It's odd to see how many votes "Humanity cannot find a replacement for earth before our resources run out" is getting. Small bands on surviving tribes don't need "resources." Even if the worst-case-scenario comes to pass, it only takes a few deer, some meltwater and some fruit trees for a human family to survive. We don't need petrochemicals to live on as a species.
And if you're counting water as a resource, you should have voted for #1. The only way Earth will run out of water is when larger cosmic events are taking place...