John Uskglass said:
Running out of fuel is a possibility, not an inevitability, and running out of space on earth? Not for a while.
Running out of fuel in the capitalist system is an inevitability (even if the world population stood at 1000) and not a possibility, the only way to avoid it being to stop using it, which is the same in effect as using it all up.
Unless you believe the great Xenu's mass-murder of aliens on earth has actually implanted a infinite-oil-making-machine deep under the earth's crust? You don't? Then the above is an inevitability.
Running out of space is not that difficult, within the capitalist system. The problem is not, again, the amount of people in dirrect corelation, though...
3 billion Asians we can survive, easy. 3 billion Asians all wanting a car we can't. QED.
See, the problem is that sociologically even within our current system our "needed growth" is finite. That's not a problem, though it becomes a problem when even over-populated countries feel the pressure to keep breathing because of some internatonal competition-bullcrap.
Economic growth is never finite inside our capitalist system, which means that even if our numbers stop multiplying we still need to grow for the system to maintain itself. By 3% a year, mind you, whereas the world population growth rate is only about 1.2%, guess which one is the bigger problem?
PS: about the "what about other planets?"-thing, I'll hold off on that one until we at least discover one that might be mildly useful, let alone travel to it. Besides, since the universe is finite, so are its resources, just on a grander scale.