How to get to 10 trillion people in this solar system?

>
> Well, some economists (like Hans-Hermann Hoppe) blame welfare for the
> declining birth rates. In old times, when there was *no* welfare at
> all, children were seen as a kind of "retirement plan": after you get
> old and beyond productive age, you would have no appreciable income,
> but your children would take care of you anyway. After
> government-sponsored retirement was invented, though, it seemed less
> and less advantageous from the personal economic planning point of
> view to have children.
>