In my view, any form of ownership has a subtle window of time attached to it, be it physical objects or human relationships. What is today yours, was owned by someone else in the past and you will have to bequeath your ownership to someone else in the future inevitably!

Within this planet earth in the past, have one set of people not conquered space from aborigines to take ownership away and wipe out the culture and civilization? Why can't the same thing happen across planets as well, if we (or aliens) are able to discoverer the technology for interplanetary spaceflights? Does anyone know for a fact that Mayan civilization was wiped out by natural forces or by capture of land by migrants?

The purport is that any form of ownership is always in transition and it will be taken away eventually by forces beyond our control! The ownership of this planet could be eventually be taken away from us (and culture and civilization possibly destroyed) by aliens or when the planet looses its ability to support lives (due to famine, drought, atmosphere loosing oxygen...).

Having said all that, it is also quite possible that when the planet becomes dead, we (homo sapiens) may have discovered the technology by then to move into other planet (along with our culture and civilization). Not to worry, when one end of the universe closes its doors for us, we will have the ability to open the doors else where in this vast endless space - aren't we survivors and so have our ancestors been! It is this small little hope that keeps us going everyday...