "I own (part?) of that."
I think on paper you can get close with derivatives although you'll never be able to liquidate them.
This reminded me of a Star Wars book where Han Solo won a planet in a game.
>Han Solo won the deed to Dathomir from Drackmarian Warlord Omogg in a game of sabacc in 8 ABY.